Author Topic: What interchangeability between A7SS Heads and A10 bl0cks/barrels?  (Read 900 times)

Offline Peter Gee

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Hi Guys,

Africa is a long way away. Appreciate infor for my 1959 A7SS which I have the crankcases and all innards including rods in v.g.c. .

I have a stock good alloy A7SS head but no block. Can I use an A10 black, sleeved if nevvessary down to A7 piston size? Or just just the A10 block and A10 pistons with the A7SS head and rods? What are mu options, or must I get a dedicated A7SS block?

The reason I have no block is someone  swapped it onto his own A10 RR when he cracked the block..therefore interchageability must be quite high!

Lastly my engine number is also stamped "HC" which means High Compression but in 1959, what was the ratio?

Many Thanks experts out there.

Offline Topdad

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Good morning Peter, (in the uk ) I'm sure I have read in the last few weeks another post re fitting a shooters alloy head on a 650 barrel and that it was a great mod , actually produces a squash effect ,whatever that maybe ! Think one of our racing members mentioned that or poss Musky who has fitted one himself. re the comp ratio that may be now dictated by what pistons you can get/what quality fuel you have available ie the modern ethanol muck does tend to pink whereas if you can obtain a regular supply of 4star (avgas ) you'd fair better . You'd best look for thick flanged barrels as the thinner earlier ones will let go if you increase the ratio to high .I'd wait for someone else to clarify that point. By the way peter we do like pictures of what you have now and what it may become also an intro would be good over in members etc  how did you get into A10s for instance or brit bikes etc , like to hear cos I'm a nosey sod (like a lot of others on here ) best of luck, Bob H
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Online RichardL

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Peter and Bob,

I'm going to stick my neck out here and just say, "no, it won't work." The A7 stroke is 11.4mm less than the A10, so, unless you came up with pistons that had that much more height above the gudgeon pin, compression would be so low as to make running questionable or plain crappy (regardless of the piston diameter). Having said this, I look forward to our extraordinary, experienced experimenters to jump in and say, "Richard, you are wrong, yet again."

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline Topdad

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Richard , could be you are right or me . I'ts in the interpretation of the question ,I've taken it to be -will the A7SS head fit A10 barrels ,which I still think will go ,whereas I think you are reading it has A7SS barrels and head being swoped complete in which you would be right they wouldn't .( Could this be a literal example of "2 countries separated by a common language "    *shh*) Sure we'll find out soon when the night shift come on  *beer* cheers my friend Bob.
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Offline Briz

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Richard's right.
If Peter has an A7 bottom-end, only A7 barrels will do. They're one fin shorter.
To use A10 barrels, an A10 crank rods & pistons are also needed.
Unless you can find some really tall pistons!

Online muskrat

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G'day Peter.
As already said you'll need A7 barrels (from 55 onwards). Unfortunately the A7 barrels only came in thin flange, but will handle 8.5:1 compression.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline Topdad

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Ho Shit, !!! It's me not reading things  correctly, note to to myself read what's there not what you thinks there ,apologises all .. Still will an A7SS head fit on an A10 head , Bobh
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Offline Peter Gee

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Wow what a lot of great responses... Well the answer is no... yes in theory they do "fit" and many thanks for those taking time to try AND GUESS WHAT i MEANT...i SHOULD HAVE USED THE WORD "MATCH".............. it would appear the answer is no, they will fit but wont "match"........ and now the slog to find  A7SS blocks............ without broken fins!

Online muskrat

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Hay Bob. Maybe you should also type what you mean.
"Still will an A7SS head fit on an A10 head , Bobh"  *whistle*
Yes an A7SS head will fit on an A10 with a bit of grinding to match the bore size. Yes an A10 head will fit on an A7 but the combustion chamber will be to wide and too large a cc so with 8:1 pistons would barley reach 7:1. If A10 barrels are put on an A7 my fart would give better compression. *eek*
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Online RichardL

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...and be more explosive.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Online Rocket Racer

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Of course if you have a thick flange A10 barrel but an A7 crank, you could always shorten the barrel to the correct height.
My A10 barrel has had 1/2" taken off it, so visibly looks like an A7 one, excepting it has a thick flange.
Some welding up/extension of the inner bolt holes is required but it can be done and was a common mod back in the day

apologies the attachment of such poor quality, copied it from a post when I did this to my A10
A good rider periodically checks all nuts and bolts with a spanner to see that they are tight - Instruction Manual for BSA B series, p46, para 2.
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Online RichardL

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Read the whole fuzzy article. Very interesting. Names a lot of places not far from where used to live. This would be a good use of barrels that have a lot of top fin damage. Takes a bit more guts in this day-and-age when you can't just ring up Hal Alzina for another set of barrels or pistons if you screw up.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.