Author Topic: cylinder head swaps  (Read 2497 times)

Offline zedman

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cylinder head swaps
« on: 15.03. 2009 09:20 »
can anyone help me. I am currently rebuilding my old a7, and I have question for some learned people out there. Can I fit an a10 cylinder head on an a7 barrel. I know a10 has a bigger bore and combustion chamber and head gasket etc. but will it have any detriment being larger than the a7 bores,or will it cause a hot spot on the 2 mill per side in both chambers, and pre-ignite.
Here is some head cast numbers to help. a7 p/no.67-381 and a10 p/no.67-251.
The reason I am thinking of mixing the two is the size of the inlet valves is very much bigger and possible more power to extract at little cost.
I have allso thought it might be beneficial  to put a slight radius on edge of bores out to head gasket and a10 chamber areas, this I feel could eliminate the pre-ignition that I think will occur.
cheers from zedman

 

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Re: cylinder head swaps
« Reply #1 on: 15.03. 2009 11:00 »
Can't tell you if it can be done myself as I've no knowedge of relative head bolt spacing, cylinder barrel thread depths and fasteners, length of pushrods, relative valve geometry and lift of camshafts etc between 500s and 650s, although there'll be many who know in this amazing corner of cyberspace.
 
But what would happen to the compression ratio, assuming it could be done? I have no idea but I'm assuming it would reduce, and the more so if you were to chamfer the lips of the bores to eliminate the step? Or were you thinking of skimming the head too?
 
What I DO know, ref another period marque, is that while some 650 heads will go on some 500 motors the straight consequence is a comp ratio of 6 to 1. Don't know about pre-ignition in that context - probably doesn't even get hot enough in there! Not saying the same applies with BSAs, not at all at all, but my hunch is, even if the bits can be made to fit, that it's a not-great idea. Very happy to be told that's rubbish though.
 
Just speaking personally and generally on (or even slightly off) the topic, I think 'tuning' is a fraught area. A machine has to be treated as an entity. Starting with a 100% verified top-notch bottom end, then looking at the piston and cam options, and the valve gear as a whole, and then thinking hard about carburation and ignition timing, gearing, clutch options etc etc etc  and trying to end up with something that'll hang together. And what about the brakes . . . and the rest of the cycle parts . . .  and the lights . . . etc?

Having recently acquired a weird beast of another marque where someone had tried to extract more power than was probably wise, had failed miserably and then sold the thing in disgust to a sucker (me), I have had to get interested in these things. In this case, not being in a position to steal the food budget for 'yet another mechanical misadventure' as it was put to me, I used what had been done as the basis. But I had to start again at the crank and spend a lot of time, and some money (from my grape-juice budget, natch), to make it work. Which it does, for the moment. But it's lucky I have a trailer - and a dustpan and brush to sweep up the bits when it lets go.
Most things are more rewarding long-term in - or close to - the state of tune they were designed to be in, and I would have thought, not having one myself, that an A7 is right at the top of the list of most-rewarding machines ever made (although my B31 might argue). The ones I know are just tremendous. The sanest classic route to more oomph is cubes, which says 650 minimum and probably says 750 (so later Bonnie or Commando really, unless you have the appetite to face an Atlas or an Interceptor). While it's nice blasting around with 40-odd bhp or 50 even, it's equally nice with a lot fewer, it's just different.
Bill

Offline 900triple

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Re: cylinder head swaps
« Reply #2 on: 14.10. 2010 16:23 »
Hi - I have just discovered the same issue on my A7 plunger basket case. The engine was in bits and I am in the process of restoring the engine and have just discovered that the head is a 67-251 A10.

I'm not really interested in tuning the bike but will the head fit on the A7 iron barrels without any modification?

Will it run okay?

Cheers

Alan

Offline Jim

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Re: cylinder head swaps
« Reply #3 on: 14.10. 2010 18:03 »
I put an alloy A7 head on A10 barrels without a problem so I imagine that they will fit the other way around.
Wimbledon, England.
BSA A10 x2
BSA B44 x2
BSA B40
BSA A65
TriBSA 500
Buell XB12R

Offline 900triple

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Re: cylinder head swaps
« Reply #4 on: 14.10. 2010 19:00 »
Thanks Jim,

I've lined up the bolt holes and its no problem. The issue will be the 2mm step between the larger head combustion chamber dia (70mm) and the smaller dia cylinder bore (66mm) giving a squish the wrong way round so to speak?

An A7 head on the 650 barrels should be okay but the other way round will produce a reverse step...and that's the bit that bothers me. How much will it affect the running?

Please help!!!!!!!!!

Offline dpaddock

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Re: cylinder head swaps
« Reply #5 on: 14.10. 2010 19:18 »
Assuming you can fit the A10 head to an A7 barrel with no other problems - and there will be plenty, I believe - a pocket of stagnant gas will be created in each cylinder which will likely cause severe detonation.

David
David
'57 Spitfire


Offline 900triple

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Re: cylinder head swaps
« Reply #6 on: 14.10. 2010 19:26 »
Thanks David - I was thinking that but needed someone to confirm...

Bollocks!

Anyone need a nice freshly painted iron A10 head?

Offline Humble Pie

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Re: cylinder head swaps
« Reply #7 on: 14.10. 2010 22:54 »
Glad I found this topic.. I have a set of A10 SA cases can I build an A7 motor with these cases using the proper internals ?
1952 A7 Plunger

A10 Trike

Offline muskrat

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Re: cylinder head swaps
« Reply #8 on: 15.10. 2010 20:56 »
G'day Humble Pie, Welcome to the forum. I love the name, I seem to eat a bit of that now and then.
 The short answer is yes. If they are late ('59-'63) cases the cylinder base studs may be too long for the thin flange A7 barrels.
To change from A10 to A7  swap over the crank, rods, pistons, barrels and push rods. The heads are being talked about here. I have an A7 head on a A10 (hi comp) but have not tried the other way. As David said about detonation, the other problem would be the ridge would hamper the inlet and exhaust gas flow.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7

Offline Rocket Racer

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Re: cylinder head swaps
« Reply #9 on: 15.10. 2010 22:51 »
Zedman,
 while I have no doubt it will fit, I just can't see it being a good idea.
Whereas putting an A7 head on an A10 makes more sense as you get a nice squish band.
You havent mentioned if you're thinking alloy or iron head, or big or small fin.
I do have a big fin iron A7 head on the shelf and will be at the Hamilton swap meet tomorrow, so could chuck it in the car, send me a PM if interested and I can give you my mobile.
Small valves with a nice cam would give a nice result for a road motor.
My alloy head swallowed $1500 (NZ peso's) in work to sort out guides/seats etc. gets expensive real quick!
Tim

can anyone help me. I am currently rebuilding my old a7, and I have question for some learned people out there. Can I fit an a10 cylinder head on an a7 barrel. I know a10 has a bigger bore and combustion chamber and head gasket etc. but will it have any detriment being larger than the a7 bores,or will it cause a hot spot on the 2 mill per side in both chambers, and pre-ignite.
Here is some head cast numbers to help. a7 p/no.67-381 and a10 p/no.67-251.
The reason I am thinking of mixing the two is the size of the inlet valves is very much bigger and possible more power to extract at little cost.
I have allso thought it might be beneficial  to put a slight radius on edge of bores out to head gasket and a10 chamber areas, this I feel could eliminate the pre-ignition that I think will occur.
cheers from zedman

 
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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Offline Humble Pie

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Re: cylinder head swaps
« Reply #10 on: 16.10. 2010 14:19 »
G'day Humble Pie, Welcome to the forum. I love the name, I seem to eat a bit of that now and then.
 The short answer is yes. If they are late ('59-'63) cases the cylinder base studs may be too long for the thin flange A7 barrels.
To change from A10 to A7  swap over the crank, rods, pistons, barrels and push rods. The heads are being talked about here. I have an A7 head on a A10 (hi comp) but have not tried the other way. As David said about detonation, the other problem would be the ridge would hamper the inlet and exhaust gas flow.
Cheers
Great thank you,,they are 58 cases
1952 A7 Plunger

A10 Trike