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After an hour or so I have found the SKF part numbers for the 500 engine. The original is 633250C and is a split inner race for the main bearing and the split inner race bearing in the gearbox is 633245A. The main shaft bearing is identical to the standard 6204 bearing except that it is a crowded split inner bearing and the inner race bore is 33mm. Ade Crimp got a bearing direct from SKF through the local rep which has the number 6204/22 which has the inner race bore ground out to fit the main bearing journal. All of the 500's had a journal diameter of 33mm to my knowledge to end of production. If you consult the any manufacturers bearing reference books you will find the dimensions of the standard bearing. Owing to the one less ball bearing in the bearing it will have a lower service rating but still more than adequate for the reduced use of these engines today. If you cannot get the bearing then you could reduce the journal diameter by turning it down, no need to grind as any competent turner should be able to turn to the accuracy required. I think you will find that the 350 uses standard bearings so will not fit and without looking up will have a smaller inner race diameter. The gearbox bearing could be a problem as the inner diameter is 21.8 and the nearest bearing is 22mm.
Had a look at my spare Omega pistons and it reminded me that the original pistons had a slit on the back face with a hole at the bottom. I think the 350's have them as well. There is a tendency for a crack to open up from the hole to the bottom of the skirt which if you are lucky will have been found in time as sometimes another crack will develop and a piece of the piston will fall of. I think Asso had this expansion slot too, but they needed them because the pistons are cast as were the originals. With the Omega pistons which are forged the skirts are solid making for a much stronger piston. The also fit narrower rings than standard reducing the radial pressure on the bore and thereby reducing friction and you all know what that means. If you should visit their place they have all sorts of piston lying around including examples of those supplied to Ducati for their race bikes. Hope this is of help to you.

Bugger! I have just re read the note from Ade and the part number seems to be for the gearbox, so await an update.

Just checked the catalogue and letters and notes, the 6204/22 is the alternative for the original gearbox angular contact bearing 633245A. The /22 indicates that the bore is opened out to 22mm. The main bearing nearest equivalent is the 6306 which needs the bore opening out to 33mm so the SKF part number for this is 6306/33.

I think that's probably the same bearing that Tom bought with identical dimensions to the ordinary deep groove 6306 ball race as fitted to 350s.

It's an FPAC, a four point angular contact bearing. They're really designed for axial (thrust) loads, but due to their construction also have a considerably larger radial load capacity.

It would fit a 350 just fine, but to little benefit apart from making the engine easier to take apart (the inner race is in two pieces).

The 500 having a longer stroke and producing more torque puts more load on it drive side bearing and Morini obviously thought to upgrade. The drive side crank journal is larger on the 500 and a bearing with a modified inside diameter was used. Did Morini think the crank needed to be stronger, or was it to use some cheap surplus bearings?

Thas bearing is marked as an Excalibur 501 part and perhaps late 501s used a smaller crank journal. I've certainly seen a 500 engine with an ordinary deep grove bearing fitted on the drive side and presumably the crank had been ground down to suit.

Thanks Paul. Only one bearing left for me to buy then - the other crankshaft bearing.

But then the question is plain bearing or roller conversion?

Crank journal diameter on the primary drive side of the 501s was 33mm. The engine in my dart is from a very late 501 Excalibur, so unlikely they went to a smaller journal late in the production run.

However I did buy some new bearings, with the correct morini part number (70250061A) listed as for 501 Excalibur/'92, in a Ducati blister pack, and when they arrived they were not the original spec 33mm 4PAC bearing (SKF 633250C) but were 30mm QJ 306MA/C3 (still 4 point angular contact)

Perhaps stock of the other bearing ran out towards the end, or Cagiva/Ducati decided it was cheaper to supply a bearing perhaps common with one of their other engines at the time?

It does at least give an extra lease of life to a crank if the journal that side was damaged, and in the absence of stock of the original spec 4PAC bearing I doubt there's a better alternative.

I bought a couple of those Ducati bagged qj306 bearings assuming there were the 33mm ones. Since they're not, and I'm unlikely to need them both I'm happy to move one on if anyone's interested. I think I paid about £60 a bearing but they're not doing me any favours on the shelf so I'm open to offers.

Tom

EVguru wrote:But then the question is plain bearing or roller conversion?

Both the original engine I'm rebuilding and the "donor" bottom-end the customer supplied are early 3 1/2 Sports and have ball-bearings on both sides. Is there a problem with retaining that configuration?

AntietamClassicCycle wrote:Both the original engine I'm rebuilding and the "donor" bottom-end the customer supplied are early 3 1/2 Sports and have ball-bearings on both sides. Is there a problem with retaining that configuration?

There is a hardened steel sleeve pressed onto the crank that's meant to be a sliding fit in the timing side bearing to allow for expansion. That's a lot of tolerances to get right; crank OD, Sleeve ID and OD and then you have the bearing ID tolerance to take into account.

Engine life was not always what Morini desired. The sleeve could wear prematurely, or fret and come loose on the crank, or if too tight in the bearing, cause it to move in the case. The change was made to a plain bearing that fitted in place of the deep groove ballrace and the oil pump worm was given an extra start.

An SKF NU206ECJ/C3 roller bearing is dimensionally equivalent, but the inner race needs to be a tight press fit onto the sleeve. I think NLM swap sleeves around to get the required fit.