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31
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.....
32
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. :)
33
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.
34
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".
35
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.
36
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.
37
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 10:59:36 AM »
Another afternoon in the workshop, step forwards, step backwards.

(Actually, this was a few days ago)
Stripped down the Concentric I bought, it's not a Premier (which has a removable Pilot Jet, where the original Concentric has a fixed size bushing, and drainable float-bowl).

It's "tired", which I expected, but all there and will make a reasonable back up if the Monoblock proves to be too much of a pain in the arse.
If I decide to try it, I'll probably invest in some updated components, gaskets, etc, and will give it a proper bzzzzzzzzzzzzzzzzzzzzzzzzzt.
The jets (at least) looks identical to the ones for the Monobloc, of which I have found quite a few in Dad's old bits so I can experiment with the main jetting at least (which is only really effective at full-bore.... ;) )

Yesterday I had a go at making the gasket for the Timing Chest

Simple enough: the ball end of the hammer sheers the paper gasket on the hard edge of the facing with only gentle taps.  These faces weren't really designed to have a gasket.  Remember this is a race-engine designed for shale and grass-tracking with a total-loss oil system... weeping cases were a feature. :D  Not great for the road or proper tracks though, so gasket making is something I feel I'm going to get lots of practise at.

Assembled up the cam-shaft and cam-followers again and its.... "better".  the cam followers aren't sticking, but there's still resistance to movement from the cam-shaft itself.  Stripped it down and smeared some R40 onto the cam-shaft and bushes and reassembled it
Liquid gold.....


That was an improvement, but still some stiction. So I stripped it again and smeared a lot more oil onto the camshaft, reassembled and just as I wsa tightening up the very last nut on it's stud.... $%^&&.  I felt the thread go.   Tried the camsaft rotation while it was all assembled and it's a tad better, but I still think it's a little tight, so will order up some 0.8mm paper (the gasket paper I had was 0.4mm) and see if that makes the difference.

In the meantime... stripped it all down again carefully... and $%^&&sticks.




"Luckily" it's the hole at 5-o-clock on the face, which means it's got plenty of wall, and it's a blind hole (so no chance of crap falling out the other end later). 
Hit the TracyTools website and found a 1/4" BSF thread repair kit which will hopefully turn up next week (grabbed some Imperial sized drill bits too, seeing as I don't have any at all, the sort of thing I'm sure to need at some point). Also ordered some more gasket paper.

Hey ho, not really that much of a surprise, but still a tad irritating.
38
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 10:59:11 AM »
Decided to actually get into the workshop yesterday!  (other things have been happening, or putting me off), and figured I'd have a tinker with the timing chest because I was advised to check the cam-wheel/cam-follower end play before I assembled the crank.

Firstly it took we a while to find the studs, then I realised that none of the threads were very clean, so gave them a good brass-brushing.


But even then it seemed that none of the studs would cleanly go into the case, and some of the nuts refused to go down the studs easily.  So I ran taps and dies over everything (took me at least 20 minutes to actually figure out the thread form and find the right taps & dies, which I DID have amazingly: turns out it they're 1/4x26tpi BSF).





Eventually I managed to get everything spun down and tight.... then realised that one of the allen head cap bolts that are used inside the timing chain case was too long for it's hole!  I can only imagine that Dad never noticed this bolt was the wrong length (nothing surprises me anymore...) so I cut them both down by about 5-6mm (well - the thickness of a nut) :  I did both of them so I didn't have to faff with picking up the "correct" bolt for the shorter hole each bloody time.



So - eventually I got the case to tighten down on the timing chest.... and the camshaft is jammed.

:wtf:

The Cam-followers are rattling around on their shaft as well - there's at least a mm of movement, if not more.

:wtf:

But it's the camshaft being jammed that is confusing the hell out of me.  The bushes that it sits in are open ended so it's not like the saft is running up against the end of a closed hole.

The timing chest case DID have a paper gasket fitted when I stripped it down.... so I'm currently thinking that "maybe" the thickness of the paper gasket is what is providing the end-play gap offset for the cam-shaft itself.

Photos from the strip-down: you can see the paper gasket clearly.





So - the next job, I guess, is to make up a new gasket and see if that frees off the camshaft, and look for a proper shim for the cam-followers.  I did find a ~1.5mm thick washer that was exactly the right internal diamater (1/2") to fit the cam-follower shaft, but the cam-followers did get "tight" when fitted - so again - if the paper gasket provides the offset.... this might work...



Oh yeah - also - over the summer I got some repairs done on the rocker-box parts:








The oil-galleries need to be machined back down to approx 3mm high and a similar thickness.
See here :




TBH, I'm probably going to do this by just fitting a dremel-esque grinding tool into the piller drill and using it as an ersatz "single-axis machine-tool" and gently take the aluminium off flake by flake...

In addition the rocker arms themselves will be ground down to be narrower (and lighter), as previously mentioned.
39
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 10:58:48 AM »
Closer inspection of the exhaust valve seat revealed a narrow part, so had another go.



I think that should be usable now.
40
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 10:57:46 AM »
Managed some time in the workshop yesterday.

Re-checked the valve springs.


Picked the two pairs that seemed to give the right spring weights and threw the into the ultrasonic.

Here's the shiny piston:



Next step is to get the valves lapped, so I prepped the seats with a Sharpie


Loaded the valves..


And did the Twist again!


After a coarse grinding...


Then cleaned off the coarse paste, and prepped the valves..


After a fine grinding.




Next job ideally would be to lap in the head to the barrel, to ensure the mating surfaces where the copper head gasket sits mate really well, but there's a complication I need to discuss with my guy.
Where the head gasket sits is in a depression in the barrel, and the head has a matching "protrusion", and where the studs run is in the"outer ring". I'm not sure if the faces on the  "outer rings" are supposed to meet or not.
Problem is that if the gasket is not in place, the "inner ring" faces don't meet when you sit the head on the barrel.
I'm not sure if I need to get some material machined off the top surface of the barrel or not.
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