A10/7 head bolts, will they fit anM20?

[Big Seven]

I have been told that some of the A10 head bolts will fit my M20. Specifically I was looking for bolts approx 60 mm below the head with 30 mm of thread. Apparently two of the A 10 set are the right sort of length.

If anyone has serviceable bolts from a rebuild or some such I would be glad to buy them. My other alternative's are having some made, or buying an A10 set (£80 !!! I don't think so).

Any replies gratefully received. Roy

[muskrat]

http://www.vintage-motorcycle.com/index.php?language=en&site=4&pid=84&id=5097
http://gmcmc.co.uk/bsa-m20--m21--cylinder-head-bolt-set-for-iron-cylinder-heads-441-p.asp

[chaterlea25]

Hi Roy,
A10 bolts have 3/8BSF threads
M20 have 26tpi cycle thread

John

[bsa-bill]

A10 bolts have 3/8BSF threads
M20 have 26tpi cycle thread

Not everyone knows that and I have barrels to prove it, (fixed now)

[Big Seven]

Thanks gents, I take it the alloy head set have some longer bolts in there? You do have to be wary of advice, even suppliers who have been in the job for 30 yrs don't necessarily know everything !!

[Greybeard]

A10 bolts have 3/8BSF threads
M20 have 26tpi cycle thread

Interesting. I bought two sets of head bolts for my rebuild from a well known supplier where some of the bolts would not go nicely into the block. They would appear to be ok but then bind up. In the end I went to another supplier and got a much better set. Makes me wonder if the dodgy ones were made for the M20. Would 26tpi enter 3/8BSF thread but then bind?

[chaterlea25]

HI GB,
Its quite easy to see the difference between the cycle and BSF in 3/8th size

John

[Big Seven]

Would be good to know the answer to that one🤔 I have just found one of my original bolts, thought I had more, but one may do for now! Have given the old gal a blast up the road and no more tweeting! But still rather gutless to say the least. Seems to me you have to thrash it rather to get some speed up, not too bad then, struggles up hill in 3rd. If you don't get a lick on, struggles in 2nd. Not had an M20 before, are they all like this? Did have an Austin Big 7 side valve, but that had 4 cylinders!!

[Greybeard]

...Did have an Austin Big 7...

I had an Austin 7 Chummy for a long time

[Big Seven]

I was reading somewhere about threads and they mentioned CEI thread, is that something to do with cycle thread?

Good fun Austins eh? I sold mine 3 yrs ago to fund my A10 purchase. I really loved it, but had no garage to protect her from the elephants (elements)😂

[chaterlea25]

Hi Big 7,
CEI = cycle engineers institute

M20 performance with I think 12 bhp?? is very dependant on mechanical condition and rider weight
People were a lot lighter in the 40's !!!
From my experience some M20's go a lot better than  others not sure why
Wear in the carbs makes a big difference, SV engines need a richish mixture to keep cool
A heat insulating spacer between carb and head will also help (dont use the soft type ) or make one from Tufnol/Paxolin

At the moment I am putting a 47 B31 engine back together, the old cams were well down in size compared to
a pair I had in stock, but they looked ok at first sight
So it may be worthwhile checking out the valve gear??

HTH
John

[Big Seven]

I have replaced the exhaust valve, and cut them both in. The exhaust valve had a ding/ flat spot on the outer/ front edge, but inlet was good.
Could I be losing compression past the rings?, If so would there be any signs to evidence this?

[Big Seven]

Thanks Chaterlea, cycle engineers institute!! Whatever next? Some of these threads sound so obscure if you're a joiner and not an engineering type🤓

[chaterlea25]

Hi Big 7
If you would like to gen up on threads
spend some time here

http://www.gewinde-normen.de/en/index.html

 
John

[muskrat]

Common names for cycle thread are BSC, BSCy and CEI.
3/8 cycle = 26tpi, 3/8 BSF = 20tpi as John said easy to tell difference.
Cheers