Thought I'd put a quick update up. I've lightened the 916 pistons to match the weight of the 748 piston I worked out that 2.3mm needed to come off the bottom of the barrel to get the squish correct, using a 0.3mm base gasket. The squish area was 1.13mm when i checked it....happy days. while this was happening i cut 10mm off of the studs as these were fouling the head i've put the engine back together and turned it over with no belts on several times and all feels good. ive also tickled the injector mounts so they match the inlet next step is to replace all of the bearings in the roller and tensioner, adjust the back plates to fit....a bit of fettling here and there to make things fit once this is done, i will have to work out what size belts i need and then find out if they exist! A call to Gates maybe in order
My 853 uses the standard 748 belts, I can't see how anything else would work, they need to have the same amount of teeth don't they, the only extra adjustment you will be able to make is with larger idler roller, I think @Dukedesmo has used them on his 916 build. Steve
Indeed, I believe there are 3 sizes of idler pulley and are different colours; black, gold & silver. Standard 916 are black but when I had the head skimmed by 1mm the belts would no longer tension so I have fitted the gold pulleys which are larger (IIRC 2mm?) and so the belt will tension as before. It does, as per Birdie's post above, move the timing as it takes up the slack in one direction only so you need to degree the cams but if you're building/tuning then it is a good idea to optimise the cam timing anyway.
@Birdie I think you went down the piston route. As I'm putting 916 pistons, barrels and heads on, it alters things slightly, it all sits a bit lower in the overall height...the distance from crank gear to cam gear is different, hence needing a different belt. As long as i use 18 tooth gears timing will remain the same and just a bit of timing to do will make things ok.......i hope. It's not the easiest route, i know, but fun for me. if it all fails, i do have a 748 engine that i will fit
Pic was taken whilst fitting the belts and was before setting the tension (I was using the pulley holder to hold the intake cam from flicking back). I don't think it ends up much closer than before once tension is correct and it's been that way for over 10yrs without any bother. I got the pulleys from Chris @ CJS racing as part of the job when he gasflowed/skimmed the heads but AFAIK they are a standard Ducati part used on a different model which has a slightly shorter pulley-pulley length, maybe 996/998?
Aren't the later ( dropped head) 748 belts two teeth shorter? 93 teeth against the earlier 95 teeth. It'll be interesting to see how it works out, there are plenty of people that have done the conversion the same way as you, so the solution is out there. Good luck. Steve
All that said, I would think that using a 748 crank with 916 pistons won't work as the stroke of the 748 is much shorter so the piston will sit lower in the cylinder and so to get a workable squish you'll need to take too much off the height to be able to get back with bigger pulleys? I think you need 'special' 94mm pistons made for the 853 conversion?
that's why i've taken 2.3mm off the bottom of the barrell, to allow the stroke to work with that piston. i've built it all up and checked the squish at 1.13mm, and then stripped it back down again. @Birdie you are indeed 100% correct, there is a 93 tooth belt available
having a little play in the garage. In an ideal world, i need a 92 tooth 17mm belt i cut two teeth, making it 93, out of the old belt, and wouldn't get the adjustment i need, but only just. I'm not keen how close the belts are at full adjustment.
Interesting, if you had the lager idler pulley, which would tighten the belt and push it towards the tensioning pulley, the anti clockwise movement of the adjustment would move the pulley slightly higher which may give you enough clearance, not knowing the difference in idler diameter doesn't help but could you try a thin shim on the idler during your mockup and try, just for curiosity. Steve
Unless you can somehow relocate the idler pulley slightly higher up so that it can come further over? I seem to recall seeing some fancy, modified mounting bracket for the idler pulley on some US Ducati forum some years back? I suppose the problem with less teeth on the belt (if they're available) is that 1 tooth is quite a big step - I managed to get the 1.2mm I needed (1mm skim from head + 0.2mm from the cyl base to get 1mm squish) with just the larger pulley, wonder if a combination of the shorter 748E belt plus larger pulley might do it? If you're struggling for compression you could skim the 916 heads as there is clearance but, of course that adds even more to your belt over-length problem. Interesting project though.
i see what you're saying, bigger idler equals the lesser side of the tensioner, and its definitely worth a mock up I have a lathe here, so may spin a couple of mock ups in aluminium to test this.
if i can't do this via the belts and pulley route, that's my next step. I will get it done one way or another
I was thinking more of a shim ie 1 mm slid between the belt and pulley, something simple, just to give you an idea. Steve
I'm thinking that if i put a slightly larger tensioner roller on and a different bearing in the tensioner to move the hole out further ( a bigger oscillation) that this may work with the 93 tooth belt. if I move the oscilation, the tension may be taken up, further up the belt, so clearing the idler a bit better. i'll try this and let you all know.
ok... it's Sunday and i love a bit of a fiddle on a Sunday I've made an aluminium mock up of a 41mm (2mm bigger over all) and moved the off centre hole by 3mm This seems to give me the tension require with 8mm between belts the belt is suprising tight with with just holding it.....i do have a grip like a Boa Constrictor though I think the problem is solved!
