Author Topic: swapping cylinder heads  (Read 4058 times)

Offline lefty

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swapping cylinder heads
« on: 17.11. 2008 17:44 »
hi all,
as the cylinder head on my '62 A7 is in very poor shape, and i'm searching for a replacement which is free of damage. i've heard of others swapping an alloy head from other models over, but which are compatible without machining? i saw a RGS head (67-1549 and stamped in it is 1208) up for sale, but suspect it won't be that easy will it?  there is also an iron head  on ebay with 1062_ cast into it.... what would this be from? is there available lists of bsa casting numbers available on the net or elsewhere?
thanks for sharing your knowledge
leif


Richard

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Re: swapping cylinder heads
« Reply #1 on: 17.11. 2008 18:33 »
Hi
I am not sure if A7 is the same as A10 but I have just fitted a iron head to my 54 Gold flash which had an alloy one fitted, first of the head bolts were different lenghts as the alloy head has more fins on it also the studs in the front and rear of the rocker box were also too long again due to the alloy head having more finning.
Nothing that could not be overcome.
The valve size on the iron head were the same as on the alloy head as was the inlet manifold size.The Super rocket has the larger inlet valves so I presume the alloy head I removed was a Road Rocket one you may have some difference between the A7 and the A7ss but I would not think this would stop you fitting it
IKBA may apply to the above but I hope this helps
Richard

Offline beezalex

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Re: swapping cylinder heads
« Reply #2 on: 17.11. 2008 18:53 »
Lefty, you should be able to fit any A7 head to your bottom end, but an A10 head, while it will fit, will not work very well.  The combustion chambers are different sizes and the A10 valves really are too big for a road-going A7.  Not sure where you are, but iron A7 heads seem to be relatively plentiful even here in the US.
Alex

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Richard

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Re: swapping cylinder heads
« Reply #3 on: 17.11. 2008 19:07 »
I hope I did not give the impression that you could swap an A7 head for a A10 head as I was trying to give a comparison as to what I found while changing an A10 head and meant that you may have the same problems between A7 heads
Richard

Offline lefty

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Re: swapping cylinder heads
« Reply #4 on: 18.11. 2008 04:37 »
i'm in Vancouver Canada, haven't seen too many going through ebay in decent condition yet. i'm tempted to go for the alloy for the improved cooling qualities, sounds like the a7ss could be relatively easy to fit...if one ever turns up affordably that is

Offline LJ.

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Re: swapping cylinder heads
« Reply #5 on: 18.11. 2008 09:24 »
Quote
i'm tempted to go for the alloy for the improved cooling qualities

I really do wonder if Alloy cools an engine sufficiently, my experience on my side valve M21, which is a very hot running engine is that it runs far better with an Iron head, sounds better too. Had you considered spraying an Iron head to make it look like an alloy one. I have seen it done recently and looks very convincing, very much cheaper too. Picture below shows an A7 crankcase with A10 internals, Iron head sprayed Silver.
Ride Safely Lads! LJ.
**********************
1940 BSA M20 500cc Girder/Rigid- (SOLD)
1947 BSA M21 600cc Girder/Rigid-Green
1949 BSA A7   500cc Girder/Plunger Star Twin-(SOLD)
1953 BSA B33  500cc Teles/Plunger-Maroon
1961 BSA A10  650cc Golden Flash-Blue
1961 BSA A10  650cc Golden Flash-Red

Offline beezalex

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Re: swapping cylinder heads
« Reply #6 on: 18.11. 2008 13:54 »
Sidevalve and OHV are two completely different animals, I don't think you can compare.  Yes, the alloy head will have better heat transfer properties and gets heat away from the valves and the combustion chamber much faster.  This is essential for running high compression.  OTOH, a low-compression motor will run more efficiently with an iron head since it will reach operating temp quicker and maintain that temperature in a wider range of environmental conditions.  Is your motor pinging now?  If not, you will not experience a gain with the alloy head.  In fact, your mileage may drop.
Alex

Too many BSA's


Offline RichardL

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Re: swapping cylinder heads
« Reply #7 on: 18.11. 2008 16:43 »
Intereting points from Alex as to when and when not to use alloy. As an additional miniscule point, regarding painting iron heads to look like alloy, black objects radiate heat more efficiently than lighter colors. Now don't go painting your alloy heads black over this, the alloy will radiate fine and the silver-painted iron, probably, also. It's just an old lesson from having built high-power audio amplifiers.

Richard L.
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Offline lefty

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Re: swapping cylinder heads
« Reply #8 on: 18.11. 2008 21:12 »
thanks for the replies everyone.
unfortunately it's more than a cosmetic issue either way. one of the carb mount drilling has split and almost sheared off back a few inches, top fin on one side long gone, and a few other missing bits of fin too. for the money it would cost to repair - thought i should just buy a replacement cylinder head. and why not get what would work best for the engine - which i still am inclined to believe is alloy. i shall be regrinding the crank, balancing, replacing bushing, seals and bearings, returning the bores to original specs with low compression pistons and hoping that it will serve me well for the next few decades. a big job, but not an unpleasant one for the reward!

Richard

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Re: swapping cylinder heads
« Reply #9 on: 18.11. 2008 22:41 »
Try looking at this on ebay  120018643640 as if you are going low compression an iron head would be best and quiter than an alloy head
All the best
Richard

Offline lefty

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Re: swapping cylinder heads
« Reply #10 on: 18.11. 2008 23:53 »
thanks for that,
i asked the first time this item was listed if they knew what years this one suited (pre or post 1954 when the fin sizing changes took place) but got no reply. just tried again though, and have my eye on one other cast head too.  fingers are crossed.

Offline lefty

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Re: swapping cylinder heads
« Reply #11 on: 19.11. 2008 00:13 »
would any of you happen to know the casting numbers for both the pre and post 1954 heads? just want to confirm whether i even have the right one with my parts.
did BSA ever publish their casting numbers?

Offline beezalex

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Re: swapping cylinder heads
« Reply #12 on: 19.11. 2008 20:44 »
It's 67-1062 for the big-fin iron head.  The one in the auction is definitely a small-fin head.  I don't know what the casting number on those is.
Alex

Too many BSA's


Online Brian

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Re: swapping cylinder heads
« Reply #13 on: 19.11. 2008 21:45 »
Lefty, the small fin A10 head is part no. 67-250 and the casting no. is 67-251. The number is cast into the underneath of the inlet manifold.

Brian.

Offline lefty

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Re: swapping cylinder heads
« Reply #14 on: 20.11. 2008 08:21 »
wow, thanks for all the replies!  After closer inspection, measuring and cross referencing....the head i have is from 1960 63 A10, spitfire and rgs models with a 1 5/32 carb inlet! glad i realized this prior to sinking money into fixing it. the extra 7/32" on the inlet would have made for interesting running!

great site and great people making it all happen

cheers
Lefty