which pistons

[alexl]

I thought I better post this here as its an air cooled question, Its also in chat so I hope I dont break any rules!

Looking at making a big bore GS550 by putting on the GS650 head and barrels, but then I saw that Wiseco used to do a 740cc kit for the 650, so It stands to reason that it will work on the 550 bottom end. Problem is they dont make the 65mm GS650/740 pistons anymore. anyone know what other pistons may work, or anyone done this ? 

[johnr]

gs750?

 

[alexl]

12 hours ago, johnr said:

gs750?

 

yes, maybe. I think the 750 pistons are 65mm, but would they be okay with the 650 head?

[wraith]

Have a look at what size the gudgeon pins are between the 650 and 750 but, I think the 750 are bigger, also have a look at kwak z650/750 as I did a 1148cc big bore on one of my gs1000 using gpz1100 pistons, so you may find something similar.

Just think on your playing with a 30+ year old motor and 30+ year old crank, so putting to much power in the top end and not looking at doing the bottom end may end up with a big bang in the bad way 

[alexl]

21 hours ago, wraith said:

Have a look at what size the gudgeon pins are between the 650 and 750 but, I think the 750 are bigger, also have a look at kwak z650/750 as I did a 1148cc big bore on one of my gs1000 using gpz1100 pistons, so you may find something similar.

Just think on your playing with a 30+ year old motor and 30+ year old crank, so putting to much power in the top end and not looking at doing the bottom end may end up with a big bang in the bad way 

Good point, what do you suggest for the bottom end?

[wraith]

If your looking at going bigger than the 650cc I'd look at getting the crank checked out, when your their you may as well put a new can chain in and at least HD clutch springs.

But if your just going for the 650 top end just check for any play in the crank and if non, away you go.

Tuning a old motor is one of them, how far do you or you bank want to go  you can keep going and going 

[alexl]

Yes, that all sounds like a good plan. I have the 550 donor engine, looks in good nick, new gaskets etc, but not looked at the bottom end yet. 650 top end on its way.  I was hoping to get hold of  an set of Wiseco or similar 65mm pistons which take it up to a 740cc using the 650 barrels, but not sure where to find them. I contacted Wiseco and they only do the 65mm pistons for the GS750 and they wont work with the 650 head (I have read that the 550/750 have hemispherical combustion chambers and the 650 has something else) So not sure where to look for them next. got a GSXR 750 front and rear end to somehow persuade to fit and a nice tank. hopefully It will turn out okay. If you know of anyone who may be good for the 65mm pistons I would be interested.