Pistons

[BSA500]

Hi,
 I know this will open a can of worms but what make of pistons do you recommend for a 1957 A7 engine. They are +20, I will be looking to get these early spring so no panic.

[Rex]

I found hardly any choice when I was looking a couple of years back. TMS merely laughed when I enquired about A7 pistons.

[BSA500]

Yeah they are rarer than the A10. I see helopite on ebay.

[muskrat]

G'day BSA500.
I'd use these https://www.vintage-motorcycle.com/en/parts/bsa-pistons-new/bsa-a7-piston-pair-020
Cheers

[Triton Thrasher]

G'day BSA500.
I'd use these https://www.vintage-motorcycle.com/en/parts/bsa-pistons-new/bsa-a7-piston-pair-020
Cheers

But don’t use 45 thou clearance.

[muskrat]

Yes a bit of a typo there. 4 to 5 thou"
Cheers

[BSA500]

What do we reckon about the other makes hepolite, IMD, Jp etc

[BSA_54A10]

Basically there is one and only one factory that routinely makes BSA Pistons, JCC in Tiawan
They do not sell retail but are the supplier for just about every brand you buy.
GPM & JP both make bespoke pistons & both have their problems
Some time ago when people were complaining about A 65 pistons John Heaney dropped the bombshel
The world demand for A 65 pistons is less than 100 pairs per year
The smallest economic production run is 1000 pairs
So to do just A 65 pistons in the 4 standard compression ratios & std oversizes would have better than $ 100,000 ( US ) of essentially dead stock sitting on your shelf for near a decade .
Being in OZ I fit quite a few JP's but you have to be very careful as they are all turned down from some standard blanks
So one set might be very light & the next set could be very heavy depending upon which blanks they had on hand at the time
On top of that JP fit rubbish rings that are way too hard so you toss them away & fit rings from any reputable manufacturer
If yo do that you have no problems with them

[muskrat]

G'day BSA500.
So to follow on from BSA_54A10. Gandini rings are the best of a bad bunch. Unless you can get original Heps.
Cheers

[Swarfcut]

 Depends how much perfection you want, compared with what you actually need. Over the years I must have wasted ££££'s discarding perfectly good pistons for the sake of a 20 Thou rebore. You can bet that sooner or later a good set of used pistons will turn up on ebay. New repro prices are well out of my aspiration and for me do not represent value for money.

 So if you can get them, Genuine New Old Stock BSA, AM, CovMo, Wellworthy or AE/Hepolite, failing that a good used pair of the same brands. Ring choice again original Hepolite, (even sad looking boxed offerings still have the same metallurgy under that patina) CovMo, Wellworthy, all  from way back, or modern Gandini's. I would steer away from the Wassell Hepolite in a Greasy Box offerings. Results from this brand have been mixed, with more reports on the Forum of disappointing results rather than success.

 If you need the block boring, consult with the engine man how much needs to come out to clean up. Then source your choice of pistons to suit, and bore the block to the recommended running clearance using the actual piston dimension as datum.

 Cheapskate  Swarfy.

[dave55]

So would it be feasable/cheaper to have new liners in and use own pistons if not at maximum wear ? Just asking for the future, Cheers dave.

[BSA500]

Thank you all very useful information. I will keep my eye out for a couple of those suggestions. I have no idea what make my current ones are as there is no marking bar the oversize. I have a set of nos Bsa rings which I  could use as well. I

[Rex]

Depends how much perfection you want, compared with what you actually need. Over the years I must have wasted ££££'s discarding perfectly good pistons for the sake of a 20 Thou rebore. You can bet that sooner or later a good set of used pistons will turn up on ebay.

Always a bit of a dilemma for me. Currently renovating a Venom with the ally Alfin barrel, and although the bore shows no signs of wear or damage the rear of the piston skirt has scoring. The business end of the piston (top, lands etc) look perfect, so given the rarety (read "expensive") nature of Alfin barrels it's a toss-up between a glaze bust + re-ring and using the existing piston, or using up another rebore size and springing for a new piston.
Given that I won't be using it to lap Brands WFO, reusing what I have seems favourite at the moment. 

[Swarfcut]

 Pistons suffer from skirt damage, and if you have a choice used,  unmarked skirts are a good starting point and the rest of the piston stands a good chance of being OK. Any cracks are also cause for outright rejection, they sometimes appear from the oil drain holes behind the oil ring. Wear to the ring grooves is also critical, but if within accepted published tolerances is fine. Piston pins generally wear more than the piston bosses, but again any ebay picture with a nicely blued up pin is another to pass by if you have a choice. Crown damage where a valve has just kissed the top is acceptable, but as evidence of misadventure, tread carefully. Pistons from reputable brands will usually have a maker's logo and casting number inside. Crown stampings are notorious for casual and illegible application. Drags has a piston chart, this also lists overall lengths so you can identify compression ratios.

 To add to Rex's thoughts. Rear face is the thrust face, and as such with a few smoothed out scores offers less friction and a little more space for lubricant. If the bore and rings are good it should be fine. If it ran without  rattle or knocks, it is well worth another try, at least until current interest rates fall a bit......and you need to investigate and remedy the cause of the scoring.

 Dave's question. It really is all down to cost and availability. Fitting liners I would consider as the last chance saloon for the block casting, but to fit liners and machine to suit a used piston is a solution, particularly with the rare breeds of classic cars where parts are simply unobtainium.  A7 and A10 lower spigots are somewhat fragile and there is always the risk of a fracture as the liner is pressed in.

 Swarfy.

[muskrat]

So would it be feasable/cheaper to have new liners in and use own pistons if not at maximum wear ? Just asking for the future, Cheers dave.

G'day Dave.
Cylinder liners are the last resort after the bore has grown to +80. Most will say that after re-sleeving your limited to +40 on future bores. My Cafe has been re-sleeved and is currently running +80 pistons! The problem is the sleeve gets very thin and might drop in the barrels. Mine were bored with a step in the skirt to stop that.
Cost wise sleeves + pistons https://www.ebay.com.au/itm/162991735707 then boring barrels and fitting sleeves another ￡100 or so.
Cheers