BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: Mike Farmer on 17 August, 2017, 13:57:28

Title: A65 Pistons
Post by: Mike Farmer on 17 August, 2017, 13:57:28


I have just bought a set of A65 barrells and pistons.

The pistons are Stamped on top  68-793 BSA.            46   6/66   L

They are also marked by the gudgeon pin hole  68-743.

I cant find anything that relates to these No's in any of my lists or on Draganfly. (In fact Draganfly says that 68-793 is a dowell)

So what have I got??? any help or advice greatly appreciated

MIke 8)
Title: Re: A65 Pistons
Post by: JulianS on 17 August, 2017, 14:49:58
Both numbers are casting numbers for 10.5:1 Spitfire Mk2 pistons. The numbers for the complete pistons drop one to 68 792 and 68 742.

Why 2 different numbers?

Around that time there were severe problems with piston seizures and the later part may be slightly different.

Attached first piston list from 1966 parts book and the second from parts service bulletin G39 from February 1967.
Title: Re: A65 Pistons
Post by: A10 JWO on 17 August, 2017, 15:28:44
My standard pistons both have marking 68-907 on each,they don't appear on the attached spreadsheet ? Mine is the OIF 1972 ????
Title: Re: A65 Pistons
Post by: JulianS on 17 August, 2017, 16:52:41
They are later than the 1966 parts book - they were introduced in parts service bulletin GA22 from November 1966. See below photo. 68 907 is the piston casting number the piston complete with rings and pin parts number drops by 1 to 68 906.
Title: Re: A65 Pistons
Post by: EDDIE SIMPSON on 17 August, 2017, 22:07:28
those pistons will give you white knuckles and tingles. various brackets will loosen then snap on a run somewhere and vibration is mad at 70mph but smooths out at 80 where you bottom end will stress like mad. but i loved them for the sound and acceleration on a spitfire.
Title: Re: A65 Pistons
Post by: JulianS on 18 August, 2017, 09:21:24
Then they changed the Spitfire pistons again in early 1967, compression now down to 9:1. From parts service bulletin G40.
Title: Re: A65 Pistons
Post by: Mike Farmer on 18 August, 2017, 21:03:41
HI Agen thanks for that. I found the info about the Spitfire very interesting. The barrels that I bought with pistons are hardly worn and  within wear limits. I'm really glad that I asked the question because I was going tojust put new rings and. re-use the parts exactly as they were. So live and learn (just in time)

Now this may be of interest. I was discussing this with a Beesa ""expert"" Yeh!! always had Beesa's mate, know all about them. So I said that I was doing a rebuild and at that stage not sure whether I would make it into an A50 or an A65.  Anyway he told me "the only difference mate is the piston" If you put an A50 piston in, you got an A50. If you put an A65 piston in you get an A65. I wonder I BSA knew that or whether this lack of knowledge contributed to their demise.

So let us dissect the word Expert
X = an unknown quantity an spert = a drip under pressure. Glad I had chance to share this with you and hope it helps in the future

Title: Re: A65 Pistons
Post by: EDDIE SIMPSON on 19 August, 2017, 09:13:41
Well mike, I m no bsa expert. But I did own a spitfire but wouldn't t again . The a10 or a7is superior as a road bike in my opinion and I do moremilage than rebuilds on my a10. The piston party numbers are a.ll listed in the parts book I remember and you can easily spot the difference between 9/1 and 10/1 are they have different crowns. The a50 and a65 was a very good looking bike but the engine did contribute to the demise of the bike arm of bsa in my opinion.