Yoshi pistons

[jizracer]

Went and picked up a bunch of surplus gs1000 bits from a friend and in there was a set of pistons, in very nice condition (without rings) with POP.Y cast in to them. two rings piston.
I am going to build a short track race bike engine and had basically decided on MTC 1085 big bore pistons. they are light, but expensive.
Are these POP.Y pistons any use? Where would I get rings?
I don't mind spending the big bucks where required but could I use the Yoshi pistons? they were basically free.

Whats the best race pistons for short track racing? Going to run Petrol. As I cant afford to go all out.
After all....we are only racing for fun.... theres no farm on it.

 

[GregT]

If they're a two ring piston - top and oil rings only ? They're a race set. I've remachined an older road set for kev to use car rings. Not hard as it's usually only the oil ring grooves and sometimes the second ring as well. Measure them about 12mm up from the bottom of the skirt to discover what bore size they are. Cliff Bond in Ferry road are the people to ask for ring sets locally but they are most unlikely to have the widths you're looking for. I'd pick that those pistons will use a 1.2mm top ring and probably a 2.5mm oil ring set. If you use car rings, you'll finish up with 1.5mm top and 3.0 or 3.2mm oil set...
There are US suppliers who carry what you want - but at a pretty high cost by our standards.
Your call. You can borrow my piston machining setup if you want.

We've used Wisecos for the race engine with no problems. My only experience with MTC pistons was for Z1's and some years back. Not that happy with them then.

 

[jizracer]

Yes two rings in total. One compression ring.
Thanks Greg.
The MTCs are a new design. slightly higher compression than the wisecos and much lighter. the wisecos are getting quite old now. But not as old as POP.Ys!
Who would know what to believe.

 

[GregT]

Yes two rings in total. One compression ring.
Thanks Greg.
The MTCs are a new design. slightly higher compression than the wisecos and much lighter. the wisecos are getting quite old now. But not as old as POP.Ys!
Who would know what to believe.

Have you handled a Wiseco to compare to the Yoshis ? Not that far back, Wiseco did a redesign and I think changed the forging dies to finish up with much thinner wall pistons in these sizes than previously. Clearances closed up too so there may have been a material change as well.
I can probably find a used Wiseco if you want a look/comparison.

 

[jizracer]

Heres a pick of two of them. The skirts actually look pretty good, I can see the original machining pattern across most of the skirt area.
They measure up at about 70.84mm with the digital Vernier so I suppose theyre 1mm over standard size and the thickness of the ring grooves measure the same as the old standard pistons I have. (with a Vernier)
What compression ratio are they likely to be?
Could I not just use 2nd over OEM rings which are still available from somewhere?
Thanks.

 

[GregT]

They look to be about 11.1 What widths are the ring grooves ?

 

[jizracer]

They look to be about 11.1 What widths are the ring grooves ?

Top groove is 1.4mm oil groove is 2.4mm. Both about the same as the OEM pistons I have here.
But the OEM 2nd ring groove is 1.25mm and the top two rings themselves that come out of the OEM pistons are both the same thickness at about 1.18mm so theres much more clearance on top ring than second ring.
Ive got the feeler gauges and micrometers out now!
I have just had a look at service specs for OEM pistons and it says top two grooves should be 1.21-1.23mm
So I suppose that means all my pistons are junk!

 

[GregT]

Top groove is 1.4mm oil groove is 2.4mm. Both about the same as the OEM pistons I have here.
But the OEM 2nd ring groove is 1.25mm and the top two rings themselves that come out of the OEM pistons are both the same thickness at about 1.18mm so theres much more clearance on top ring than second ring.
Ive got the feeler gauges and micrometers out now!
I have just had a look at service specs for OEM pistons and it says top two grooves should be 1.21-1.23mm
So I suppose that means all my pistons are junk!

Of course not
The top groove can be machined to accept 1.5mm rings and the oil ring groove can be machined to suit whatever is in the 71mm ring set you'll find...

What all your measurements are pointing to is that they've had more use than you thought. They've shrunk a little and there's wear in the grooves.
I'd get them crack tested before buying rings.

 

[jizracer]

Of course not
The top groove can be machined to accept 1.5mm rings and the oil ring groove can be machined to suit whatever is in the 71mm ring set you'll find...

What all your measurements are pointing to is that they've had more use than you thought. They've shrunk a little and there's wear in the grooves.
I'd get them crack tested before buying rings.

Could do. Might be easier just getting new pistons. Ill mull over that one and have a look about for rings in the mean time.
I'm still just ****ing about at moment. Don't want to tear down motor till I am ready to focus on the job. (I have a fairly small workshop)
Tried to send you a PM a couple of days ago Greg but don't think it sent?
Cheers, Steve.

 

[GregT]

Yes, got it twice...Have you got a contact for crack testing ?
I use Glen Durie Engine Reconditioners. Can do dye penetrant and magnaflux crack testing.

I'd get them done anyway as it'll answer questions...