Author Topic: BSA M20 cylinder head bolt sizes and thread  (Read 169 times)

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geraldhattrick75

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BSA M20 cylinder head bolt sizes and thread
« on: 30 June, 2022, 21:15:34 »
For my sins I am at the point of preparing an AA patrol bike and found that most of the head bolts have had their threads stripped (or possibly stripped the barrel threads...). Not sure if it is the new stainless bolts that are or have caused the problem, so I don't want to assume that the bolts are the correct thread and or length.

I would greatly appreciate if someone could let me know the correct bolt sizes and thread please?


Also, do you know of any thread inserts that I could use to remedy the problem...

Would it be safe to assume that the barrel needs to be removed in order to effect a proper repair?

Any advice would be greatly appreciated.

Many thanks,

Dave.
« Last Edit: 30 June, 2022, 21:19:09 by geraldhattrick75 »

spaceman

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Re: BSA M20 cylinder head bolt sizes and thread
« Reply #1 on: 01 July, 2022, 11:12:33 »
I'm not very familiar with the M20 but looking at the 1949-58 parts list, the 9 x cylinder head bolts are 3/8 x 20 tpi BSF. The correct length appears to vary between suppliers but may be 1 7/8" overall length including the hexagonal head.

I assume the threads are stripped in the cast iron barrel. I've done lots of Helicoil repairs but none into cast iron. Drilling into it will not be a problem but I am not sure how easy it will be to cut the Helicoil thread? Assuming a Helicoil will work then providing you can get enough clearance for the drilling stage, it shouldn't be a problem to do the repair in situ?

Dean Southall

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Re: BSA M20 cylinder head bolt sizes and thread
« Reply #2 on: 01 July, 2022, 12:06:26 »
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Mike Farmer

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Re: BSA M20 cylinder head bolt sizes and thread
« Reply #3 on: 01 July, 2022, 14:55:24 »
 :) :) :) :)

This reply is entirely a personal opinion. I wouldn't try to do it in situ, in truth I wouldnt attempt it as a DIY thingy. It may well be fine but you can only get it wrong once.

I would chenck as positively as possible where the problem lies. If its defo the cylinder threads gone-----Take the barrell to a reliable engineer and have it done. Even if its 10/15 per hole its a lot cheaper that a barrell. Thats even if you can get an half decent one.

I know its a bit boring having to strip and reassemble---but to me that is preferable.

Best of etc

Mike 8) 8) 8) 8)

geraldhattrick75

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Re: BSA M20 cylinder head bolt sizes and thread
« Reply #4 on: 01 July, 2022, 19:23:59 »
Once again folks, many thanks for your advice.

To clarify, the bike is not mine- I am helping a guy who turns non-running  'classics' into runners for onward sale as 'projects'.

I took the head off today and found that 4 of the barrel bolt holes had been quite badly butchered and we will 'helicoil' them (Probably insitu) and purchase new bolts. 

Thanks very much for your suggestions and in particular about taking care with thread cutting cast iron......


Regards,

Dave.






 

Dr_Pinky

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Re: BSA M20 cylinder head bolt sizes and thread
« Reply #5 on: 25 July, 2022, 00:04:59 »
Hi Dave,

I agree with previous answers: threads are BSF, helicoiling is a good fix, take the barrel out to do it. Beware the bolt length: iron heads have a mix of long and short bolts. 'Not sure about later alloy heads. If you replace them, make sure they're high tensile. And solid copper gaskets are better than the composite type.

Don't know where you are, but I gave my barrel to T&L Engineering, Bedford, who did a great job. (I'm not on commission.) They can also repair lost fins.

Lots of info on the WD BSA forum, if you use the search function.