Author Topic: High compression pistons or not?  (Read 3743 times)

Offline Samson

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High compression pistons or not?
« on: 16.01. 2010 10:33 »
Hi all.
I am rebuilding a A10 engine bought partly dismantled. It has an alloy head and a spitfire cam.
I have a choice of two pistons, one 8.75:1 and is in poor condition the other is in excellent condition but is obviously a much higher compression.
The guy I bought the bike from (a dealer) said "don't use those you'll break the crank".
Appart from the obvious pinking, what engine problems could this create? I am thinking difficult starting!
I am more interested in reliablity than performance and would welcome your views and comments.
Regards Paul

Offline MG

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Re: High compression pistons or not?
« Reply #1 on: 16.01. 2010 13:53 »
Hi Paul!

Those look pretty HC indeed. If I had to guess I would say somewhere around 11:1?
Like I have already stated elsewhere, I am running my A10 on forged Wisecos (9:1). It goes reasonably well and I haven't had any problems up 'til now, although it's an iron head model and its actually 9,3:1 (head and barrels skimmed). The A7 runs on 9:1 (with 8,75:1 JP pistons and skimmed alloy head).
If you want a reliable and sturdy engine I would not go any higher than somewhere around 9:1.
By increasing the compression ratio, you impose more stress on the bearings, the conrods, the conrod bolts and you have a higher combustion temperature = more thermal stress on the cyl. head material. And naturally starting gets a bit more difficult. And as you are using the 357 cam the effective compression will raise, too.

Maybe you could have those HC pistons machined off a bit on top if the material is thick enough? I would not reuse the other ones, they look like they have seized and probably the ring grooves aren't any good either? Have you checked them?
1955 A7 Shooting Star
1956 A10 Golden Flash
1961 Matchless G12 CSR

www.histo-tech.at - Restoration, Repairs, Racing

Austria

Offline rocket man

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Re: High compression pistons or not?
« Reply #2 on: 16.01. 2010 14:05 »
i would get new ones i wouldn't risk it after all the work rebuilding it

Offline muskrat

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Re: High compression pistons or not?
« Reply #3 on: 16.01. 2010 20:44 »
G'day Samson,
                    They look like 10:1 to me and would be shy to use them on a small journal crank. They also look tricked up with the holes in the crown. They should go to the back of the top ring groove to improve top ring sealing on compression, but create more places for carbon to build up increasing the chance of pinging. High comp doesn't mean hard to start if it's all tuned right, I run 10 1/2:1 and starts 1st kick hot or cold. but does produce more heat and timing needs to be retarded a few degrees.
But in saying all that, a full rebuild should get new ones.
With the 357 cam you will find most of the power up high in the rev range 4500 + so once again no more than 9:1 on a SJ crank. I know because I have snapped a few cranks.
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, .
Australia
Muskys Plunger A7

Offline A10Boy

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Re: High compression pistons or not?
« Reply #4 on: 18.01. 2010 20:23 »
Dont use those ridiculously high compression pistons, you will regret it - unless you want to go racing.

With your sporty cam you probably need some Super Rocket pistons, this will be a good set up.

If money is tight you can get a set on ebay for £40 or so and another £30 will get a new set of rings. You could sell those HC pistons to fund some suitable new ones.

Try Cake Street Classics or Burton Bike Bits, one of those sells new italian pistons for Super Rockets.

Good Luck
Regards

Andy

1960 A10 - Black Golden Flash
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1974 Kawasaki Z1a
Yam XJR 1300

Offline Josh Cox

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Re: High compression pistons or not?
« Reply #5 on: 19.01. 2010 04:35 »
My opinion

If you want to go fast buy a rice burner.

I you want lots of bling, to be able to pull old tattooed bikers chicks at the pub and have a nice ride, go the BSA.

I have had the great pleasure of riding all of Orabanda's bikes.

Some with high compression, due to the combination of high compression and me being a pussy, I had trouble starting the bikes.

When I do my next BSA, I'm going 334 cam and 6.5:1, even the wife will be able to kick it over.
Black 1953 Golden Flash Plunger

Offline olev

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Re: High compression pistons or not?
« Reply #6 on: 19.01. 2010 06:45 »
'pull old tattooed bikers chicks at the pub'

Josh, You've been down Hay street again. Did Orabanda take his eye off you for a minute.
or was he involved too.
when you get back to Cairns give me a call.
cheers

Offline Josh Cox

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Re: High compression pistons or not?
« Reply #7 on: 19.01. 2010 09:03 »
Back in Cairns until Friday.
Black 1953 Golden Flash Plunger

Online BSA_54A10

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Re: High compression pistons or not?
« Reply #8 on: 19.01. 2010 09:11 »
I had a pair of those in my first A10.
Did 120+ mph ( got the criminal record to prove it).
Also ended up tearing the thick flange barrels right off the cases by removing 90% of the flange.
Would not run on anything else but 105 octane with 10% nitro benzene.
Broke windows from 20 paces, set of every alarm ( house or car ) that it passed.
Cost me a fortune in fines & harrassment, lots of fun but will never do it again.
Get some flat tops
Bike Beesa
Trevor

Offline Samson

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Re: High compression pistons or not?
« Reply #9 on: 19.01. 2010 15:01 »
Hi all
Thanks for the response, looks like the tatty pistons are for the bin and the high comp. ones will be on ebay.
Most prices on the net seam to be about the £75.00 each but will give burton a ring.
Paul

Offline mike667

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Re: High compression pistons or not?
« Reply #10 on: 19.01. 2010 18:18 »
lots of fun but will never do it again.

Trevor - that's not a very ton-up attitude ! never say never!

Online trevinoz

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Re: High compression pistons or not?
« Reply #11 on: 19.01. 2010 20:17 »
I tried those 10.5:1 pistons once.
I could stand on the kick starter and it would slowly go down.
Like Trevor, I had to use high octane fuel which became a problem once out of the city.
I didn't use them for long as I also destroyed the barrel.
I find the 8.25:1 pistons to be right for my Rocket engines and 7:1 in my Flash engines.
The 8.75:1 pistons pinged badly but at that time I was using the ridiculous factory advance of 13/32".
Trev.

Online Brian

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Re: High compression pistons or not?
« Reply #12 on: 19.01. 2010 21:31 »
Samson there is a place in Austria that seems to have a good supply of BSA pistons. I have never dealt with them so know nothing about their service and quality etc but they might be worth checking out.

http://www.vintage-motorcycle.com/index.php?language=en&site=4&pid=113

Online BSA_54A10

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Re: High compression pistons or not?
« Reply #13 on: 20.01. 2010 03:04 »
Quote
Trevor - that's not a very ton-up attitude ! never say never!

Back in those days you could look all doe eyed at the magistrate and get away with lines like "really I did not know that" .
Can't get away with that now.
Also takes a lot longer for skin to heal at my age.
Also have radar guns, laser guns, arial patrols to deal with.
Down hear they think that they can compensate for very poor rider ( & driver) training by hitting "offenders" with bigger & bigger sticks.
Get nabbed at 100 mph (160 kph to some) in NSW and it is mandatory 3 months in the slammer, no license for 12 months and a minimum fine of $ 1200 so yes, it is a case of never.
Bike Beesa
Trevor

Online groily

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Re: High compression pistons or not?
« Reply #14 on: 20.01. 2010 08:31 »
Big sticks indeed. Wonder why they're so paranoid - and arbitrary - down your way -I thought it was all a bit more relaxed and robust than in prissy Europe?
Makes our classics more attractive frankly, as 100mph+ on most of them isn't a serious option while 80-odd feels like about 120 on a modern . . .
Bill