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Thread: Winter Layup - 1995 Ducati 900 Supersport

  1. #661
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    1,152

    replacement of a failed Helicoil

    A wee bit more work over the last few months... stripped the engine down (again) and had a very close look at everything. As part of that I found a head nut was loose. That particular stud anchor had failed earlier, I'd helicoiled a repair, and it had seemed fine on that earlier reassembly. It wasn't any more... The thread supporting the helicoil in the casting had completely torn out.

    I'd drilled and tapped by hand. The stud hadn't been perfectly aligned. The under-tension part of the helicoil was only as long as the stud's threads, into soft casting alloy. It's possible that the helicoil had an issue with neighboring threads twisting relative to each other, since it's essentially a rolled-up spring. The thread profiles aren't particularly well defined.

    I turned up a solid bung, internally and externally threaded, with a much longer engagement length than the helicoil. It's nothing fancy in terms of material, just some mild steel. That's deliberate. If anything in the chain fails, I want it to be this bung rather than the engine casing again.

    The other part to the job was getting the relevant hole drilled on center and straight. I made a special tool for this, to guide both the drill and the tap. The idea is to transfer accuracy from the machine tool to the engine in its stand, then work from there.
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  2. #662
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
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    Lower Hutt
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    A few pics of the rest of the job. The one-off tool was a pain to fabricate but the proof was in the reassembly... the head went straight on with no problems.
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  3. #663
    Join Date
    28th January 2015 - 16:17
    Bike
    2000 Ducati ST2
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    Lower Hutt
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    1,152

    gasket crush issue with Ducati engines

    During the rebuild, I encountered a lot of trouble trying to correctly set preload on the main bearings.

    Ducati engines use pre-loaded, rolling element, opposed taper main bearings. These engage on a nodular cast iron crankshaft, within aluminium alloy casings. Due to differences in thermal expansion between crankshaft and casings, some main bearing cold assembly preload is required. Once things warm up, the casings expand more than the crankshaft. These main bearings shouldn't open up once the engine is at operating temperature since there'll be impacting and skidding on the rolling elements and raceways.

    Anyway, there's been a lot of debate over the years about what the best preload should be. Figures of anywhere between 0.3mm and 0.12mm (or even 0.05mm) are bandied around. I'm going with 0.15 to 0.18mm, after reading widely.

    Setting this preload is a bit of a pain, you have to delete some shims from the crankshaft, reassemble loose, and work out free clearance using a dial gauge. You then throw in more shims to get the desired preload. Simple, if laborious... or is it?

    Despite shimming it correctly according to the dial gauge method, I kept running into symptoms of over-loading. These symptoms were grindy main bearings and excessive out-of round on the tip of the crankshaft, where the alternator outrigger bearing runs. The crankshaft is squashed between the mains, there's compression across the big-end journal, so the crankshaft flexing because of this preload is plausible.

    I'd checked the crankshaft for straightness earlier, btw. The between-centers method specified in the workshop manual is limited at best; I'd found it better to turn up some plain half-shell journals out of scrap aluminium for the bearing surfaces on the crankshaft ends, mount these half-shells on posts and a baseplate, and place a dial gauge on a free stand underneath the crankshaft to test all relevant surfaces. The crankshaft came in fine, within the 20 micron specification for maximum out of round anywhere.

    So, what was going on with the preload?

    After much time reassembling and testing, I finally realised that there's an issue with the cases behaving in a non-linear fashion, due to gasketing between the halves. When the cases nip up - particularly with old gasketing - they bow inwards. There's reasonably significant movement at center, on the main bearings, because of this flexing of the cases - approx 0.10mm, give or take. In a setting as fussy as mains preload, this is a real problem.

    It happens because there's a centerline between casing bolts, and there's more gasket surface outside that centerline than inside it. The bolts crush the gasket off-center, so it then crushes more on one side than the other. Hence tilt between mating surfaces and dishing in the casings.

    The photos are of the old gasket. The off-center nature of the sealing surface is clear, once you look for it, and there's clearly more crush on the gasket surface near the oil pressure relief - the closest part of the gasket to the center of the engine.

    I think it's because of style. Yes, in an engine casing. Nobody wants to see knobbly bolts sitting outboard, so they're tucked in.

    This is probably the reason that Ducati went to silicone gasketing such as Three-Bond. The problem evaporates once the casings engage directly, face to face. I've stayed with the gasket for this rebuild - I like the fact that PTFE impregnated paper will come apart again reasonably easily - so I've set the crush using the old gasket, then fitted the new gasket. They're the same type, torqued to the same specifications, so whatever behaviour the first had, the second should repeat it.

    So far so good. Mains spin nicely and the alternator outrigger is out of round by roughly 50 microns, which isn't perfect but a significant improvement on the 120 microns observed when I went back into the engine to look for where the vibration was coming from.
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  4. #664
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    Main bearing shims 1

    Right, this is probably a bit left field... I've had a go at making my own main bearing shims.

    This was done after noticing a few things about the crankshaft. Clearances between the crankshaft shoulder and main bearing inner races have opened up, particularly on the transmission side. The shim-locating flanks of the crankshaft are mirror-polished. There's what appears to be some sort of wear or cutting on the radius of the crankshaft shoulder, as if the shim has been working its way in there... and the inner diameter of the shims appears to be fairly shiny and smooth too.

    This could happen if the crankshaft has been rolling within the inner race of the main bearing. This would happen on any open clearance between the two, I think, but would get progressively worse with wear and tear. The driver is the crankshaft being pushed one way and then another, relative to the bearing race (like pulling the piston down from TDC at the beginning of the compression stroke, then having the piston pushed into the crankshaft during the power stroke). Or it could happen if there's a constant push sideways, via the transmission gears. Certainly a lot of black oil had accumulated in this area when I took it apart. The crankshaft shoulders are polished, as well.

    So, if there's relative oscillation and rotation happening (admittedly very small amplitude), then the shim/s find themselves caught between two elements (bearing inner race and crankshaft) which move relative to each other. So the shims move. They'd probably not want to stay on center, and there's nothing mechanical keeping them there in the original design.

    If this happened (big if) then the shim edge could end up riding on the crankshaft's shoulder radius. This would increase shim preload, possibly by quite a bit, thus flexing the crankshaft and leading to out of balance behaviour on the flywheel. It wouldn't do anything good for main bearing lifetime, either.

    Ducati appear to be aware of this. All engine designs from '99 onward use thick shims with a deep chamfer, which feature an inner diameter which will locate centrally on the crankshaft's bearing shoulder. Pre '99, though... it's thin shims, maybe stacks of them. On the alternator side, they're held by friction and the stack of components ending in the alternator rotor nut, which is torqued up to around 180 Nm. They're clamped pretty firmly at all times. On the transmission side, though, it's just whatever force is exerted by the engine cases, which will fall as the engine warms up while in use.

    There's no positive radial retainment of the shims, in other words. There's no place where a diameter matches another diameter. They're just loose.

    My idea was to make shims that located on a diameter, either crankshaft or bearing. In the end the best place was the outer diameter of the main bearing's inner race.
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  5. #665
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    Main bearing shims 2

    Carrying on...

    Making these was a bit of an education in why commercial shims are flat discs, finished on a magnetic table surface grinder. It's just about the only way to do this in volume, to acceptable quality, in an acceptable time.

    The method I used was just stupid hard work: lathe bar stock to size, send resulting blanks off for heat treatment, find that they come back buckled and warped, then flatten and re-diameter everything with whatever grinding facilities are to hand. Yep, had to spend a few hours trying to sort these out. In the end I tried to make up a rotating plate mount for emery paper, fittable to a tool post, and basically sand down to size in the lathe via passively driven orbital face sanding. There was a lot of trying stuff for size or measuring, while making sure that abrasives didn't go anywhere near anything sensitive. It got fiddly.

    The material used was 4340 alloy steel, hardened and tempered to 42 Rockwell C (as per OEM shims). I never got there on the alternator shim. It simply warped too much to be corrected properly. Due to the hardening, it wouldn't spring flat either, so the thickness went badly out. The transmission side shim was useable, so that's in the engine now. I've had to use the original Ducati shims on the alternator side. As mentioned, this side is nipped up tight and should be OK.

    A bit of basic learning happened on this one. Steel will expand slightly once hardened and tempered. There's a phase change in the material, resulting in a different blend of crystalline structures. The new structure is more rigid, hence the material gets harder. The expansion isn't predictable, or uniform: it depends on the material, the grain orientation and dispersal, the quench medium and how quenching was done, then the temper and cooling programme applied. Hence finishing to precise size via grinding becomes essential. There's little or no chance of keeping fussy precision worked into soft material prior to hardening. The precision just won't survive.

    In the previous post, there are a couple of photos of the shims at various stages of work: as arrived back, bead blasted, then plain sanded against a surface plate, then orbital sanded in the lathe. That's the sequence I used, unfortunately the photos aren't in that order. If I'd had access to some sort of proper grinding machine I'd have used that.

    The heat treatment company I used also bead blasted everything. I really hadn't expected that but it does make sense, given that the metal will get blackened and oxidised from being heated.

    Grinding parallel planes really does want bonded grinding wheels, pumped coolant and magnetic work holding. I managed, via very laborious single-use workholding clamps and use of lots and lots of wet'n'dry paper, but there's no way I'm doing it this way twice.

    Anyway, the transmission shim is in the engine now - the only way to be sure if this works is to use the motor.
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  6. #666
    Join Date
    28th January 2015 - 16:17
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    2000 Ducati ST2
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    Lower Hutt
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    1,152

    Removing rear shock spherical bushings

    Just a quick post for anyone else with a good shock but loose bushings.

    The bushings themselves are an odd size: 12mm bore, 22mm OD, the shell is 9mm wide and the sphere is 11mm wide. The only aftermarket part I could find matching those sizes was an IKO spherical bushing part # SB12A, which has lubricant channels and requires regular servicing via greasegun.

    Getting the old ones out is a bit of a chore, partly because it looks like they were never intended to be changed. The photos show the sequence.

    1) 10mm collet type bearing puller is used as a punch, to drive the underside top hat down wards.

    2) .. this gets the top hat just far enough outboard to get flat bladed screwdrivers underneath it, and lever it out the rest of the way.

    3) 12mm pin punch drives the other top hat straight out.

    4) Jeweller's screwdriver / s for the semicircular clips, these have to be lifted out of their grooves and then up and out.

    5) a socket can be used to drive the bushing out, a proper press and lathed up press tool would be better here though. It's too easy to go sideways with a hammer.

    6) a quick look at the groove I put in with a Dremel, earlier - greatly helps with getting the screwdriver blade underneath the semicircular clips.
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    Last edited by OddDuck; 10th December 2019 at 19:34.

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