Author Topic: high compression pistons in early A10  (Read 1768 times)

Offline GFlash

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high compression pistons in early A10
« on: 17.08. 2011 21:58 »
Does anyone have experience with high compression pistons on a A10 from 1951 with the small-diameter crankshaft, it is a high risk of failure?

Offline taroha10

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Re: high compression pistons in early A10
« Reply #1 on: 18.08. 2011 00:33 »
Hi Gflash.
This may not exactly answer your question, but having had 2 A10s with flat top pistons I would choose that every time.
My current Super Rocket (A10 no 3)is excellent and I will leave well alone all the time it is running well.However,if I ever take it apart I will put flat top pistons in as it will be less stress and to be honest I don't think the extra performance  of H/C pistons makes a huge difference unless you are maybe racing..
If I don't get the starting correct ( which I seem to most of the time now I have lived with it for a while ),by about the third or forth kick my knee starts to give out (over 50 you know.)I don't know what pistons are in it but they are fairly serious.
My first A10 engine was probably the best.I had a set of spare barrels and flat top  pistons which were  +60.I put in new rings,fitted a new set of standard shells in the crank,filed the high spots off a camshaft and followers(I was young )It went like a bomb,I used to ride it fairly hard.It was no effort at all to start.
Other people in the Forum may say different but especially with a early crank I would steer clear of H/C.
Good luck with whatever you choose.

Online muskrat

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Re: high compression pistons in early A10
« Reply #2 on: 18.08. 2011 02:10 »
G'day GFlash,
                  I would say about 8.5:1 would be the max safe limit for long life. Most here are quite happy with 7.25-7.5:1.
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Muskys Plunger A7

Online Brian

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Re: high compression pistons in early A10
« Reply #3 on: 18.08. 2011 02:17 »
Depends on just what you call "high" compression. STD was 6.5-1, these were the pistons with the concave tops. Next was the 7.25-1 which are the flat top pistons, then raised flat top which are 8.5-1 etc.

Personally I would go with 7.25-1 with the 356 cam. This seems to be a good combination and will give you a nice tractable engine capable of 65 mph cruising. I have two plunger A10's running this combination and I am very happy with both bikes.

Any higher and you will start to stress the motor plus you wont get much benefit unless you go to a larger carby, which means port work, and a different cam (357).

I have a 61' Flash with iron head that has a 357 cam in it and 8.5-1 pistons, a 1 1/8" monobloc. It goes very well but of course being 61' it has big journal crank and thick flange barrels. I wouldnt go any higher compression without a alloy head which I would like but one hasnt come along at my price, ie. Cheap !

Offline A10Boy

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Re: high compression pistons in early A10
« Reply #4 on: 18.08. 2011 11:49 »
I agree with all the above. Mine is a 1960 with 356 cam and 7.25:1 flat top pistons. That combination makes for a very sweet and tractable motor. It will cruise at 70 mph all day no problem.

As a general rule, the more it is stressed the shorter its life. Another consideration is the Pxxx water we are fobbed off with when we think we are buying petrol. HC will make an iron topped engine run hotter and the hotter it runs the more liable it is to pre-ignition [pinking or pinging].


1958 Super Rocket
1974 Kawasaki Z1a
Yam XJR 1300

Offline beezalex

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Re: high compression pistons in early A10
« Reply #5 on: 18.08. 2011 22:14 »
The compression isn't going to harm the crank, but it can be rough on the top end.  I'm running 8.25:1 in my Golden Flash and the all-iron top end gets hotter than two hamsters f*&$ing in a wool sock when pushed.  It also suffers from chronic pinging.  Next time I get in there I'm going to take it down to stock compression ratio.  Don't get me wrong, compression is the easiest thing you can do to make your bike faster.  I dig being able to hit the ton on a plunger A10, but if I do it for more than a few seconds, the exhaust clearance closes up and it starts to detonate through the exhaust.  Long flames shooting from the muffler sure are cool but I can't say it's good for it.  Also, once in the mountains after pushing up a long grade it pinged so bad that it broke the electrode off a spark plug.  Took me a while to figure out why it would only run on one cylinder below 3000 rpm. ???

Anyway, if you're going to run an iron head and street gas, I wouldn't recommend going over 7.5:1.

Too many BSA's