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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 10:56:32 AM »Progress thus far:
First, the dinosaurs came, but they got all fat and sick and died and turned into oil, then came Mercedes Benzes....
ahem.
The visit up to the JAP guru in Ipswich went very well indeed. We got on very well and it was a fun morning of fettling.
I took up a few parts to get his opinion on their worthiness and discovered a few things.
Firstly - the 2nd head I bought is badly pocketed: the valves have been sunk into the head so far (by use and grinding in) that they're actually now "outside" the original hemisphere shape of the combustion chamber. I also found out the other day that the original head has had the valve-seats replaced (I think I knew this), so it's actually in better condition than Head 2 (despite Head 2 being cleaner... ).
In addition we dicovered that the exhaust valve for Head 1 is bent. That was a puzzler - because there's no evidence of it having hit anything within the chamber (the other valve, or the piston). It's why it wasn't sealing. Luckily he had a 2nd valve that slotted right in.
We also looked at the loose inlet guide, and he replaced it from his stock. I didn't know this... but the valve guides can just be hammered in and out! :wtf: The original valve fits in the new guide perfectly, so Head 1 will be back in action once it's fully cleaned up and re-lapped in.
I'll probably by some flap wheels for my Dremel and have a go at the carbon build ups in the exhaust manifold myself, and then as a final resort take it to a media blaster of some sort. (walnut or vapour probably).
We also examined the rocker box : This is the best pic I could get of the issue :
What I thought was wear caused by too much endplay in the rocker arms, is actually caused by the fact that the rocker arms are the wrong arms for the box! Well - not actually "wrong" - but they've not been fettled properly! The original forging patterns for the rocker arms make them too big for the rocker-box, and it's necessary to grind off a lot of the material. When he's doing race-engines apparantly he takes off quite a lot of the material on the rocker arms, including drilling lightening holes and a hole down the ball-end! (sounds painful!). So - I'm going to take off the material necessary from the rocker arms to ensure that there is clearance for the oil-gallery (shelf), and get that oil-shelf rebuilt up - that should ensure that the rocker-bearings get plenty of oil in the future. I'm hoping that I'll be able to do that to a reasonable quality using hand-tools... we'll see.
He also explained a good trick for checking the valve-spring weights, which I'll take photos of as I do it.
He checked the piston - it's fine (needs a clean), and the cam-shaft - which is a "standard", not a race item (which is fine), and also explained a few tricks for getting that aligned correctly should it need it. All stuff I'll be checking when I reassemble it.
Then we got onto the crank (and I remembered I had a camera.... )
Separating the big end shaft from the flywheel
https://youtu.be/poGPWyDRhGg
Big end rollers - all looked ok
The big end cage is just ally - but it looked ok.
Inner faces:
At this point there was a slight bit of confusion due to the distinct lack of balancing holes having been drilled in the inner faces, until I remembered it's a 350 crank (not a 500). Obviously new cage and rollers were fitted.
Pressing the tapered big-end shaft back into the flywheel.
With some minor adjustments to get the outer diameter faces parallel :
(I now want a brass hammer!)
Things are much easier when you have the right kit....
More minor adjustments to the get the shafts aligned :
This was very much an iterative process: take a measurement, squeeze the wheels, take a measurement, squeeze again... etc. etc
The for the first few checks the nut wasn't even done up as tight as it could be ... just f-ing tight. After getting it pretty damned close, he did the nut up even tighter (using a 4-foot breaker bar and a lot of bodyweight....) then spent another half-hour getting it just right: toed IN 3/4 of a thousandth of an inch on one side, and about 1 thou on the other side. Apparently under heavy load the crank can toe "out" and pinch the con-rod, so he sets them up toed in slightly.
At the end of the session he declared that it was a "good set of wheels". Which I'm very happy about! They should now be good for quite a while, and with the use I'm expecting from the bike (pottering about locally annoying the purists) they should outlast me.
As we were tidying up and nattering (another half hour at least... ) he realised that I didn't have a mag-platform steady bracket: it should look like this :
And this is how it's mounted : to the underside of the mag-platform, and to the crankcases.
The problem this fixes is engine vibration cracking the mag-platform supporting structure. So, while I have no idea why the engine didn't have one, it looks like a good idea to have one fitted so I'll try to either source one (I really should have asked him about that - so I probably will), or make one up. It needs to be good strong steel. (or whatever that fancy bronze-looking one is).
I now have all the parts ready, but still a lot of cleaning to do, and a few other optimisation jobs (eg lapping in the timing cogs to their shafts, and even lapping in the head to the barrel!).
First, the dinosaurs came, but they got all fat and sick and died and turned into oil, then came Mercedes Benzes....
ahem.
The visit up to the JAP guru in Ipswich went very well indeed. We got on very well and it was a fun morning of fettling.
I took up a few parts to get his opinion on their worthiness and discovered a few things.
Firstly - the 2nd head I bought is badly pocketed: the valves have been sunk into the head so far (by use and grinding in) that they're actually now "outside" the original hemisphere shape of the combustion chamber. I also found out the other day that the original head has had the valve-seats replaced (I think I knew this), so it's actually in better condition than Head 2 (despite Head 2 being cleaner... ).
In addition we dicovered that the exhaust valve for Head 1 is bent. That was a puzzler - because there's no evidence of it having hit anything within the chamber (the other valve, or the piston). It's why it wasn't sealing. Luckily he had a 2nd valve that slotted right in.
We also looked at the loose inlet guide, and he replaced it from his stock. I didn't know this... but the valve guides can just be hammered in and out! :wtf: The original valve fits in the new guide perfectly, so Head 1 will be back in action once it's fully cleaned up and re-lapped in.
I'll probably by some flap wheels for my Dremel and have a go at the carbon build ups in the exhaust manifold myself, and then as a final resort take it to a media blaster of some sort. (walnut or vapour probably).We also examined the rocker box : This is the best pic I could get of the issue :
What I thought was wear caused by too much endplay in the rocker arms, is actually caused by the fact that the rocker arms are the wrong arms for the box! Well - not actually "wrong" - but they've not been fettled properly! The original forging patterns for the rocker arms make them too big for the rocker-box, and it's necessary to grind off a lot of the material. When he's doing race-engines apparantly he takes off quite a lot of the material on the rocker arms, including drilling lightening holes and a hole down the ball-end! (sounds painful!). So - I'm going to take off the material necessary from the rocker arms to ensure that there is clearance for the oil-gallery (shelf), and get that oil-shelf rebuilt up - that should ensure that the rocker-bearings get plenty of oil in the future. I'm hoping that I'll be able to do that to a reasonable quality using hand-tools... we'll see.
He also explained a good trick for checking the valve-spring weights, which I'll take photos of as I do it.
He checked the piston - it's fine (needs a clean), and the cam-shaft - which is a "standard", not a race item (which is fine), and also explained a few tricks for getting that aligned correctly should it need it. All stuff I'll be checking when I reassemble it.
Then we got onto the crank (and I remembered I had a camera.... )
Separating the big end shaft from the flywheel
https://youtu.be/poGPWyDRhGg
Big end rollers - all looked ok
The big end cage is just ally - but it looked ok.
Inner faces:
At this point there was a slight bit of confusion due to the distinct lack of balancing holes having been drilled in the inner faces, until I remembered it's a 350 crank (not a 500).
Obviously new cage and rollers were fitted.Pressing the tapered big-end shaft back into the flywheel.
With some minor adjustments to get the outer diameter faces parallel :
(I now want a brass hammer!)
Things are much easier when you have the right kit....
More minor adjustments to the get the shafts aligned :
This was very much an iterative process: take a measurement, squeeze the wheels, take a measurement, squeeze again... etc. etc
The for the first few checks the nut wasn't even done up as tight as it could be ... just f-ing tight. After getting it pretty damned close, he did the nut up even tighter (using a 4-foot breaker bar and a lot of bodyweight....) then spent another half-hour getting it just right: toed IN 3/4 of a thousandth of an inch on one side, and about 1 thou on the other side. Apparently under heavy load the crank can toe "out" and pinch the con-rod, so he sets them up toed in slightly.
At the end of the session he declared that it was a "good set of wheels". Which I'm very happy about! They should now be good for quite a while, and with the use I'm expecting from the bike (pottering about locally annoying the purists
) they should outlast me.As we were tidying up and nattering (another half hour at least...
) he realised that I didn't have a mag-platform steady bracket: it should look like this : And this is how it's mounted : to the underside of the mag-platform, and to the crankcases.
The problem this fixes is engine vibration cracking the mag-platform supporting structure. So, while I have no idea why the engine didn't have one, it looks like a good idea to have one fitted so I'll try to either source one (I really should have asked him about that - so I probably will), or make one up. It needs to be good strong steel. (or whatever that fancy bronze-looking one is).
I now have all the parts ready, but still a lot of cleaning to do, and a few other optimisation jobs (eg lapping in the timing cogs to their shafts, and even lapping in the head to the barrel!).
