A7 star twin pistons

[Mikeredfern]

I am rebuilding a 1949 star twin from boxes of bits. The engine came with two +20 split skirt pistons in excellent condition so I had the barrels re-bored to suit. When The new piston rings arrived I measured the ring gaps and found them to be 0.12. Further investigations found the pistons to be 63mm rather than 62mm. As far as I can see, the 63mm pistons look to be for a C11. I re-ordered 63mm rings and they fit fine. According to the Draganfly website the compression height for C11 pistons is 1.19 and A7 are 1.24, 1.31 and 1.38 depending on compression ratio. I fitted the barrels this morning without any problems but noticed that the piston crowns seem higher than the flanges on the barrels.
QUESTIONS:
Does this sound ok
If not Would it be advisable to fit a decompression plate to lower the piston height
Or possibly fit 2 cylinder head gaskets to achieve the same result
I have a spare barrel, but would need to source 62mm pistons and rings if I were to start again so this is an expensive option!

Advice would be much appreciated
Thanks MIKE

[RDfella]

Don't follow this at all.
Longstroke bore is 62mm. At + .020" that would be approx 62.5mm, not 63. But you say 63mm rings fit OK. Were the pistons marked +020? Then you say you may have C11 pistons? And that when fitted they're too high - but you also say the comp height for C11 pistons is less than for A7 - meaning pistons would be short, not too high. Can you clarify / confirm the various measurements you've given?
Finally, you refer to piston crown height - are these pistons flat like C10 or raised (my hepolite catalogues are in the office so can't check at present).

[JulianS]

For me the question is will there be sufficient clearance between piston and valves. The C11 piston is domed without valve cutouts, the long stroke A7 flat bevelled with valve cutouts, according to the Hepolite catalogues.

The casting number on the inside piston skirt would identify your pistons.

[Swarfcut]

 BSA Service Sheets 701 & 702 give the following data...

                             Bore           Stroke        Small End Bush ID

 C10/C11/C12         63mm         80mm          0.6255/0.625"

 A7/A7ST L'Stroke   62mm         82mm          0.6881/0.6878"

 With a standard bore of 62mm on the Longstroke Star Twin a 63mm piston represents a plus 40 Thou oversize. Unless someone has been extremely clever in the past, C10/11 pistons might fit the bore but the gudgeon pins certainly don't fit a standard A7 rod.

 If the barrel was bored using those pistons as a datum we assume the running clearance is correct if the engine man knows his stuff, and the correct rings should  fit fine.

 The pistons should carry some identification marks, casting number or manufacturers stock number, together with some indication of oversize. For example...

 To hand I have 2 Longstroke pistons, both plus 40. Both have a flat crown, with a deeply bevelled top edge to clear the hemisherical combustion chamber, together with large valve cut outs as Julian describes.

 !.   Casting ref AM 2814  &  AE cast inside the skirt. Crown stamped 10030    +040   Research shows this to be an original Hepolite Piston, CR 7:1

 2   Casting  ref 67 32    Cast into the underside of the crown. Genuine BSA  Piston, 67 108 and 1mm/OS stamped on the crown. This turns out to be 7.5:1

 The raised crowns will rise into the combustion chamber. With the correct pistons all should be well. Kids Plasticine or Blu Tac on the top of a well oiled piston were the backyard ways of checking internal clearance. If the piston pins were correct for the rods, good chance the pistons are correct. As noted in previous posts C and A series pistons differ in crown shape and machining. Identifying what pistons you have will solve the conundrum, plenty of images on the web.

Swarfy.

[KiwiGF]

I am rebuilding a 1949 star twin from boxes of bits. The engine came with two +20 split skirt pistons in excellent condition so I had the barrels re-bored to suit. When The new piston rings arrived I measured the ring gaps and found them to be 0.12. Further investigations found the pistons to be 63mm rather than 62mm. As far as I can see, the 63mm pistons look to be for a C11. I re-ordered 63mm rings and they fit fine. According to the Draganfly website the compression height for C11 pistons is 1.19 and A7 are 1.24, 1.31 and 1.38 depending on compression ratio. I fitted the barrels this morning without any problems but noticed that the piston crowns seem higher than the flanges on the barrels.
QUESTIONS:
Does this sound ok
If not Would it be advisable to fit a decompression plate to lower the piston height
Or possibly fit 2 cylinder head gaskets to achieve the same result
I have a spare barrel, but would need to source 62mm pistons and rings if I were to start again so this is an expensive option!

Advice would be much appreciated
Thanks MIKE

Assuming the piston clears the valves and as you are using non std pistons I recommend you check the compression by measuring the combustion chamber volume at TDC as best you can, you might find you have to fit a compression plate anyway.

There are free comp ratio calculators on the net but from memory an approximate figure is:
(chamber volume + swept volume)/chamber volume.

Swept volume being around 249cc in your case, a better calc takes into account piston clearance etc

If I have the calc wrong no doubt someone will correct me!

[BSA_54A10]

If you use the plasticine method the usual way is to place sheet of plasticine around 0.040" over the complete crown
This allows running clearance and take thermal expansion into play
Too many times I have seen people use a really thin layer on the piston only to find it spins freely on the bench but bent the valves when running.

[Mikeredfern]

Many thanks for all your help and suggestions.

I am now certain that I have fitted A7 pistons, they are flat topped rather than domed and have a chamfer round the edge to clear the cylinder head. Also the gudgeon pins were standard A7 rather than the smaller C11 ones. I will also check the clearances using the blue tack method. Better to have checked, rather than inflict damage on the old girl!

Thank you all
Mike

[trevinoz]

Could be Triumph pistons as well. The pre-unit 500cc will fit.

[muskrat]

G'day Mike.
To follow on from Trev. These Trihard pistons have the same gugeon pin size and compression height.
https://draganfly.co.uk/shop/103859/piston-set-5t-1938-58-std-pair/#10334/STD
Cheers