Anyone know these pistons? Cylinder advice...

[CrazyCloud]

I won these and the accompanying cylinders on e-bay, they came out of a 2 valve GS750. Obviously aftermarket, they measure an oddball ~69.45mm. They only markings I can make out are 'IA' and '(M?)ATREX'.
Anyone recognize them? The seller hasn't responded to requests for info.





The dome is about 9mm high.

I have cylinders that go with them, they seem to be bored to correctly match the pistons; but the liners are under 2mm-- being bored out that much. Would you guys run these in the bored out 750 cylinder that came with them? I also have stock gs850 cylinders I could bore out to match them, and have a little more 'meat' left on the liners. This is for my GS750, I started to do the 850 piston/cylinder swap and got sidetracked by these... So what would you guys do?

[Steve]

I am NOT an engine builder, but I think I have seen several times that you really want at least 3mm thickness on the walls. Since you already have the 850 cylinders, I would use them.

I have not checked to see what the actual piston diameter is, vs cylinder diameter, but an 850 with a 0.5mm overbore will have a displacement of 855.9, compared with the stock 843.6.

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[CrazyCloud]

For comparison--stock GS850 piston on left, mystery piston on right:

[Steve]

Yeah, those are going to make a bit more heat than stock, I would definitely go with the 850 liners.

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[Nessism]

You might want to look for someone with a GS1000 w/73.5mm big bore kit and have them measure the liner thickness so see how it compares. As I recall the 73.5 kit is pushing the limit but doable. If you didn't pay too much for the kit it may be worth a shot at rolling the dice. Question: what are you going to do about the rings? Run the used rings that come on the pistons?

[CrazyCloud]

Not sure about rings yet, was hoping to be able to track down the piston mfr. to help in the search. If not, I will contact Sureseal to see if they can help. I need to make sure these are not too much compression for a street bike before starting to build an engine around them though. To me the dome looks similar to the some of the old yoshi pistons, does anyone know what compression those were? Anyone able to give a ballpark CR guess based on the pics above? I will try to measure the dome volume of these vs. stock. to get some idea of CR.

Assuming these pistons work out, I will probably go with the bored 850 jugs just to be safe.

[TeamDar]

First of all, are you measuring the pistons at the bottom of the skirts to get the diameter? Back in the day both MTC engineering and RC engineering offered 870cc big bore kits for the 750 that were 70mm and made it 870cc. Both were recommended to bore the stock sleeves. MTC also marketed a 72mm 920cc kit that required resleeving. I have two GS1000's with the 73.5mm overbore and have had zero problems with either. Wiseco would not recommend the kit to be bored for stock sleeves if it was a problem. If the bore is indeed 69.45mm, for rings, you might get lucky with the Kawasaki ELR has a stock bore of 69.4mm, just check the thicknesses. My guess on compression would be 12:1 when you take into account the displacement increase plus the dome height. Dar

[CrazyCloud]

Thanks for the good info Dar. I will check out those rings. Yes, that measurement was 1/2" from bottom of skirts. I assume 12:1 is too much for pump gas? For a street bike, I'm guessing I would want 10.5 or 11:1? The domes are pretty thick, so I'm thinking I could have the domes milled once I know what I'm working with.

[Nessism]

Just noticed that the Wiesco big bore kit for the 750 is 70mm. They wouldn't recommend this size if there were issues boring the stock liners this big.

[TeamDar]

Just noticed that the Wiesco big bore kit for the 750 is 70mm. They wouldn't recommend this size if there were issues boring the stock liners this big.

The 70mm Wiseco kit is for the 16 valve model which is 3mm oversize. The 8 valve Wiseco kit is 69mm or 4mm over, the same bore as a GS850. Dar

[CrazyCloud]

Any ideas for what to use for a head gasket? I was thinking the OEM GS850-1100 MLS? I think it will work with the 750 cylinder/head, right?

[TeamDar]

Yes it will work. You will not use the square "O" ring that surrounds the cam chain tunnel with this gasket. Dar

[Big Jay]

We use 2mm as an absolute minimum wall thickness. The Wiseco 750 2 valve pistons are 69mm.

[Chuck78]

Did you measure them up higher near the rings to see if they are still 69.5mm diameter?

Those are some TALL domes!

I don't think those are MTC's but could be RC's as far as I know