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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on January 30, 2026, 11:07:33 AM »I'm starting to think that I really need to get in the workshop a bit more often - there's still a lot to do, and I'm definitely taking my time with things.
So - last night I went in and made gaskets. ... I know, it's soooo exciting!
Some of the parts that need gaskets, with existing old ones:
I started with the output side shaft seal plate gasket - this is a "non-standard" fitment anyway to slow down the loss of oil around the output shaft.
The outer and the fitment holes were relatively simple - but I need to break out the school-boy drawing tools for the inner circumference.
Note to self... for f*cks sake draw on the OTHER side....
But a reasonable finished product.
Next - the oil-drain chest cover.
Should be functional:
Funnily enough - the oil-collection chest gaskets can be bought, but this is an older one that's been used, and I can use it as a "master" template: it's a bit out of shape, but not enough to be a problem:
Much easier to work on the non-printed side of the paper!
With the old one that I pulled off: I'm going to leave this one as is, and not bother to trim out the internal "web" - it's not necessary.
The last gasket is the base gasket, but first I wanted to get the surfaces a tad smoother, so I dug out the fine grinding paste....
It's an improvement (I hope).
The initial circles I drew out with a compass, then tested for fit on the barrel
I then put the barrel onto the cases and marked the cut-outs for the bolts that tie the head & barrel down, then trimmed them out and checked the fit
I also trimmed out a smalle cutout on the inner of the gasket to match the slight cutouts which allow the con-rod to not-hit the cases.
I've put them in "storage" for now, (under a couple of books as well), and will very likely make up another complete set, using these as templates.
I then decided to make another fix:
The timing chain cover has another little cover over the mag-cog to allow you to adjust the timing without taking the entire cover off: you can lock the mag-shaft, then loosen the pinion nut and tweak it through a hole in the timing chain cover.
BUT - the threads on the tiny holes were tired - so I found some new set-screws with wide heads in the box of bits: figured out what thread they were M6 as it happened, ideally it would be nice to use Imperial all over, but there's other Metric threads on the bike so I'm kinda beyond caring at this point; re-cut the threads in the timing chain cover and hey presto!
I cut the set-screws down to ensure they don't interfere with the timing chain
And there'll be another paper gasket going under that cover to reduce any oil weep (the timing chain isn't lubed much, I don't think..., so it shouldn't be an issue really).
Actually quite pleased with the evening's progress.
Like I say - I'm going to make up another complete set of gaskets and will use that 2nd set for the build, and keep the first ones (the ones made directly from the mating faces) as the "master-templates". I may well find there's other places where a gasket might be useful, but we'll cross those bridges when we get to them.
So - last night I went in and made gaskets. ... I know, it's soooo exciting!
Some of the parts that need gaskets, with existing old ones:
I started with the output side shaft seal plate gasket - this is a "non-standard" fitment anyway to slow down the loss of oil around the output shaft.
The outer and the fitment holes were relatively simple - but I need to break out the school-boy drawing tools for the inner circumference.
Note to self... for f*cks sake draw on the OTHER side....
But a reasonable finished product.
Next - the oil-drain chest cover.
Should be functional:
Funnily enough - the oil-collection chest gaskets can be bought, but this is an older one that's been used, and I can use it as a "master" template: it's a bit out of shape, but not enough to be a problem:
Much easier to work on the non-printed side of the paper!
With the old one that I pulled off: I'm going to leave this one as is, and not bother to trim out the internal "web" - it's not necessary.
The last gasket is the base gasket, but first I wanted to get the surfaces a tad smoother, so I dug out the fine grinding paste....
It's an improvement (I hope).
The initial circles I drew out with a compass, then tested for fit on the barrel
I then put the barrel onto the cases and marked the cut-outs for the bolts that tie the head & barrel down, then trimmed them out and checked the fit
I also trimmed out a smalle cutout on the inner of the gasket to match the slight cutouts which allow the con-rod to not-hit the cases.
I've put them in "storage" for now, (under a couple of books as well), and will very likely make up another complete set, using these as templates.
I then decided to make another fix:
The timing chain cover has another little cover over the mag-cog to allow you to adjust the timing without taking the entire cover off: you can lock the mag-shaft, then loosen the pinion nut and tweak it through a hole in the timing chain cover.
BUT - the threads on the tiny holes were tired - so I found some new set-screws with wide heads in the box of bits: figured out what thread they were M6 as it happened, ideally it would be nice to use Imperial all over, but there's other Metric threads on the bike so I'm kinda beyond caring at this point; re-cut the threads in the timing chain cover and hey presto!
I cut the set-screws down to ensure they don't interfere with the timing chain
And there'll be another paper gasket going under that cover to reduce any oil weep (the timing chain isn't lubed much, I don't think..., so it shouldn't be an issue really).
Actually quite pleased with the evening's progress.
Like I say - I'm going to make up another complete set of gaskets and will use that 2nd set for the build, and keep the first ones (the ones made directly from the mating faces) as the "master-templates". I may well find there's other places where a gasket might be useful, but we'll cross those bridges when we get to them.
)
