1962 BSA A10 Super Rocket Pistons

[bl**dydrivers]

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

[Tomcat]

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)

[Slippery Sam]

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

[trevinoz]

You will probably find that 9:1 was standard for the U.S. Rockets. Some were 10.5:1.
I prefer 8.25:1.

[Clive54bsa]

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.

[terryg]

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.

[chaterlea25]

HI All,
Seems I'm getting the "blame " again

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 
It's a bit of a snarling beast 

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

John

[terryg]

No 'blame' John.

Credit where credit is due!

[bl**dydrivers]

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

[RichardL]

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.

[chaterlea25]

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

[edboy]

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.

[chaterlea25]

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

[edboy]

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.

[trevinoz]

1549 heads are also "big valve."