GT piston possible substitute

B

Basketcase

Senior member
Looking at a friend's restoration of a Yamaha TX750, I measured up a piston.

Pin diameter 20mm

Compression height 33mm (above pin centreline)

Flat top with a very small dome at the centre.

Cheap and freely available in 80.0, 80.5 and 81mm at a whopping US$80 a set from Japan.

Of course it is a basic sand cast lightweight piston but I am wondering if anyone had heard of these being used as a substitute for the GT ?

Cheers,

Rob
 
Hi Rob

piston to bore clearances on 70's/80's Japanese motorcycle engines are often much much tighter than you would expect, at least based on my fairly extensive experience with air cooled Kawasaki multi cylinder engines.

Whatever Kawasaki makes their pistons out of, the piston doesn't expand nearly as much as we allow for in Laverda engines. I don't have specific experience with Yamaha pistons, but I would be inclined to check and use whatever tolerance for piston/bore clearance Yamaha specifies if you stuff a Yamaha piston in a Laverda bore.

From the measurements you listed, it might be worth a try if you can't get Laverda specific pistons, or you think there may be a benefit from a lighter Yamaha piston, if it is lighter.

Paul LeClair
 
Hi Paul,

Maybe early Japanese pistons used a lot of silicon to cast better. I think I would use the TX750 clearances as a guide.

Reminds me of a story of a guy trying to weld the crankcases of a sandcast Honda CB750 and they just would not fuse.

He had a metallurgist work out that there was huge amounts of lead added to make the aluminium flow into the sand castings better !

Cheers,

Rob
 
Basketcase said:
Reminds me of a story of a guy trying to weld the crankcases of a sandcast Honda CB750 and they just would not fuse.

He had a metallurgist work out that there was huge amounts of lead added to make the aluminium flow into the sand castings better !
Click to expand...

We had a long and fruitful debate a little while ago and discovered that these Hondas were not sandcast but gravity cast.
 
Back to Rob?s question about these pistons. If the specs you mention are on the mark as replacements for Laverda pistons there is no reason why they couldn?t be used. There are lots of parameters though. Valve pockets, crown shape, squish band etc. Otherwise I am sure they are good quality and standard clearance for cast pistons would be fine, as they are intended for an air cooled motor with cast iron liners that generally runs  hotter than the well finned SF motor.
 
Tippie said:
Back to Rob?s question about these pistons. If the specs you mention are on the mark as replacements for Laverda pistons there is no reason why they couldn?t be used. There are lots of parameters though. Valve pockets, crown shape, squish band etc. Otherwise I am sure they are good quality and standard clearance for cast pistons would be fine, as they are intended for an air cooled motor with cast iron liners that generally runs  hotter than the well finned SF motor.
Click to expand...

The metallurgy would be better...Yamaha were well into high silicon alloys for pistons by then. Only caveat I have is longevity....I've never heard of a TX750 lasting long enough to need a rebore so piston life is a mystery....
 
I used a 1200 Laverda yamaha tx pistons maybe 60000km,
when a replacement from the, condition was excellent.
Experience, I can say that working second solution.
 
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