T500 Redline with GT750 pistons...

[Frank]

I've heard the stock T500 pistons weren't safe over 7K rpm but am curious how the GT750 slugs do? I'm using 1st o-size in my rebuild. Can anyone make a recommendation?

thx
Frank

[tgrogan]

The 'official' limit for late T/GT500 and hence GT750 pistons is 8200 rpm. At which time the back of the piston is supposed to break off. I experienced this break-off at some high rpm which was not really measured, but well over 7k. Unless you are running some pretty radical porting, there is not much use in going over 7500 anyway.

[Frank]

Thanks - I have mild porting but have to run stock pipes so 7500 it is then!

[water cooled]

I also have 1st OS pistons with very little modification to them. Peak HP is 8515 rpm and I go through the traps at or above 9000 rpm. Stock rods (balanced) and stock crank with minor mod for bearing lubrication. I've run the same pistons for 3 years and have not seen any signs of cracking so far.

[Frank]

Well that's another interesting data point. I'll be land speed racing mine so extended periods at WFO just creeping up in revs I expect. I've gone up one tooth on the c/shaft sprocket to help the situation. I've also ordered a cheap ebay induction type tachometer as I don't trust the stocker. What do you use for a tachometer water-cooled?

[water cooled]

I use an Autometer Phantom II digital w/ shift light which is wired into an MSD multi-channel ignition (total loss system). I never used the stock tach...

[tgrogan]

I also have 1st OS pistons with very little modification to them. Peak HP is 8515 rpm and I go through the traps at or above 9000 rpm. Stock rods (balanced) and stock crank with minor mod for bearing lubrication. I've run the same pistons for 3 years and have not seen any signs of cracking so far.

Are you talking about a 750? (I noticed the radiator cap on your tach pic.) If so, you can probably get away with slightly higher revs due to the tighter piston/cylinder clearances not allowing the piston to cock in the bore quite as much as with stock air-cooled T500 clearances. My T500 race bike was set up with piston/cylinder clearances 1/2 way between T500 and GT750. I used 8500 rpm, fairly often but not for sustained periods, with no issues.

[joethebike]

Way back when I was bumped out of B and C grade racing and into A grade my options for road racing other than production racing were limited, as I could not see how buying a Yamaha TR was an economic proposition.

The answer on a shoestring was an old 500 Cobra with massive holes carved into the ports, running on methanol through Amal carbs. A shortened swing arm, a couple of frame braces, a Honda CB750 disc brake tack welded to the steel forks and a fibreglass fairing, seat and fuel tank.

El cheapo total loss battery ignition sans heavy rotor, some expansion chambers and ready to go.

Ah the ignorance of youth. It handled like a bucket of sh*t on Dunlop triangular tyres and every other tyre but with big enough jets it went like a scalded cat - for a while at least.
The intel about GT750 centre pistons never filtered through and the number of times it promised good placings before silence are still a distressing memory.
The piston skirts always ended up lining the insides of the chambers as it revved like hell until either holing a piston or breaking the skirts.

At my only ride on it at Bathurst in 1971 the first run down Conrod Straight saw the carb bowls run dry as the float valves could not keep up with demand. Amal speedway methanol parts were fitted then the main jets had to be reamed out so much it might as well have run on the jet holders.

So my advice is not to rev it too hard for too long.