GT750 Crank Rebuilds

[sav0r]

My father and I are finally getting his home built GT750 sports racer back together for some vintage racing. The chassis work is closing out and it's now time to go through the engine. He has the engine torn down, we've read about some various modifications and changes to the crank to achieve greater revs, but we are also sure that things have changed drastically since he built the motor in the early 70's. It's got pretty drastic porting modifications with the cylinders raised and decked and has hand built chambers and some large VM's. Seems like the intake rubbers are totally shot as expected, so those will need to be replaced, though they aren't stock dimension. He created an adapter plate the widens the stud spacing for the larger intake boots. Anyways, it's a bit of project...

Seeing as this car won't see any truly competitive racing we don't need crazy performance, but something reliable and a bump from stock would be good. So, who is building water buffalo cranks and what configuration should I be looking for/ordering?

[Evans Ward]

Bill Bune in Minnesota is who did mine. I believe he is one of the "go to" guys fir this.

[Eric P]

Bill Bune in Minnesota is who did mine. I believe he is one of the "go to" guys fir this.

I second Bill Bune. He is THE Suzuki triple crank rebuilder. I had good luck with him back when I rebuilt my GT380.

[teazer]

I have bought "rebuilt" cramks that had supposedly been rebuilt by all sorts of experts around the Midwest - and they were all pretty bad.

I have also had three or four rebuilt by Bill Bune including a couple of race cranks and I will continue to send them to him.

In theory you could lighten the crank and add stuffers to the crankcases but rumor has it that they work best stock.  Just remove the slinger plates, open up some oil feed holes and run it on pre-mix. 

How much power are you looking for and do you have a decent cylinder head?  Even the TR750 used a quiescent head, but there are much better shapes available in replaceable domes. Or you can weld up a head and start again.

John McCann in Cali had a beautiful GT750 based Ocelot race car with full 750 and 850 Ocelot race motors IIRC.

Do tell us more....

[sav0r]

Seems like Bill gets the unanimous vote.

I don't believe the domes are modified, but I honestly didn't look at them. I believe the compression was raised some. We probably won't do a lot of extra modifying, it's not particularly in the spirit of vintage racing, but necessary things will be refurbished and generally it's okay to do so to modern standards. The chassis is an all aluminum semi-monocoque that's been retro fitted with a steel roll cage for safety purposes. The car is around 800lbs with a driver.

My dad used to get beat up on by the Ocelots rather often, he always felt he had a chassis advantage but he never made the HP that the Ocelets did. I believe they did a 6 speed conversion as well where my fathers car is still 5 speed.

I'll try to get some pictures of the car, especially as it goes back together. Right now the steel roll cage is out for power coating and we have bunch of welding and carrying on around the chassis at large. We hope to get the crank sent out and while that's underway get the chassis finished up.

[bitzz]

Any idea where the 6 speed came from?

[sav0r]

I believe it comes from a GS but I could be mistaken.

[teazer]

Chris,
Barton in the UK used to make and sell a 6 speed cluster.  I have looked at a number of other transmissions and for sure a GS750, GSX750, 1000 and 1100 will all fit with different degrees of modification, but a GS550 6 speed is too wide as are all the GSXR transnmissions I have looked at so far.  Doesn't mean that there isn't a suitable 6 speed, but I have yet to locate it. 

Early GT's had a low first gear which is great for getting off the line and then has a huge jump to second.  That was changed in later years and they also changed to a lower top gear, bringing it closer to 4th.  That is the same gear ratios as the early GS750.  Later GS had the same gearing but different dog designs which are better for racing and can be undercut by someone like R&D in Clearwater FL.

Ocelot used to offer 750 and 850 motors with basically TR750 porting.   

I have just finished porting a GT motor for an owner in CO and with that we machined 4mm off the top of the block and slipped in a 3mm spacer plate plus additional base gasket to get a zero deck height.  Then took off an additional 1mm off the head and matched the bores to the block - that's really important.  You also want to get a good head gasket.  All aftermarket gasket sets have oversized head gaskets that create a ring around tops of the bores that promote detonation.

We ended up with a vestige of a squish band but for a race motor I'd recommend a full squish head.  Flannery in MI or RK-Tek offer them.  Or weld up your head and machine it to your own specs  and make sure the chambers are concentric with the bores. Stock they vary a lot and in general are a terrible match.  http://pinkpossum.com/GT750/GT750head.htm

For a car you need to have enough torque, so high compression works for you. If you plan on letting it rev, you may want to consider forged pistons with thin rings that won't flutter.  You could also consider a programmable ignition for Ignitech, Zeeltronic or HPI. 

Vintage racing is still racing and we all want to have the best opportunity to come close to the front.


 

[sav0r]

Great information, Teazer. I really appreciate it. I'll definitely be showing my father this thread.

My father is a capable TIG welder and I could machine new domes on my cnc mill without too much trouble. Fixturing would be the tricky part.

I know my father built the engine to published specs, perhaps those of the TR750(?), but apparently before my father did his revisions he had Erik Buell build the motor when Eric was still living here in Pittsburgh.

Suffice it to say, I need to review some of the build specs with my father and then circle back to this thread as there are more questions than answers currently.

[sav0r]

So we sent the crank out and it was returned in like 10 days. Excellent service from Bill.

We also sent the cylinder to Millenium Technologies. They've had it for more than two months. We decided to have it nikisiled to help deal with some of the heat issues the engine had before, they've continued to botch the job. Apparently they've tried plating it 7 times, never once did they call us about it. We are 7 days out from track time, they aren't returning our phone calls. Not sure what to do about these blokes.

The car, asides from the cylinders and some adjusting, is basically done. Without cylinders we won't be going anywhere.

[teazer]

I could send you a block but it would still need to be bored and honed to match your pistons. Not sure if that could happen in time for your race given that things are closed today.

For overheating, I'd suggest a small Bosch or Davis Craig electric pump and an ebay new radiator. Or keep the stock pump and still use a new radiator. Jet one jet richer than "perfect" for cooling and don't use too much ignition advance.  I like tons of compression and advance but that causes overheating if not perfect. You may need a programmable ignition to retard it at higher revs and if you add a TPS, you could use 3d mapping. I could let you have one of my Ignitech ignitions with fully programmable mapping if you need one.

TR750 used fixed timing, but in a car with less airflow around/through the radiator you may be better off with a modern ignition and decent race gas. If you can use it, I'd recomend U4.4 and if that's not legal for that class, try good old C12. It doesn't have the throttle response of 4.4 but it works.

[sav0r]

I certainly appreciate the offer, but given the extensive head modifications and customs chambers I'm not sure a different cylinder would play nice with our setup. The port heights have been changed quite drastically.

My father converted it to a hall effect sensor ignition which is fixed. Compression is high but I assume we will keep ignition slightly safe, as we will with the mixture. There's not much point in walking the line when there's no prize to win. We do this for fun.

The car has two radiators, each is about 8" by 12". You can see the radiator outlet on the left side, it has the NACA style shape. Basically the radiators are fed via high pressure air inside the front wheel well, it's a fairly common arrangement for sports racers, it's antiquated for several reason, but it is effective (just not as effective as other arrangements).

I did all the fixturing and measurement to machine toroidal domes (a very conservative design in my opinion) for the heads, unfortunately we just haven't had time weld the heads up for machining. Too many other projects, things like brakes and uprights. That said, the fixturing is novel in that there are no clamps or bolts that protrude above the head gasket surface. It means that the domes can be shaped, a very small step over is used much like in mold making, and then the head surfaced. This ensures the proper head volume and achieves a nice transition from the gasket surface into the combustion chamber. There are a number of other tricks we could throw at it from here, but we kept the initial design simple. Hopefully we will just get the thing running in time for the upcoming race. The head work will have to wait for winter.

I spoke with Millenium Technologies on Wednesday. They said the head was done. It's realistically about two days too late, but I'm prepared to pull an all nighter on Tuesday to get the car on track Wednesday. That means the head has to show up Monday or Tuesday. I had to make a racket on Facebook to get them to call us back. It's a shame, they have a great reputation, but they've failed us mightily. Communication was nearly zero, delivery time was (assuming it shows up next week) three entire months. They promised twenty days.

Anyways, the beat goes on. This GT750 will get back on track eventually. The engine isn't incredibly radical but we expect well above 100hp. The chassis is actually legal in modern terms, the safety upgrades were massive, but it still weighs in under 900lbs wet. For a full bodied car that doesn't utilize composite structures it's pretty decent.