balancing crank with new pistons ?

stian buksa · Post by **stian buksa** » Fri Aug 13, 2010 6:42 am

I rebored my -72 GT 750 this winter and changed to 81mm Ø pistons.
Now it vibrates more than before, and I wonder if I need to dynamically balance the crank.

Has anybody experienced this, or may the vibrations come from something else ?

Also does anybody know the recommended balancing factor ?

Stian, Norway

Steve Foster · Post by **Steve Foster** » Sat Aug 14, 2010 2:52 am

.... and on a related matter, can the original 750 GT pistons be swapped for 750 Sport pistons (which are marginally lighter) without having to balance the crank etc? Has anyone done a straight swap? My parts book shows different part numbers for the GT (0755.47.100) and Sport (0755.47.108) crankshafts.

Regards, Steve.

machten · Post by **machten** » Sat Aug 14, 2010 7:26 am

I have 80.4 original Mondial Sport pistons in my GT. It also has Ducati Sport con rods. No balancing issues at all. and there has been no changes to stock balancing.

I'm just doing some work on my Sport, however, which has 80.4 GPM pistons. It's time to rebore due to a previous poor boring job. The current 80.4 pistons are in very good nick and I can use them some time later for my GT.

I can get 80.5 pistons, but that doesn't give enough bore metal to work with, I think. The next pistons I can find are 81.0. I'm interested in peoples experiences with that size piston. I have some very good mondial 80.0 pistons from a 74 SS. Should I bore out to 81, or resleeve and go back to 80.0? Resleeving is about 20% more expensive.

Kev

machten · Post by **machten** » Sat Aug 14, 2010 7:28 am

A correction - the GT has 80.0 Mondial pistons - hence I can use the 80.4 s later.

Kev

machten · Post by **machten** » Sat Aug 14, 2010 8:11 am

I should also point out - I have a non standard, non Ducati con rod in the GT. I don't know for sure what it is, but is in the style of the SS conrod (but isn't one). Don't know if this matters. Pic's attached....

Kev

wdietz186 · Post by **wdietz186** » Sat Aug 14, 2010 10:27 am

I put Sport pistons in my 750GT back in '77 with the 1st valve guide that went. Noticed a slight increase in vibration but not enough to be any bother. Could be due to the increase in compression as much as piston weight.The rods are the same between the GT and Sport models as far as I know. All that was about 50k miles ago. The rings are now a bit tired so it uses some oil but it runs just fine.You will enjoy the extra snap the higher compression provides,go for it.

Steve Foster · Post by **Steve Foster** » Sat Aug 14, 2010 9:12 pm

Kev, your mention of fitting the 80.0 mm Mondial Sport pistons with Sport conrods to your GT made me go back to my parts book to check: there are indeed different part numbers for the GT (0755.47.350) and Sport (0755.47.355) conrods. Like wdietz186 I was under the impression that they were the same. I'd like to know exactly how they differ. I'd assumed that the crankshaft was all that was changed to accommodate the different pistons.

So that means that the Sport had a different crankshaft, conrods and pistons. Hmmm. Thinking twice about a Sport piston swap now.

The other part of the equation is the carbs - but they can be upgraded to 32s relatively easily.

Steve.

machten · Post by **machten** » Sat Aug 14, 2010 11:48 pm

Steve,

I'll see if I can get some photos from parts and bikes I have apart, but from Ian Falloon's Ducati 750 Bible describing the differences from the GT engine...

"The most significant engine development was the use of 80mm Mondial slipper type higher domed (9.3:1) pistons. At 425g (with gudgeon pin and rings), these were 70g lighter the the 750 GT pistons. The distance from the gudgeon to crown was 42mm and the gudgeon pins were shorter. The 750 Sport crankshaft featured machined crank webs to maintain the identical balance factor with the lighter pistons. The forged steel con-rods were similar, but also machined for additional lightness, and the overall crankshaft assembly was 140g lighter than that of the GT"

Kev

machten · Post by **machten** » Sun Aug 15, 2010 2:43 am

Took some pics of the various rods...So this first pic is an old style GT rod. Note that is is just forged and cleaned up, minimal machining on the webbing...

This one is a Sport rod. Note that it is machined on the flat and the side of the webbing.

There is also a different webbing at the small end. Later GT con rods may look the same as the sport, I'm not sure. The Sport rod is on the left, GT on the right.

Sport rod...

GT Rod...

And it wouldn't be complete without.....Greenframe rods...

Hope that helps.

Kev

Steve Foster · Post by **Steve Foster** » Sun Aug 15, 2010 3:26 pm

That is brilliant Kev - thanks very much. (And the "Enjoy Responsibly" message in the first image is great .... dunno about you but I always try to be responsible when I use my conrods but I don't always succeed.)

Cheers, Steve.

machten · Post by **machten** » Sun Aug 15, 2010 5:52 pm

Steve,

I only noticed the "Enjoy Responsibly" message after I posted the photos. What can I say? I just used the nearest surface to hand!

Kev