Author Topic: New Pistons  (Read 1376 times)

Online Russ

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New Pistons
« on: 26.07. 2013 04:30 »
What pistons are recommended for my 51 A10 Gold Flash.
The barrels appear reasonable and measure at 70mm. (Standard?), however, if they do need to be re-bored, what is the minimum oversize for a first re-bore.
What brand of pistons and compression ration should I go for.
Thanks Russ
1951 A10 Plunger.
Australia

Online Brian

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Re: New Pistons
« Reply #1 on: 26.07. 2013 05:32 »
Pistons go up in increments of .010" so the first rebore for you would be +.010" providing the bore is not worn too badly, if it is then you will have to go to +.020".

Now that is all very well but finding a pair of +.010" pistons could be a problem, plus 20's will be easier to get.

There are several brands you can go for, JP's from here in Aus, GPM's from Italy or Wassel (Hepolite from Tiawan). I have used GPM's and I think they are a well made piston and will use them again. I have not used JP's in a A10 but have in a single (B31) with no problems. I have also used two sets of the Tiawanese pistons and both bikes are running well, I like the piston but there are issues with the rings. Both sets I have used I replaced the rings with Hepolite ones. Having said that I believe that the ones Wassel are selling under the Hepolite brand are being supplied with American Hastings rings which should be ok.

http://www.ebay.com.au/itm/Hepolite-piston-kit-11062-20-To-fit-BSA-A10-650cc-/330909738730?pt=UK_Motorcycle_Parts_13&hash=item4d0bc1e6ea#ht_1966wt_966

The first thing you need to do is get the bores checked to see if they require a rebore or not, can you feel a lip about 1/4" down from the top ? You will need to get a machine shop to have a look and measure the bores to know for sure, they could be worn to a oval shape.

If the bores check out ok you then you can put std pistons back in. For a 51' I would recommend flat top pistons which will give you a compression ratio of around 7.5-1, that ratio in conjunction with a 356 cam will give you a very nice engine.

Online Russ

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Re: New Pistons
« Reply #2 on: 26.07. 2013 06:24 »
Brian thanks very much for that very informative post, I'm sure others will gain a lot of info from it as well.
Greatly appreciated
Russ
1951 A10 Plunger.
Australia

Online Russ

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Re: New Pistons
« Reply #3 on: 26.07. 2013 08:17 »
I have just had the bores checked and they are fine. Will be honed but do not require a re-bore so standard pistons should be OK.  There is 6 thou play in the old pistons, however, I assume there will be less with new pistons. How much play is acceptable or for that matter necessary. I'm hoping to keep it standard.
Thanks Russ.
1951 A10 Plunger.
Australia

Online Russ

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Re: New Pistons
« Reply #4 on: 26.07. 2013 08:27 »
Just found a previous post from Brian which answered my questions.
Thanks Brian.
1951 A10 Plunger.
Australia

Offline chaz

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Re: New Pistons
« Reply #5 on: 26.07. 2013 09:27 »
word of warning, if you have to rebore, keep away from GPM pistons. Ive had several issues and advice from engine rebore specialist about them.
allways willing to learn
United Kingdom

Online Brian

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Re: New Pistons
« Reply #6 on: 26.07. 2013 10:52 »
Chaz I would keep an open mind with these sort of things. I'm not claiming GPM's are the best piston in the world but I and friends have used many sets without trouble.

Over the years I have heard many stories about various brands of some item being no good but in my experience 95% of the time it is due to incorrect fitment or clearances or some similar issue.

Remember that you shouldnt own a A10 BSA because everytime you start the motor the timing side bush wears out, if I had a dollar for everytime I have heard that I'd be rich. The thing is it pays to be very cautious of stories or things you are told.

Another issue is with rebore specialists, most of them mean well and are usually quite skilled at what they do but unfortunately they deal with modern liquid cooled engines which when it comes to rebores means they are not able to come to terms with the clearances required in our old air cooled engines.

Having said all that I should say I always get cylinders bored to the upper limit of the recommended clearance. I dont believe in riding along at 40mph for the first 1,000 miles or whatever. I put my motors together and while I am carefull not to let them "labour" and I dont flog them I ride them at highway speed and treat them like normal from the word go.

Offline trevinoz

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Re: New Pistons
« Reply #7 on: 27.07. 2013 00:39 »
I agree with Brian re GPM pistons.
I am currently running them in my Atlas and did many miles on a set in my RGS and have no problems whatsoever.
My Flash has flat top Taiwanese pistons and rings fitted with no issues.

Trev

Offline Triton Thrasher

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Re: New Pistons
« Reply #8 on: 27.07. 2013 11:50 »
Why do you think you need new pistons, Russ?

Online Russ

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Re: New Pistons
« Reply #9 on: 27.07. 2013 23:24 »
1 piston has a piece broken out at the bottom, which possibly is still useable, however, they both have slight score marks where they have nipped up at some stage.
I feel more comfortable replacing them and as it seems the Taiwanese made  Hepolite pistons are OK so I will go with them on price.
Russ.
1951 A10 Plunger.
Australia

Online Brian

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Re: New Pistons
« Reply #10 on: 27.07. 2013 23:38 »
If you use the Taiwanese pistons Russ make sure to check the end gap of the rings before you install them.

I'm pretty sure those pistons are now being supplied with Hastings rings which will be ok. The rings that were originally supplied with those pistons were absolute rubbish.

Online Russ

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Re: New Pistons
« Reply #11 on: 28.07. 2013 08:19 »
Thanks Brian, I will make enquiries about the rings before I buy them. What gap should I have, or more importantly, what gap shouldn't I have in a set of rings.
I have seen oil rings for Vincents which are 2 and 3 piece and am told they are much better than a 1 piece oil ring. Has anyone had experience with these?
Thanks Russ.
1951 A10 Plunger.
Australia

Online Brian

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Re: New Pistons
« Reply #12 on: 28.07. 2013 08:28 »
You should have about .012" Russ, a couple of thou more wont matter though. The crook rings I had with the Tiawanese pistons had about .030". If the gaps are anymore than .015" you might have a problem.

I have not used three piece rings in any A10 but I believe they are available.

Offline muskrat

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Re: New Pistons
« Reply #13 on: 28.07. 2013 09:34 »
 3 piece rings ( two very thin rings and a spring) are great but a real bugger to get in properly. I've done quite a few but every now and then a rail will pop out un noticed. There goes another bore. *problem*
Cheers
'51 A7 plunger, '57 A7SS now A10CR,  '83 CB1100F, 88 FXST .
Australia
Muskys Plunger A7