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21
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:04:26 AM »
Did some more tinkering on Friday night:

This is the dural bar I mentionted previously:


The intention is to use these two holes - yes, yes one of the bolts is the upper shock mount, but it'll be fine.... ;)


Just a shot of the general arrangement of what's under the saddle unit:


Current shock mounting bolt on bottom... hmmmm
Two found 3/8" bolts above - much better shanks!


"Found" bolts is pretty much the right length (2"), but not completely convinced that the shank is long enough.


Ideally the shank of the bolt should interface with the inner part of the shock mounting plates as well as the full width of the shock eyelet.  This is to reduce shear forces on the bolt - thread forms are a weak point when dealing with shear forces.

The Allen-head bolts looks trick.... so if I end up searching for custom bolts I "may" go back to Allen heads - but the hex-head is a loer profile which bh, I think I prefer.




I was cleaning up the botls and looking for possible bits and pieces to convert into spacers, but I had a bit of a blocker happen when I used one of the new Cycle thread Dies to clean up the thread on one of the bolts.  They were a bit manky (but good), so I cleaned them in the usual style with a bras brush, but then decided to spin the Die down them as well.  Loaded the die in the stock for the first one, and started running it down the thread. 

Hm - this is a bit stiffer than I expected... but maybe the gunk in the bottom of the threads is harder than I though.... carry on...

Got most of the way down the thread and took a much closer look - and realised in shock that the thread had been destroyed!  The top half of the thread form had been scraped off uniformly all the way down!  I ran a cycle nut down the thread and it IS a cycle thread, checked the other found bolt against the TPI guage, yup - 26TPI, and the mutated thread on the bolt also checked out as 26tpi.  Checked other known 26tpi bolts and nuts....


the friggin Die is damaged, or badly made.  :angry-cussingblack:  :angry-cussingblack:  :angry-cussingblack:  :angry-cussingblack:

Checked the other dies from the same purchase, and where I had a known good bolt - the dies ran fine (1/4" and 5/16" seems to be all I have in known good bits and bobs).

So - going to have to ring the supplier today and see what can be done.  Thankfully I've only wrecked one bolt... but it's a tad frustrating.  :roll:


I've also been thinking about these bolts and where to get bolts with a proper long shank, and I think I'm probably just going to have to get the longest 3/8" bolts I can find, with the longest shanks and turn down the thread to match what I need.  Buying custom made bolts will most likely be ruinously expensive, I don't really need 1000+ of the bloody things. :D
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:04:07 AM »
Hmmm - looking at this....


Those suspension mounting brackets are not standard...

The wideline frame doesn't have cupped shock mounts either :





Anyone got a lathe?
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:03:34 AM »
I'd been dong some general tinkering the last couple of evenings, it's frigging cold out there so anything that requires proper thought is being put to one side.

I was going to reassemble the head, and made a start: I'm only going to put a couple of mica washers on each spring assembly because the spring weights are good:


But then I discovered that the valve-stem circlip I put on looked very tired, so I thought, bugger it - should replace them, and tidied the head away again for later.

I've been looking at the engine cases trying to figure out the best way to get the upper-surface as flat as possible (ie where the cylinder sits), and realised that the two small engine case bolts that are the first two that get used, and which hold the cases together while the engine is mounted etc really should be proper shoulder bolts, and I've got 2 set-screws.  So I'be been trawling the net for a 1/4" BSC bolt with a long shank (7/8").  See my other thread  for details : https://revtothelimit.co.uk/viewtopic.php?t=11380 .

So - while I was in the workshop last night I had a good furtle through my collection of Imperial bolts from random sources..... and lookee here!

"Bolt" ( aka "Partially-threaded-Bolt") on left, "Set-Screw" on right.

No idea where this came from... but I have a feeling it may actually be the exact bolt that Speedway Service sell!


Knowing my Dad, he probably didn't fit this because it's actually too long, and would look odd.  It's longer, to allow the Magneto Platform Support Bracket to sit on it!

Win!

So - I spent the rest of the evening cleaning up the engine studs:


And then, because the Mag-Platform bracket was on my mind and something was feeling off.... I bolted up the cases (empty) and fitted them into the bike to check a few things :

This is how I was initially envisaging the Mag-Bracket sitting :




But... there's an issue:
The engine stud isn't long enough.


I'm going to have to rethink that Mag-platform bracket.  Even though it will be a pain making sure it clears the engine plates - I think I'm going to have to use the smaller bolt as per the usual practise, so I need to remake the bracket.  With any luck I can use the existing piece of metal, but if not, I do have more of that stuff, which is quite handy.

This is the rider's view when in the usual riding position :

The cylinder axis is directly in-line with your head! :D

Incidentally: You can see the longer 1/4" engine bolt in that photo.

While I was looking at less-fundamental things I also tinkered with the "saddle-bag" fitting that I have never been really happy with: it's a cruiser-style "yoke-bag" that uses two thick leather straps and I had it hanging by the side of the saddle, banging against the shock:


I was never really happy with that arrangements, so spent some time trying to figure out a way to improve it, and found a length of Dural in the box of metal bits that Dad had drilled a large number of holes in: no idea what it was for - maybe an early attempt at a brake-tension-arm or something, but it's absolutely ideal for mounting this bag onto the bike. I'll do more photos when I do more on it, but I spent some happy moments on the grinder tidying it up somewhat for use.

So - as part of that I looked into where it would be mounted, and discovered something that made me both laugh and curse.

The bike has always been "bangy" at the rear end when riding.  It crashes about and is noisy, not just from the engine.  I now know why....





The shock uppers have at least 1/2" of movement side-to-side!!  :wtf:

I need to look into what a regular Featherbed frame setup uses here - but at the very least I need a couple of 1/4" spacers to hold the shocks in place properly!  I also think that the bolts should again have longer shanks, so we'll see what can be found, or bought to bring these shock support mounts up to scratch.

$%^&&in ell....
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:02:59 AM »
The Cycle taps and dies turned up yesterday..... :D  (I ordered a set of 5 of each, 26 TPI : 1/4", 5/16", 3/8" 7/16", 1/2"  as a loose set, far far cheaper than ending up having to buy them separately).

I first cut the end off the stud that I had found, tidied it up a bit and span the 7/16 Die down it to clean the threads.  Then cut a nice deep slot in the top.

It fits perfectly.


Because the thread in the case doesn't go full depth, the plug does go tight.


Internal view.


Hmmmm decisions decisions.....


In the end I went with the 248, and slathered it all over.  Should keep it place!

Buoyed by that success, I turned to the magnesium engine case:

First I made up a sacrificial wooden block to stop the drill plunging into the cam-follower shaft:


hmm - that ain't gonna fly: :(


So it got mounted in the vice.



Drilled the hole using a very specific imperial size (which I've now forgotten, albeit it is written down in the notes in the workshop) after about 20 minutes trying to find out what the correct tapping drill size should be for a 7/16" cycle thread.
My Zeus book is a Metric edition, so while it does have "some" Imperial information (ok, a lot), it's not comprehensive.  And the tables in this book https://www.sunbeamland.com/wordpress/wp-content/uploads/2019/04/The-Vintage-Motorcyclists-Workshop.pdf which I am using quite a lot, have a gaping hole in the Cycle tapping drill sizes table. (see page 8) :(

I initially found information that suggested 13/32" but then found something else that suggested the next size down (XX/64 iirc) was suitable, and even that was a conversion from a metric measurement : ie the given measurement was a metric one, and I had to figure out the closest imperial drill.

So - drilled the hole, slowly and gently and when I hit the wood, it protected the shaft beneath adequately.

I re-oriented the cases for tapping the hole and remembered why I hate doing jobs like this:


It looks a lot worse than it actually is....
TBH I think the hole was a little narrow and maybe I could have used the larger drill size and the tapping would have been easier - but then I am not sure if the thread would be actually deep enough (ie the peaks of the threadform could have been flattened if I'd used a larger drill bit).

Next time I buy taps I'm going to try to buy the drill-bit from the same supplier and order over the phone to make sure I get exactly the right bits.

So - anyway, this hole isn't used for anything, it just needs plugging so a slightly off-axis thread isn't going to stop that.

I moved to plug.

Because I had a stud to cut for this, I screwed the stud in all the way until it ran out of thread and started to go tight in the hole.  Then marked the untheaded part that was sticking out, and the excess that was protruding into the timing chest, and cut the plug out.  Ran the Die down the threads to tidy them and cut the slot in the top:


I seemed to have cut the plug a little shorter than I intended:



But it descends to the right depth:  Because it was cut from a stud, the plug threads don't go full length of the plug - so it can be done up "tight". (Given that the head of the plug is below the level of the casing outer this would indicate that the threads in the hole aren't 100% at the top of the hole - probably due to my cack-handed tapping :( But - it's a plug, it's tight, it's good enough and a learning experience for the next time).


Fitted and loctitied, after cleaning the cases out with (iirc) carb-cleaner and a lot of blue paper.



By that time I was reasonably happy with the evening's work and headed in for chicken kiev and chips. :)

Next job is (I think) either to re-assemble the head: I'm pondering whether to have another go at grinding the valves in but I had a look at them last night and I think they're ok; or to get the crank shimmed up and then assemble the crankcases together.

I may have to look for a taller pillar drill so that I can get large objects under it properly: if I'd had the tap in the pillar drill to begin with, that hole in the engine cases would have been tapped much straighter.  My drill was a random find at an impromptu garage clearance and is a hobby-ist wood drill really.
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:02:24 AM »
Made up the new Timing chest gasket last night.

Started out with locating the bolt holes




Punched them using a 6mm punch (closest thing to 1/4")


I hammered the edges round for about 20 minutes, and got the inner shape punched out, then tidied up the outside with scissors.
This is why kids get taught to make crafts....


Perfect fit.


The active components installed


All perfect!  The camshaft rotates freely and there's a little bit of play on the cam-followers, but not enough to worry about.


I'm still pondering on getting some cap-head bolts to use instead of the studs and nuts.  Dad did have that on one of the earlier incarnations of the bike, but I've no idea why it's back to studs and nuts.


So - here's another mystery.  I dissassembled the timing chest again because there's still a lot to do before final assembly, and while I was doing that, a washer dropped out of nowhere into the chest.

w.t.a.f. ??

Where did that come from?

Cue about 20 minutes of utter bafflement and poking and prodding.  The workbench was utterly clear before I started, and this washer definitely "fell" into the timing chest - I saw it and heard it happen!

It's NOT one of the washers from the outer casing - there's 4 of them and besides the fact that I count them all on, and count them all off again each time I assemble and dissassembly the thing: it's not the same diameter.  It's 1/4" alright, but it has a slightly smaller outer diameter.

EVENTUALLY I worked out that it had come from the socket cast into the outer casing which the cam-follower-shaft pushes into.

In fact... there's 2 of them in there.


I can only surmise that they were "stuck" in the by a little bit of oil that was still in there, but still baffled as to "WHY" they are there.  The cam-follower-shaft is pressed into the main cases, it's going nowhere fast (or at all!).  I reassembled the chest with the washers removed to see if they were having any effect on the gapping: none whatsoever, then reassembled it again with them in place: no difference that I could see to anything at all.

I am confuzzled.

Given that they were there to begin with: I'll likely replace them on final assembly... maybe they stop vibration or something??

So.  I stopped thinking about that, it was giving me a headache....

And I looked at my stock of old bits of metal, and decided to make up a mag-platform bracket.

I found a length of steel that looked about right - I think it was originally part of a foot pump...

Faced it up to the cases a few times and realised it just need a straighten and a twist.


5 minutes of heat (gas-can-torch) and some judicious heaving and belting with a hammer....


Checking the fit with a stand-in bit of engine plate


Of course I've made it so that it's bolting to the WRONG engine bolt.... the one I was shown by the Guru actually bolts to the smaller lug you can see on the photos. D'oh!  BUT - it should do the job, and may even be a little easier to fit and means I won't need to compromise the engine plates (any further).

Obviously it'll be tidied up and (probably) polished once it's been proven.

Next, I turned to finding a plug for the valve-lifter-cable-hole.  I'm not going to bother with the valve-lifter, at least to start with.
It may become necessary later, so I'm not throwing it away, I'd need to get the exhaust cam-follower modified again by someone with a proper machine shop.
However - all last year I managed to turn the bike over without the valve-lifter working (I always thought was as innefective as hell) so, despite a dodgy right knee, I'm hoping I'll be ok.

Rummaging through all the stuff I've got, I eventually found something that matched the threads in the casing:


No idea what this stud was from originally, but it was in the "odds and sods" box and the only person who might know isn't with us ( :( ).

it's 7/16" x 26tpi.... which makes it a British Standard Cycle thread. (anybody got Bingo yet?)

It's tempting to look for a bolt in that spec... but tbh, I'm going to just cut the stud down, saw a slot in one end to make it a flat-head-screwed grub screw and be done with it by using a lot of loctite. :D  TBH, the threads at the bottom of the hole are knackered, so the stud only goes in about 3/4 of the depth, so it does "go tight" by itself.  The loctite will be to guarantee permanance. (obviously I'll be using blue.)

I'm also going to redo the plug in the top of the main cases that is over the cam-follower shaft: the plug that's in there is loose and is only being held in place by silicon sealant.....
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:02:00 AM »
Managed to fix the stripped thread in the timing chest last night.

I've done this fix a few times now and it's normally relatively simple:

Drill out the hole - I took it slowly, the cases are (iirc) magnesium alloy, so they're not hard to work (and not ferrous...)


Tap the hole



Wind the insert in: Instructions say to use a light downward pressure.


Then ding the end of the insert off, invert the workpiece and the debris all falls out: simples!


Tested a stud in it... won't go in.  eh?

Look closely:


f******* c*********!

The damned insert jumped the threads.

This is one of the other holes - you can see the threads clearly



Here's the "fixed" thread : you can clearly see the "threads" are twice the pitch



So - sat back, started searching t'internet for "how to remove a thread repair", and found a lot of bollox about other types of thread inserts and repairs - but then finally came across this photo :


And I thought: Hey, I've GOT a tool like that somewhere!

Yup - dug it out and presto!





Came out easy. :D

The tapped threads were still useable.


Wound in a new insert (there were 20 in the box!) this time with no downward pressure at all, just gently rotated the tool, and it wound in easily and properly. :)


After tidying up all the swarf, which took a while, (I must sort out that air-compressor), I started gathering the bits for making a start on the new gasket, but realised I was running out of time for the evening so packed away again. 

I also discovered that the oil-box cover (the volume under the timing chest), screws are also 1/4 BSF, so if any of them go, I know I've got repair inserts. :)
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:01:40 AM »
Update from Tuesday:

Got a lot more done on the rocker box.

Working on the inlet side oil-gallery




Assembled the rocker arm bearing assemblies (which is a right faff) to check the clearances.








This is the rocker arms pushed as fair "in" as possible - which is the "worst-case scenario", I now have clearance here (just!)





I lost count of how many times I assembled, checked, stripped, filed/ground, cleaned off, assembled, checked, swore, stripped.......

But eventually ended up with a satisfactory clearance.

I was going for a "not-touching" clearance if the rocker arms ever closed up to the galleries, but when you assemble the box, the end-float is initially set by the pressue from the "lumps" on the inside of the cover.  You set up the box with the bearing liner on the push-rod side as far "out" of the box as possible, and then do up the valve-side of the box tight, but leave the push-rod side screws slightly loose, so the rocker-arm bearing liner can be pushed in as the cover is screwed down, and that sets the "end-float" of the rocker arm.  The photos below show roughly what clearance you get now.





Should be good! :D

You can also just make out that I chamfered off the outer edges of the oil-galleries to give a bit more clearance.  (you can see the chamfer grow through the photos :D )


And finally, gave the little buggers a polishing:








The other ends (the valve ends) now look a little 2nd hand.. :D  So I "might" give them a gently polish if I can be arsed.

I may also borrow the kitchen scales to see how different they weigh, but I'm not sure it'll be much of an issue.
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:01:20 AM »
Got into the workshop on Friday evening....

Made some progress on the Rocker box: I figured grinding down the inlet rocker arm wouldn't take too long, and it didn't, (well, not that much!)

Inlet on left, unmodified.  Exhaust on right, modified.


What need to come off mostly, is the material on the "underside" (as seen above) so that the rocker arm clears the little oil-galleries.

Closer pic beforehand


Working the underside of the arm with files.  I did the material around the "head", where the screw thread is, on the grinder, then tidied up with files.


Afterwards:

I think I probably need to take a bit more off in that inner curve near the shaft, but that will depend a lot on how the clearance looks relative to the oil-gallery once that is cut-down.


This is all "rough-work" just to get the required clearances.  The rocker arms will be flap-wheeled smooth and polished before re-assembly.  I'm only modifiying the push-rod end. The other half that pushes down on the valve stem doesn't need any work: I'm only going for "usable" rather than "light as possible for racing".
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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:01:03 AM »
oh - another thing - I'm now kicking myself for not picking up a rocker box I spotted at Ardingly last weekend.  The seller wanted 200 for it, and was full of a story where he'd just sold one for 250.  But simply having another one, with another set of rocker arms would remove a lot of the "peril" involved in what I'm doing right now.

I was looking back at some old photos of the stash of parts my Dad had before he died, there was at least 80% of another engine there: 2 rocker boxes, another head, another barrel, cases, a single crank wheel... but becuase I had no idea what parts were needed at that time, it was decided to send them off to auction. :( 

So: I'm going to start stockpiling what I find from now on.
30
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:00:27 AM »
For someone with the same issues as Arthur Dent, I do seem to spend a lot of Thursdays in the workshop....

Decided yesterday to stop faffing and get on with some stuff.

First, I cleaned up the drive-side crank case where the main crank bearing liner was removed:
Before.


After a few minutes with a brass brush and a wipe down


I rigged up the hot-air blower in an articulated grabber (a digital tablet holder in fact), and took a preparatory reading


After only a few minutes I turned it over to get a good spread of heat:


Then I popped inside and retrieved the new liner from the freezer, where it had been hibernating for a few months....
That's the OLD bearing liner in the background of the photos.
When I got back, the temperature was even higher than this....


The inside was a little cooler after I took the heat off:


The new liner literally fell straight in, I was so surprised I didn't even remember to give it a tap, it didn't need it.
The little locating screw went straight in too.



Light at the end of the tunnel!

This is the "feed" for oil to the main crank bearing....

While it cooled off, I pondered what to do next..... then decided to go for it and dug out the new bearing cages and pins.
oh, and the grease.... LOTS of grease!

To be honest, I think I used more grease just on these bearings in ten minutes than I've used in any single year of working on bikes....

For some reason the original size roller pins aren't made any more so the accepted practise is to use 2 pins for each slot in the cage, and alternate the positioning - as you can see in the photo.

Almost as soon as I'd taken that pic and was pondering on the pins:  I realised: hang on: the old pins I took off are the OLD ones!  I actually have the proper old full-length bearing pins... a complete set!  But, given that I have no idea just HOW old they are, I decided to stick with the fresh ones.

So - the cases went together relatively easily for the dry-fitting, to see what the crank end-float is:
Crank pushed fully to the left:


Crank pushed fully to the right:


I measured the difference using a micrometer tail and it appears to be a movement of about 0.065" - I measured a few times, obvously, and I think the extremes were 0.078" down to 0.050" - so the consensus is about 0.065. 
I checked what I'd written down earlier and read "crank end float : 0.12" - 0.15" ".

Hang on.... (I thought).

How can my end float be LESS than the desired??   Did the Liner not go in properly??  Have I got to take the liner out again??  $%^&&sticks!!

So - I ended up having a quick call with the Guru, and while I was talking to him I took another look at the book....

which read:

Crank End Float : 0.012" - 0.015".

GAAAAH!!!!  what a muppet!!!  I apologised profusely to the Guru for wasting his time, he was very nice about it and we had a nice chat about things anyway, and I got on with it whilst kicking myself!!

In my defense - I'm a metric kid, I've no concept of what thousands of an inch "look" like!

I agreed with the Guru that all the crank needs is some gentle shimming (phnar) and dug out the collection of shims I've got : some new, (iirc supplied by the Guru) and some from the original build.
0.050, 0.040, 0.023(*2), 0.020, 0.010. 
So - with any luck - the 0.023 pairing should be exactly what I need! :D

Required thickness of shims: 0.065" - 0.015" = 0.05". 
Desired shim width if using 2 shims : 0.05" / 2 = 0.025"

I can also potentially use the 0.010 and 0.020 to get it a bit tighter if required.  but I'll have an afternoon of tinkering with that no doubt!.

I'm probably going to reassemble the cases again, dry, and double check that end-float, before deciding fully, but the crank looked pretty central so that should do the job.

This whole job was made MASSIVELY easier because I found a pair of legs for the engine last time I was in Dad's workshop:


While I as pondering things I also noticed that the cases have another set of numbers on the rear faces  :


I have no idea what these would be (they're not the engine numbers, those are on the front, on the rhs), and I'm tickled somewhat that they match, given that the cases are from totally different sources!

After I'd dissassembled the crank again to put it out of the way, I figured I'd make a start on the rocker box...
Measuring up the excess that needs to come off...

Answer: a lot.  These little shelves are supposed to be about 3mm high.....

Setting up the ersatz "single-axis-milling-machine" (dremel grindstone in the pillar drill!)


I used the grind stone first just to test the concept, and it worked well.
So I dug out a pot of cutting wheels. :D



I should state that there was a good long session of faffing with a bubble level to quadruple check that the everything was as flat as possible (the drill stand, the drill plate, the clamp, the workpiece, etc etc etc)

The main faff with it is replacing the cutting wheels every few minutes, but the rod that holds the wheel is chamfered about half-way down, so it's actually very easy to put it back into the drill in exactly the same place each time: with the chamfer butted up against the outer face of the drill-clamp jaws.

I got the first shelf (the exhaust side as it happens) cut down somewhat roughly and put the rocker arm together so I could check it against the shelf.  There was still a of interference, so I decided to have a go at the rocker arm as well :


This was filing down the sides of the arm, to help it clear the oil-gallery/shelf.  I also took it over to the grinder and shaved off quite a bit of the metal from around the bolthole where the ball-fitting is fitted.

I'll take more photos of the rocker arm next time I'm in there.  Still a lot to do, but some good progress made I think.
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