31
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Vreagh on March 22, 2026, 11:11:50 AM »Interesting, I always thought the petrol engine would require more finning because it burns hotter than cool running dope.
Recent Posts
32
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on March 22, 2026, 10:57:14 AM »Looking good. I notice there's not too many fins on that cylinder head. Was your dad running it on petrol?
Leon
Yes, the flywheels are 350, so the piston travel is only 80mm, compression is down to about 9:1 (ish), it was put together as a road bike, not a full blood race engine deliberately to make it easy to ride and to use petrol.
33
British Bikes / Re: Waratah article
« Last post by cardan on March 21, 2026, 01:07:24 AM »Thanks - satisfying to read to the end.
Leon
Leon
34
British Bikes / Re: Troubleshooting a JAP Special
« Last post by cardan on March 21, 2026, 01:04:18 AM »Looking good. I notice there's not too many fins on that cylinder head. Was your dad running it on petrol?
Leon
Leon
35
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on March 20, 2026, 09:55:09 AM »I think things are going well... (and a lot faster now the weather is warming up!)
So - last night I got another couple of hours in the shed and....
Dribbled a bit of oil into the cylinder to help the initial ring seal and checking of compression
I got the engine spinning a bit by hand and then wiped most of the excess that had gathered at the top of the barrel.
Then it was time for some heat!
The last time I tried quenching copper it was some brake-line crush seals and I massively over-did it and turned them into slag....
So this time I played the heat over the gasket until I had a glow chasing the heat around the ring, then chucked it straight into the water
I only did one side, mainly because I really did NOT want to over-heat the thing and warp it out of shape, so, we'll see how it goes. If it starts blowing very quickly... at least I now know how to get at it.
Shiny side up!
Buttoned up again.
I did try rotating the engine with my thumb stuck over the plug hole..... and there's definitely compression there! Whether there's enough compression only time will tell....
So - time to assemble the rocker box:
All the bits:
First I cleaned up the threads for the rocker adjusters:
Stuck the cap-heads on the valve stems : (This is exactly the sort of thing I would forget if I hadn't put ALL the parts into little trays of associated parts, as seen above, I would highly recommend this practise if you're doing a job like this).
The exhaust rocker had a little bit of corrosion on it's shaft, (phnarr), so I gave the shaft a little polish (ooer!). The inlet seemed fine.
Assembly of the rocker shaft with all their annoying little roller pins in situ:
I dribbled a little bit of oil into the roller-pin bearing assemblies to get them started, they definitely felt dry when I was first checking their movement.
As described earlier in the thread: the Rocker box assembly is done by pushing the bearing liner for the outboard (push-rod-side) as far out as it will go, fitting the top-cover of the box, then donig up tight ONLY the inboard screws, so the inboard bearling line is locked in place, and the outboard liner is "just" moveable ... then screwing the outer cover on, and the bosses on the inside of the outer cover then push the rocker arm into the box: pushing the outboard liner into the box just enough. This means the rocker arm "end-play" is automatically controlled and is hopefully minimised. I may actually re-do this process with a lot more photos next time I'm in the shed just to make sure I've done it right....
First thing to go on is the lower halves of the valve-covers.
I had to spend a bit of time trying to figure out where the washers on the bolts that hold the rocker box to the head (only 2 of em!) were supposed to go. I didn't make an exact note of this when I disassembled the bike (d'oh!) but I think I worked it out eventually. The complication is that there's also a bracket fitted above the rocker box that attaches to a head-steady which bolts to the bike's frame at the headstock, so I was trying to figure out if there should be a washer under the bracket, or 2 above.... I'm pretty sure there's no washer under the rocker box, it mates directly to the top of the head.
And that is where I ran out of time for the evening. :roll:
I'm currently pondering the idea of trying to seal the valve gear with the 3Bond - it's not designed to be sealed (it's a shale-track engine, weeping oil all over the place is a design feature), so it's a problematic process at best. Especially since I may well be placing and removing things repeatedly....
Food for thought over the weekend anyway.
NEXT TIME: Rocker box fitting and TIMING! (well, hopefully!)
So - last night I got another couple of hours in the shed and....
Dribbled a bit of oil into the cylinder to help the initial ring seal and checking of compression
I got the engine spinning a bit by hand and then wiped most of the excess that had gathered at the top of the barrel.
Then it was time for some heat!
The last time I tried quenching copper it was some brake-line crush seals and I massively over-did it and turned them into slag....
So this time I played the heat over the gasket until I had a glow chasing the heat around the ring, then chucked it straight into the water
I only did one side, mainly because I really did NOT want to over-heat the thing and warp it out of shape, so, we'll see how it goes. If it starts blowing very quickly... at least I now know how to get at it.
Shiny side up!
Buttoned up again.
I did try rotating the engine with my thumb stuck over the plug hole..... and there's definitely compression there! Whether there's enough compression only time will tell....
So - time to assemble the rocker box:
All the bits:
First I cleaned up the threads for the rocker adjusters:
Stuck the cap-heads on the valve stems : (This is exactly the sort of thing I would forget if I hadn't put ALL the parts into little trays of associated parts, as seen above, I would highly recommend this practise if you're doing a job like this).
The exhaust rocker had a little bit of corrosion on it's shaft, (phnarr), so I gave the shaft a little polish (ooer!). The inlet seemed fine.
Assembly of the rocker shaft with all their annoying little roller pins in situ:
I dribbled a little bit of oil into the roller-pin bearing assemblies to get them started, they definitely felt dry when I was first checking their movement.
As described earlier in the thread: the Rocker box assembly is done by pushing the bearing liner for the outboard (push-rod-side) as far out as it will go, fitting the top-cover of the box, then donig up tight ONLY the inboard screws, so the inboard bearling line is locked in place, and the outboard liner is "just" moveable ... then screwing the outer cover on, and the bosses on the inside of the outer cover then push the rocker arm into the box: pushing the outboard liner into the box just enough. This means the rocker arm "end-play" is automatically controlled and is hopefully minimised. I may actually re-do this process with a lot more photos next time I'm in the shed just to make sure I've done it right....
First thing to go on is the lower halves of the valve-covers.
I had to spend a bit of time trying to figure out where the washers on the bolts that hold the rocker box to the head (only 2 of em!) were supposed to go. I didn't make an exact note of this when I disassembled the bike (d'oh!) but I think I worked it out eventually. The complication is that there's also a bracket fitted above the rocker box that attaches to a head-steady which bolts to the bike's frame at the headstock, so I was trying to figure out if there should be a washer under the bracket, or 2 above.... I'm pretty sure there's no washer under the rocker box, it mates directly to the top of the head.
And that is where I ran out of time for the evening. :roll:
I'm currently pondering the idea of trying to seal the valve gear with the 3Bond - it's not designed to be sealed (it's a shale-track engine, weeping oil all over the place is a design feature), so it's a problematic process at best. Especially since I may well be placing and removing things repeatedly....
Food for thought over the weekend anyway.
NEXT TIME: Rocker box fitting and TIMING! (well, hopefully!)
36
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on March 19, 2026, 10:21:37 PM »Nice going.
Probably an optical illusion, but is that piston circlip fully seated in it's groove all round?
I've been caught out before where the supplier fitted round section circlips in a square section piston groove, and they didn't seat well.
It is yes, i had to look 3 times though...
37
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rex on March 19, 2026, 09:50:20 PM »Nice going.
Probably an optical illusion, but is that piston circlip fully seated in it's groove all round?
I've been caught out before where the supplier fitted round section circlips in a square section piston groove, and they didn't seat well.
Probably an optical illusion, but is that piston circlip fully seated in it's groove all round?
I've been caught out before where the supplier fitted round section circlips in a square section piston groove, and they didn't seat well.
38
British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on March 19, 2026, 07:51:56 AM »Next stage.... the barrel.
These are the barrel studs. The little discs above them go into the blind holes in the cases first to act as a "bump-stop" and (I think) to help against galvanic corrosion (or something)
Each one gets a small amount of copa-slip (One day that tub will run out, and I will be left hopelessly bereft!)
All in situ, and tightened down hard:
Cleaning up the excess copa-slip that was left
Ready for the piston: already have new rings and gudgeon pins.
The first bit is easy - piston and pin slid together nicely.
The circlips though were a bit of a fight to get in - but eventually I worked them in with a large pair of needle pliers (which is probably where that scratch on the piston came from. :roll: hey ho.
The second one was just as hard to get in.
Imprompto hook (from an old bungee net) bent up to keep the piston from flailing around.
Sika Wonder Wipes and meths again to get the base gasket surface clean
3Bond application, tried to put the minimum on this time, so just dabbed it on lightly and spread it out thin
The base gasket was fitted and more 3Bond lightly applied to the top surface of the gasket, then I checked the ring gaps: they just need to be "opposite" each other, and from memory, they shouldn't be directly on the "thrust" sides of the piston (ie, fore/aft), so I made sure they were lined up on each side of the piston:
Rather nice little ring-clamp from Dad's collection :
Much easier and lighter than some I've seen
And this is where I envy Zaphod Beeblebrox..... a 3rd hand would be helpful at this juncture.
GAH! First attempt and the friggin bottom ring managed to slip out after the ring-clamp had slid away!
But, after a couple more attempts (it was a bit of a fight to get the rings into the bore!)
Yay!
Up
And down she goes!
The piston moves nice and freely in the bore, must remember to drip a little oil in there just to help it slide before I do the head gasket.
I pushed the barrel down and wiped up the worst of the 3Bond excess
Then stuck the head on so I could pull the barrel down tight and let the 3Bond go off properly in compression.
Here's the head sitting on the top of the studs so you can kinda see how the studs work.
The upper part of the stud assembly is free to rotate so it pulls the head down onto the barrel. With studs like this, you can remove the entire barrel and rockerbox assembly without having to split it all up (should you want to)
However: doing up the studs is a long process because you can only rotate the stud with a spanner about 1/3 of a turn each iteration..... so I broke out my new secret weapons....
Wera Joker spanners: they're good.
You can flick the spanner back and forth like a ratchet spanner: Took a few tries before I really got the hang of it, they do tend to drop away unless you use your other hand just to keep them aligned on the nut correctly, but by f~ck do they speed up something like this! The studs go into the head by about an inch (approximately) on a relatively fine thread, so it's a LOT of 1/3 turns to do - the Joker cut the time down I'd estimate by at a half, if not more. I was pressed for time while I was doing this last bit as well, so I was very grateful for having the Jokers! (I can't seem to put an Amazon url into here - so just search for "Wera Joker Self-Setting Spanner" I have a set of 4, each one covers a smallish range of sizes.)
I left the head tightened down so the 3Bond has time to go off and the base gasket should be all set by the next time I get into the workshop.
Coming up!
Fire! Heat! Water! Imminent probability RB burns himself!
These are the barrel studs. The little discs above them go into the blind holes in the cases first to act as a "bump-stop" and (I think) to help against galvanic corrosion (or something)
Each one gets a small amount of copa-slip (One day that tub will run out, and I will be left hopelessly bereft!)
All in situ, and tightened down hard:
Cleaning up the excess copa-slip that was left
Ready for the piston: already have new rings and gudgeon pins.
The first bit is easy - piston and pin slid together nicely.
The circlips though were a bit of a fight to get in - but eventually I worked them in with a large pair of needle pliers (which is probably where that scratch on the piston came from. :roll: hey ho.
The second one was just as hard to get in.
Imprompto hook (from an old bungee net) bent up to keep the piston from flailing around.
Sika Wonder Wipes and meths again to get the base gasket surface clean
3Bond application, tried to put the minimum on this time, so just dabbed it on lightly and spread it out thin
The base gasket was fitted and more 3Bond lightly applied to the top surface of the gasket, then I checked the ring gaps: they just need to be "opposite" each other, and from memory, they shouldn't be directly on the "thrust" sides of the piston (ie, fore/aft), so I made sure they were lined up on each side of the piston:
Rather nice little ring-clamp from Dad's collection :
Much easier and lighter than some I've seen
And this is where I envy Zaphod Beeblebrox..... a 3rd hand would be helpful at this juncture.
GAH! First attempt and the friggin bottom ring managed to slip out after the ring-clamp had slid away!
But, after a couple more attempts (it was a bit of a fight to get the rings into the bore!)
Yay!
Up
And down she goes!
The piston moves nice and freely in the bore, must remember to drip a little oil in there just to help it slide before I do the head gasket.
I pushed the barrel down and wiped up the worst of the 3Bond excess
Then stuck the head on so I could pull the barrel down tight and let the 3Bond go off properly in compression.
Here's the head sitting on the top of the studs so you can kinda see how the studs work.
The upper part of the stud assembly is free to rotate so it pulls the head down onto the barrel. With studs like this, you can remove the entire barrel and rockerbox assembly without having to split it all up (should you want to)
However: doing up the studs is a long process because you can only rotate the stud with a spanner about 1/3 of a turn each iteration..... so I broke out my new secret weapons....
Wera Joker spanners: they're good.
You can flick the spanner back and forth like a ratchet spanner:
Took a few tries before I really got the hang of it, they do tend to drop away unless you use your other hand just to keep them aligned on the nut correctly, but by f~ck do they speed up something like this! The studs go into the head by about an inch (approximately) on a relatively fine thread, so it's a LOT of 1/3 turns to do - the Joker cut the time down I'd estimate by at a half, if not more. I was pressed for time while I was doing this last bit as well, so I was very grateful for having the Jokers! (I can't seem to put an Amazon url into here - so just search for "Wera Joker Self-Setting Spanner" I have a set of 4, each one covers a smallish range of sizes.)I left the head tightened down so the 3Bond has time to go off and the base gasket should be all set by the next time I get into the workshop.
Coming up!
Fire! Heat! Water! Imminent probability RB burns himself!
39
British Bikes / Re: Whatsit?
« Last post by murdo on March 18, 2026, 10:05:14 PM »Could it be the ring (with a spring) that holds the headlight bulb in place?
40
British Bikes / Re: Waratah article
« Last post by murdo on March 18, 2026, 10:01:57 PM »They have sent an update on the Phil Irving article.
https://mcusercontent.com/8d8ea490ca17c83e195d0d40f/files/80eebd3d-80f3-5f99-479a-5baeec168cb4/Phil_Irving.pdf?utm_source=Old+Bike+Mag&utm_campaign=079f4c2002-EMAIL_CAMPAIGN_2024_09_10_05_32_COPY_01&utm_medium=email&utm_term=0_-08fc9335e8-569925615&mc_cid=079f4c2002&mc_eid=fd765f182e
https://mcusercontent.com/8d8ea490ca17c83e195d0d40f/files/80eebd3d-80f3-5f99-479a-5baeec168cb4/Phil_Irving.pdf?utm_source=Old+Bike+Mag&utm_campaign=079f4c2002-EMAIL_CAMPAIGN_2024_09_10_05_32_COPY_01&utm_medium=email&utm_term=0_-08fc9335e8-569925615&mc_cid=079f4c2002&mc_eid=fd765f182e