Piston Question

[Happyhenry]

Hi all, hoping for a little advice.

I'm rebuilding a longstroke and have a pair of barrels that were on 62mm original bore with signs of a seize. I bought a pair of +20 pistons (GPM/Gandini). However, the rebore chap says that at +20 there is a score in one bore that is still showing and really it needs to go to +40. I can't get +40 pistons anywhere - I can get +60.

The Hepolite reference for longstroke pistons is 10030/xx.

However, the chaps at Draganfly have said that +40 pistons are as Triumph 5T ref 11563/std (Triumph part 70-1563 - you can see what they did there!) i.e. 63mm which is 62mm +40 thou. The gudgeon pin is same 11/16". Draganfly would have supplied me with a set, but their stockist is out. I have sourced a pair of Harris pistons.

The compression height (pin centre to crown) is 1.7/16, which is a tad over the 1.5/16 for a longstroke 6.6:1 piston or 1.3/8 which I think is the higher compression version. I think it'll give about 7.5:1cr.

My only concern is the overall piston height, which is 69.85mm as opposed to 65mm for the cooking A7. Accounting for the 1/8" (1.5/16 to 1.7/16 = 3.2mm) additional compression height the piston is still a tad longer.

Has anyone tried this?

[Swarfcut]

   Henry. I reckon you're ahead of the pack on this and as original type pistons become rarer, adapting components from other motors offers a way of getting a runner.

  The overall piston length needs consideration as the "extra" may be below the pin centre and as such has a good chance of fouling the crank flywheel. Shaving this off alters the reciprocating mass, changes the balance factor and as always there's a 50:50 chance of it working out. Will the Triumph Pistons clear the combustion chamber, is there any chance of valve/piston interference?

   In view of the costs involved I'd be inclined to give it a try first with the existing parts. In practice the scored bore may not present such a problem and there is a good chance it will carbon up over time. Of course this depends on whether it is just a light score or a major excavation. Only you can judge, but perfection will certainly cost.

 Swarfy.

 

[muskrat]

G'day Henry.
Taking a few mm off the bottom is of little concern. As Swarfy touched on, valve to piston clearance is a problem. There must be 1.5mm clearance (not necessarily at tdc as the valve may come closer just before or after tdc). It will mean a few trial fits. I wouldn't take more than a mm off the crown as it will weaken the part that takes the brunt of the combustion.
When I put T150 pistons in my A7SS I had to use A10 rods and shorten them by 2.5mm. Taking that off both the cap and rod mating faces and re-boring the hole.
Making eccentric little end bushes is another option. Thicker head gasket would help. So it may take a combination of a few things.
Probably the best option would be to sleeve the bores to take the pistons you have.
Cheers

[Happyhenry]

Update:

I now have an old 6.6:1 piston and a new Triumph T5 std side by side with the gudgeon pin through both.

The rim of the piston top (where it goes from vertical side to the crown) is in exactly the same place on both, the valve pockets are the same position, width and depth. The BSA piston has a chamfered edge to the crown and a flat crown. The Triumph piston has a rounded edge and a slightly domed crown, the centre of the crown is abut 2mm higher than the BSA piston. The skirt of the triumph piston is about 1.5mm longer than the BSA.

They look pretty damned similar to me and I'll be very surprised if this doesn't work.

[muskrat]

G'day Henry.
Looks like your on a winner.
While you got it in the lathe to take a bit off the bottom, put a good chamfer inwards. This does two things. Reduces drag and shoots oil up to the little end and cools the crown.
With the crown I'd leave it and cc the combustion chamber at tdc to see what comp ratio it gives you. 7.5:1 would be ideal which I think it'll be pretty close.
Cheers