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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on March 25, 2026, 09:41:25 AM »Didn't really feel like going into the workshop last night, but nevertheless I did.
Firstly I cleaned off the timing side case:
To check exactly what is hitting what I improvised some "engineers red"
(It's a red paint pen
)
Gently fitted the crank and rotated it slowly a few times (only a few)
As I thought.....
I think the crank-pin nut is also slightly scrapping the slightly proud area of the casing, the ring around the bearing liner. Hmmmm. But it's definitely hitting that screw head.
For Reference: that test was done with a 0.023 and a 0.010 thrust washer fitted (which is how it was setup when I discovered the issue).
I found a nut for helping to hold the screw in place in the vice:
Then gently and slowly took some material off the head of the screw with a file:
The (nearly) end results:
Fitted back into place:
That should be an "improvement", but I think the clearance is still going to be somewhat tight....
I then took it out again and cleaned up the slot a tad: (not very well unfortunately, but it's still functional)
I then played with thrust washers for a bit, trying to get the clearance: even with a 0.050 thickness thrust washer there was a slight scraping still happening, so I added the 0.010, and it's clearing now. This was done with the timing side case lying on the blocks, so the full weight of the crank was pushing down on the thrust washers/bearings. In practise, that won't happen, because the engine isn't mounted lying on it's side! (IE, it's the "worst case scenario" for this clearance: heat not included....)
I'm aware that when the engine gets hot this clearance may reduce, but I'm hoping that by setting the clearance as large as I can, that danger will be mitigated at least somewhat.
I'm still trying to figure out why this has become an issue when I can only assume it wasn't a problem before. I've had a look back at the photos from when the crank was rebuilt, and the ONLY nut that was removed during that process was the output side crank pin nut, nothing else was touched: so - this problem nut is the original one, and it's in exactly the same state/position as it was before.... so I'm no more enlightened as to why this nut is striking the cases as I was before. Hmmmmm.
Anyway: I fitted the 0.060 thrust washers on the timing side, and a 0.023 on the output side..... and checked the end float:
Base:
Float:
That's not bad!! There IS float with the thick timing side thrust washers, and I have a 0.020 washer that will bump the float up to between 0.012 and 0.015, so looking good so far.
I need to fit that 0.020 washer, check the float again, and check the piston is at least vaguely central in the crankcase mouth and the piston is central in the barrel, but dinner was calling me....
Bloody thing!
Firstly I cleaned off the timing side case:
To check exactly what is hitting what I improvised some "engineers red"
(It's a red paint pen
)Gently fitted the crank and rotated it slowly a few times (only a few)
As I thought.....
I think the crank-pin nut is also slightly scrapping the slightly proud area of the casing, the ring around the bearing liner. Hmmmm. But it's definitely hitting that screw head.
For Reference: that test was done with a 0.023 and a 0.010 thrust washer fitted (which is how it was setup when I discovered the issue).
I found a nut for helping to hold the screw in place in the vice:
Then gently and slowly took some material off the head of the screw with a file:
The (nearly) end results:
Fitted back into place:
That should be an "improvement", but I think the clearance is still going to be somewhat tight....
I then took it out again and cleaned up the slot a tad: (not very well unfortunately, but it's still functional)
I then played with thrust washers for a bit, trying to get the clearance: even with a 0.050 thickness thrust washer there was a slight scraping still happening, so I added the 0.010, and it's clearing now. This was done with the timing side case lying on the blocks, so the full weight of the crank was pushing down on the thrust washers/bearings. In practise, that won't happen, because the engine isn't mounted lying on it's side!
(IE, it's the "worst case scenario" for this clearance: heat not included....)I'm aware that when the engine gets hot this clearance may reduce, but I'm hoping that by setting the clearance as large as I can, that danger will be mitigated at least somewhat.
I'm still trying to figure out why this has become an issue when I can only assume it wasn't a problem before. I've had a look back at the photos from when the crank was rebuilt, and the ONLY nut that was removed during that process was the output side crank pin nut, nothing else was touched: so - this problem nut is the original one, and it's in exactly the same state/position as it was before.... so I'm no more enlightened as to why this nut is striking the cases as I was before. Hmmmmm.
Anyway: I fitted the 0.060 thrust washers on the timing side, and a 0.023 on the output side..... and checked the end float:
Base:
Float:
That's not bad!! There IS float with the thick timing side thrust washers, and I have a 0.020 washer that will bump the float up to between 0.012 and 0.015, so looking good so far.
I need to fit that 0.020 washer, check the float again, and check the piston is at least vaguely central in the crankcase mouth and the piston is central in the barrel, but dinner was calling me....
Bloody thing!
I think the 3Bond had soaked into the gasket, or maybe it just wasn't thick enough?? Don't know: but when I fit the new base gasket (I'm going to make another one because this one is very tight around the barrel base), I think I'll use Blue Hylomar instead of 3Bond, maybe, don't know, we'll see. I'm not impressed because this was weeping with zero provocation whatsoever, there's not even any crankcase pressure at the moment - the timing side is still completely open so this was weeping simply through capillary action. It is interesting though that the weep lined up with edge of the barrel base that is broken (see earlier photos). I think pulling the gasket away from the barrel base may help, hopefully!
