The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: bl**dydrivers on 23.02. 2017 02:28

Title: 1962 BSA A10 Super Rocket Pistons
Post by: bl**dydrivers on 23.02. 2017 02:28
What's the standard compression ratio?

I've read in places 8.25:1, 8.5:1 and 9:1.

It has the 357 cam (if that matters).

What's the best pistons to use? Hepolite NOS +040" is hard to find.

Thanks in advance
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: Tomcat on 23.02. 2017 07:32
8.25:1 is standard. I put 7:1 in my SR and am happy with performance (356 cam) and touch wood, durability on 95 octane. I purchased new Hepolites from Priory Magnetos (7:1 was Andrew's suggestion)
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: Slippery Sam on 23.02. 2017 16:45
I use 9:1 hepolites and have no problems. Not done many miles but the motor runs fine with spitfire cam.
I use super unleaded.
Raymond
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: trevinoz on 23.02. 2017 20:21
You will probably find that 9:1 was standard for the U.S. Rockets. Some were 10.5:1.
I prefer 8.25:1.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: Clive54bsa on 24.02. 2017 14:53
I used 8.25 in my '61 Super Rocket with a 357 cam, I tried 9:1 but I got pinging with our crappy California fuel.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: terryg on 24.02. 2017 15:37
The option to fit lower than 'standard' compression ratio pistons can lead, it did with me, to a crisis of conscience.  Something along the lines of 'I shouldn't be changing a thing'.

If it helps at all it was member Chaterlea who not so long ago reminded me that with rebores there is a slight increase in compression ratio.  It's not monumental but it might help ease a troubled conscience.

My '59 Super Rocket now has 7.25:1 (I think) pistons and is a delight to ride.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: chaterlea25 on 24.02. 2017 19:56
HI All,
Seems I'm getting the "blame " again *red*

I have built a few different spec "Super Rocket" engines with varying specs depending on customer
wants and also using what came with the engines that was still fit for service
So after riding the different bikes maybe I can comment ??????

My own SR has a 357 cam and 8.25 pistons, the head has a 1 3/16ths inlet port 30mm Mikuni carb and is fitted with valves from the cylinderhead shop that have thinned stems and penny on a stick heads  :P
It's a bit of a snarling beast  >:D

Another SR bike built for an older gent has flat top 7:1 pistons a 356 cam, std SR ally head and monobloc carb
Its a pussycat compared to my SR
Riding it is "pleasant" :! it has  much flatter exhaust note event though both bikes have very similar exhaust systems

Another A10 fitted with an alloy head and 7:1 pistons but a 357 cam and separate exhaust system 
sounds a bit sharper  (no miles on this yet)

The RGS project engine has 9:1 Wiseco pistons MAP Rods dynamic balance, roller conversion
+ all the bells and whistles,
Its all sittng there awaiting the attention it deserves when time permits *sad2*

John
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: terryg on 24.02. 2017 21:17
No 'blame' John.

Credit where credit is due!
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: bl**dydrivers on 01.03. 2017 13:35
Had not checked this for awhile and just noticed all the responses. Thank you

All the literature I've come across in the last week point out that 9:1 is the standard compression ratio.
I'm in Florida and the highest octane is 93, as long as it could run on that with no problem I'll be ok and don't plan to take it on long trips.

How about piston brands? JP, GPM etc
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: RichardL on 02.03. 2017 05:02
Bd,

I run 9:1 with a 356 cam and 93 octane gas. All going well with no pinging. Sometimes I add some octane booster just for fun, but I always run lead substitute.

Richard L.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: chaterlea25 on 02.03. 2017 18:47
Hi,
Just a reminder that the octane ratings in USA and Europe are different
As far as I can find out 9:1 pistons were a USA fitment  *????*
UK SR's were 8.25:1 as standard

John
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: edboy on 02.03. 2017 19:28
sorry to disagree  john, its 8.5:1 in the service sheets, and the R.G.S were listed 8.75:1 and my guess is the R.G.S. head was skimmed to suit and same piston [just guessing] used. the 8.5:1 is the shooting star type piston crown but the dome type 9:1 was the common aftermarket fit. go faster goodie.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: chaterlea25 on 03.03. 2017 15:12
Hi Edboy
I must go and read up the service sheets again *????*
I have never come across a reference to head skimming or high comp heads
There are waay too many 1571 heads floating about, that some think were exclusive to RGS
THE RGS engines were taken from the SR production line , the same 1571 heads were fitted to late SR's
signifying the "big valve" designation

John
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: edboy on 03.03. 2017 20:15
i am only trying to contribute to your great wealth of bsa knowledge john. i forgot to add that the 8.5 : 1 pistons had bsa part numbers on the crown usually but i have never seen part numbers on the dome  9:1 piston crowns which is why i suspect they are aftermarket. RGS head skimming is a guess as i have never been knighted, won the lottery or handled a RGS cylinder head.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: trevinoz on 03.03. 2017 22:02
1549 heads are also "big valve."
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: chaterlea25 on 08.03. 2017 00:12
Hi Trev,
Yes I Know that but neglected to add it into the conversation *red*

I have a mint low mileage 1549 head destined for my RGS,
Of course some "knowledgable" twat will tell me its Wrong *razz*
Now should I swap the 1571 head from my SR to the RGS  *????*  *dunno*

John
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: trevinoz on 08.03. 2017 20:25
I would use the 1549 head as I am pretty sure that was fitted to the RGS as well as the SR.
Anyway, said "knowledgable twat" would need a mirror the tell what you have.
I am sure that you can handle the "experts", John.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: bl**dydrivers on 19.03. 2017 05:42
Hi all, it turns out the Super Rocket was fitted with 8.5:1 ratio as standard and was lucky enough to have sourced NOS BSA +.040" pistons. Now can have my bores suited for the pistons.

None of my sources mentioned 9.1:1 ratio.

Thanks
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: JulianS on 19.03. 2017 11:06
The 1960-62 parts book piston chart shows piston numbers and compression ratios.

Parts service bulletin G23 from March 1963 shows a change of piston for the SR and RGS to a common piston but does not say what ratio the new piston was.

And the Hepolte catalogue of the same time gives more choice of 8:1, 8.5:1, 9:1 or 10.5:1 compression.

Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: edboy on 19.03. 2017 20:02
i remember finding the flat top 8.5/1 were unavailable in the 80,s. only the dome 9/1 which were considered inferior due to overheating and accurate timing problems with a mag. back then almost every bsa shop i frequented recommended 7.5/1 flat tops for an easier life.
Title: Re: 1962 BSA A10 Super Rocket Pistons
Post by: worntorn on 24.03. 2017 03:19
I recall reading on the BSA page that there were three types of Super Rocket each year, UK, East Coast USA ( same as export bikes to other countries) and finally West Coast USA as demanded by the big Californian BSA Dealer, Hap Alzina.
The UK bikes had less chrome than the East Coast USA bikes but the same compression ratio. I believe it was 8.3 to on in 1962, however different articles list slightly different numbers.
The Hap Alzina bikes after 61 were 9 to one. They also had the peanut tank, more chrome yet and cowhorn bars, all to suit the California Hot Rod style of the day.

Of course my memory could be way off on this. I did quite a bit of reading on that site about two years ago when shopping for a Super Rocket.

Glen