jakepeewee
Hi there,
My new old engine is coming along nicely but I am running into problems I don't see a simple solution for. Hope some one can shine a light.
I was anticipating I would run into tensioning problems because I did mill 1.2 mm from the base of the cylinders. That's why I bought a pair of golden fixed pullies which are supposed to be a bit bigger than the original one (SP3 engine). Unfortunately with these pullies I am still not able to tension the belts like they should (95 teeth). Don't want to use a thicker base gasket, squish is currently 0.95 mm.
SP3 fixed pullies have a diameter of 39.5 mm, the gold coloured ones I bought are 40.5 mm.
I also tried some 748 pullies which have an even larger diameter (42.05 mm), but that doesn't help either.
Also tried to fit 93 teeth belts from an 748 low cam Ducati, but that's too tight.
Does somebody know if there are 94 teeth belts available?
Heard that the Exactfit belts might be a bit tighter? Does somebody know about this?
Any other ideas other than longer conrods and/or pistons?
TIA,
Jacco
Jasper
I have exact belts on my SP5 and they are definitely tighter than the oem when putting them on the pulleys.
griff851
Ah yes, floppy belt syndrome, a fly in the ointment for sure. I had a nasty bout of it during my build. It got so bad I even looked at plasma spraying. The solution did appear but. ST4 belts and pretty sure ST4 pulleys, but it could be ST2. The belts for sure, it's just the pulley model. I know they were off the shelf, and I looked back at 'Engine Rebuild” Griff851 page 14, but I neglected to write down the type of pulley there. It is hiding somewhere.
jakepeewee
Yeah, tried different pullies. The gold fixed pullies I bought have a 1 mm larger diameter than the regular pullies (40.5 instead off 39.5), could be from an ST4S or 996S I believe. Didn't do enough to get them tensioned unfortunately.
Good to hear the ExactFit belts have a bit tighter fitting, so I just ordered a pair for my bike. Hope this solves my issues.
Thanks all!
Cheers,
Jacco
brad black
the exactfit belts are closer to the spec tooth form. they're not going to fix your issue.
1.2mm is a shitload. has it got 916 pistons in it?
jakepeewee
Brad,
No, JE Forged pistons. They were, when I bought them some 20 years ago, advertised as being 12:1 pistons. Now that I am rebuilding my engine I found out they were way less, more like 10.5 - 11:1
I believed this was caused by the deep valve pockets, probably needed for the early cam types. Since I wanted to use 748R cams I decided to machined the deck of these pistons to reduce the depth of the valve pockets with 1.3 mm. Then took 1.2 mm of the cylinder base to get the correct squish. Squish measured before this all took place was about 0.75 mm. So I figured I would end up with 0.85 mm after the machining was done. Didn't turn out his way. Still measured 0.75 mm. I added 0.2 mm base gaskets to raise the squish and tension on the belts. Didn't do enough. Lets say I learned a lot.
One forum member stated that with the gold coloured pulley he was able to tension his belts after removing 1.3 mm from the cylinder base. Doesn't work for me, but the changes made were more than just the 1.2 mm machining. Also fitted the carrillo's and changed to S4 cases.
Sorry for the long answer.
Speedmade
I wonder if you could try the 20T pulleys from a ST3. The upper pulleys have the correct ID for the desmoquattro cams unlike the 999 pulleys.
jakepeewee
Interesting thought. Didn't know the ST3 has these 20T ca pullies, I always thought it was Testastretta related.
Don't know if those could fit, but I guess you will also need to change out the driving pullies, otherwise the cams won't rotate at 50% of the speed of the crank.
