Hi guys, the thread of the screw plug from the primary chain case is ruined. Its a Bonneville T140V from '74. Eventually some of you know that problem and can give me some advice to solve it.
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Helicoil insert is the usual answer. Good luck
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Thread is 7/16" UNC. As suggested by @Dawsy get yourself a helicoil kit to repair.
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Hi guys, thanx a lot for the suggestions. I was already thinking about this solution, but I m afraid there is not enough material. On the outside wall only 1,5 mm.
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The drill supplied in a kit will take the thread to the next thread size. In this case it should take the hole from 11.13mm (7/16") to 11.6mm.
You could go for the smaller helicoil size which, I think, is 3/8 (you’ll need to check). The access for primary chain adjustment will obviously be an even more difficult task. Never a fun job at the best of times.
I’ve seen crankcases cracked when the access/drain bolt has been overtightened due to an annoying oil leak.
A few other choices - have the crankcase welded and redrilled/threaded (obviously complete engine dismantle required), buy a new left hand side crankcase (will obviously have the wrong engine number and a complete engine dismantle) or seal up the hole completely - use the old bolt and JB Weld (the primary cover will then need to be removed to carry out any primary chain adjustment).
If you are worried about material thickness and still want access through the hole then I’d go for the smaller helicoil. Best of luck, let us know how you get on.
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Thanx Iron for your creative suggestions. I am indeed flirting with the smaller size … seems clever.
Keep you guys updated
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Hi guys 
as I promised, my solution for the leak Primarychase plug.
For helicoil 7/16 isn’t enough material beside, for 3/8 the existing hole is a tad to big, even the metric system doesn’t work … in the end I bought a new plug cleaned the thread and clued it in with sealing silicone. Looks trustworthy …
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Good job with the silicon, time will show if the leak is cured. The advantage with silicon is that the access bolt will be removable, then clean and re-silicon.
The brown staining on the pistons is burnt on oil. There looks to be some carbon deposits between the rings as well. The scratches on the piston above the top ring also look to be carbon deposits.
These bikes run fairly hot when in traffic, hot weather, running a bit lean or low on oil.
How do the cylinder bores look?
The pistons should be removed so the gudgeon pins and small ends can be checked for any movement. The conrods on T140s don’t have bushes in the small ends and are usually fine. You will also be able to see any oil burnt stains underneath the piston. I assume the big ends feel fine - there should be some side to side movement but no up and down movement (pull the piston up hard and while holding it, hit the piston on it’s top with a rubber mallet, you do not want to hear a click)
The pistons look like they’ll clean up ok but the rings will need to come out to check for sticking, end gap and any cleaning required of carbon in the piston grooves.
Hone the barrels and if the bores/pistons/rings etc look and measure to spec put it all back together.
I would personally - given how far you are already into this engine - split the cases and clean out the sludge trap.
Enjoy.
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Thanx Iron for your fast and well founded assessment. Probably a good idea to check the sludge trap … but I never split the case … which parts of the machine are necessary to dissmantle and special tools are required?
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Rather than describing the process, have a watch of Todd (of Lowbrow) rebuilding a T120 engine.
I’ve started you off at part 3 with the barrels off.
The engines are almost identical so the special tools and methods are the same on both engines.
The T140 engine has a 5 speed gearbox (which is actually easier to rebuild than a T120 four speed), the front engine mount hole has a hollow dowel that needs to be hammered out before separating the cases otherwise it’s almost exactly the same process with the special tools shown that are easily available and fairly inexpensive.
There’s no need to remove the cam shafts or the cam shaft pinions if they don’t feel sloppy or the lobes aren’t damaged.
You’ll need a parts catalogue ( https://partsbooks.britishonly.com/ ) and a workshop manual for the correct timing marks.
It’s really worth the effort if you are planning to keep the bike. These engines are now over 45 years old and if the sludge trap has never been cleaned and blocks up it’ll kill the engine. But it may have been cleaned previously and last another 45 years. Who knows…
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Thanx a lot Iron a further time … I m going to dive into it 
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Excellent choice 
Keep us updated and, obviously, we’re all here to give a helping hand and advice when we can.
Get stuck in and enjoy. No rush.
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This dude is marvelous, it s great fun to watch him work. I wish I would have that motorstand/holder. Thinking about an alternative …
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