SV650 Pistons

[Roger P.]

Does anyone know if a stock 81mm SV650 piston would work in a 1216? I was curious if it might give more compression than a stock Hayabusa piston. Pin height seems to be different and the dish seems less pronounced.

[spiderpig]

The sv lads are using busa pistons to get higher comp in the 650, so they'd be useless for us I'd imagine.

[Roger P.]

I read it as they were using Busa pistons to save weight and allow the engines to spin up quicker, then they pulled out a leaf from the muli layer gaskets to up compression at the same time. Seems like it was more lore than actually useful, as 17grams per piston hardly seemed worth it. The mod seemed to fall out of favour a few years ago.  Anyhow, just looking for a cheap 1216 that doesn't waste the CR increase the dot head gave me. Some back of a napkin bore x stroke calculations got me 12.5:1 in an 1157 with a dot head but I am probably making a few too many assumptions.

[Oilyspanner]

Good bit of lateral thinking RP -not heard of this mod before -  reckon you'd need a lot of dry builds and measuring to make SV pistons work, sounds like they're a chunk heavier too, so would need stronger rods.

17g of piston weight saving is a lot, you can notice a wheel that's 15g out pf balance, a piston has to rev quicker than a wheel and go from full speed to a dead stop and back again - even a couple of grams is a lot for a piston/valve. Working out comp. ratios can be difficult, as  how 'lumpy' the crown is makes a big difference and where you might have piston to valve welcome parties ! - plastigauge and burette would sort out many of the questions/problems.

[Roger P.]

Well after acquiring an SV piston from that site, I did some measurements and got a compression height of 24.8mm, which is .01mm less than what I have seen quoted for a stock Busa piston. That may just be tolerances or measurement error, but even so, I calculate the extra clearance at approximately 0.5cc if correct. Now, if a DOT head has a 22cc chamber, and an SV piston has a 4 to 4.5cc dish, I am getting  a CR of up to 12.7:1. That seems a bit much for street use. Here's my calculations:

 

8.1 x 8.1 x 5.9 x .7854 + (22 + 4)  = 330cc

 

330cc / 26 = 12.7

 

If I throw in the extra half cc I get 12.47.

Would either be useable in the street?

Someone want to check my numbers?

This is before the head gasket though. SV has bigger valves than a DOT head, so the valve pockets may not need much work, if at all.

I am wondering how much heavier this complete piston is than a stock 1157 one.

Edited August 20, 2017 by Roger P.

[Roger P.]

10.5:1 assuming 1mm head gasket with 81mm gasket bore size. Sounds a lot more reasonable. Probably a small drop in power over my current 1157 stock bore + DOT head.

 

Edited August 20, 2017 by Roger P.