UK Monster Owners Club Forum » .: Technical :. » Engines, Clutch, Gears » 750 engine re-commission

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Old 30-06-2022, 07:03 PM   #1
350TSS
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750 engine re-commission

I was fortunate enough to bag the 750 engine Nasher was offering on here a while back.
Yesterday I collected it, enjoyed a good coffee and chinwag and admired Nasher's stable of bikes then brought it home.
I am not sure really why I bought it as I have 4 projects ongoing and 3 more waiting in the wings, these are
1. Slow Monster - bike has been transported to Benjy Straw's (electrical wizard and guru for Morinis) for him to see if he can find a spark,
2. 3 1/2 Moto Morini also at Benjy's for replacement of kick starter spring and set up of carbs,
3. ZX9R running on 3, 3.5 and sometimes 4 cylinders at low revs - a consequence of leaving ethanol petrol in the tank over the winter. I have had change the fuel pump and had the carbs off 5 times to put them through an ultrasonic cleaner - its 31 fasteners and 6 pipe clips to disconnect so a right pain,
4. replacing the crankshaft in a MZ TS125
This is what the 750 engine looked like
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Old 30-06-2022, 07:45 PM   #2
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At this point it looks better in the pictures than it actually is. Once I got it onto my stand I gave it a cursory examination, I think it must be out of an 750SS as it has the mechanical rev counter drive on top of the horizontal cylinder head. The ignition pick up leads have definitely seen better days, worryingly the gearbox sprocket has about 6mm slop radially around the centre line of the output shaft but only about 2 - 3 degrees circumferential slop, so hopefully it is the sprocket splines that are worn and not the shaft
Today I attacked it with the dead petrol drained out of the ZX9R. Whilst petrol loosens the grease and grime it is not very good at actually de-greasing as it evaporates as it runs off and merely spreads the crud more widely. It was all I had to hand and I thought I ought to get some value out of stale E10 petrol.
The engine original colour was I believe silver but it had been brush painted with another silver that barely survived the petrol wash. The barrels and head had been also brush painted with black using a bog brush by the look of it.
After about an hour I was able to examine what I had more closely without being afflicted with congealed chain grease.
The good news was that the swinging arm bearings had been removed from the back of the crankcase as they can be an absolute swine to get out. The bad news was that I found this.

The hole in the drive side crankcase for the swinging arm is at least 1mm oval measured 3 o'clock to 9 o'clock compared 12 o'clock to 6 o'clock. The right hand side measured with a digital caliper is still round. I do not think anyone extracted the drive side swinging arm needle roller bearing I think it probably fell out.
Many would at this point simply break the engine for parts but somehow I do not want to do that and anyway I love a challenge. I think what I will need to do is make a jig so I can bore the drive side case out and fit an oversize bearing, the difficulty will be in ensuring that the hole is at right angles to the split in the crankcases and parallel to the rear wheel spindle. I do not have a milling machine and only a small lathe, it will certainly exercise the little grey cells (both of them).

Last edited by 350TSS; 30-06-2022 at 07:48 PM..
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Old 30-06-2022, 10:25 PM   #3
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From conversations I had with Darkness I've got a feeling that the engine is from a Cagiva Elefant. The rev counter drive alone does not mean it was from an SS.
Some Elefants had a kickstart and you can see where the shaft would have come out of the cases on the rh side of the engine under the clutch cover. I can't see if the boss is unmachined or blanked off. Maybe all those early 750 engines have that boss in the casting as a remnant of the kickstarter?

It may not have had swinging arm bearings in the cases? I think the Elefant had mountings on the frame for the swinging arm, but a spindle could well have still gone through the engine?

Not sure what else had the 750 engine. I'm pretty sure all the Pantah and Alazzurra had the rear exhaust exiting from the front of the head with a carb behind? The 750 Paso was water cooled I think? Indiana perhaps?

Not sure if any of that is correct at all but food for thought and investigation.
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Old 30-06-2022, 11:46 PM   #4
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An interesting piece of engine archaeology there! You may be able to determine both the year of manufacture and original model type from the engine number - try giving Moto Rapido a ring and see if they can identify it for you.
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Old 01-07-2022, 12:05 PM   #5
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Quote:
Originally Posted by 350TSS View Post
The good news was that the swinging arm bearings had been removed from the back of the crankcase as they can be an absolute swine to get out. The bad news was that I found this.
.
Well if indeed that motor did come from an Elepfant then it would not have had bearings in the crankcase IIRC the bearings wear in the swingarm and the pivot was the frame, the pin mearly went through the hole in the engine suggesting the wear was caused by a worn swing arm
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Old 01-07-2022, 01:31 PM   #6
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Hey 350TSS, was good to meet you on Wednesday and enjoy a coffee and chat.

I'm really sorry about what you have found with the oval hole, I don't suppose even Mark had seen that before putting it under his bench.

I think I did mention that he had a 750 Elephant, so this may well have come out of another one.

Seeing it like that the holes in the cases don't look beg enough to have ever had a bearing in them to me, so I think I'm going to agree with Kato.
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Old 01-07-2022, 01:40 PM   #7
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Thanks for confirming the Elefant swingarm arrangement Kato. The sprocket cover also looks more like the one from that era.

It turns out that the 750 Paso was air cooled, but it had the dry clutch (as did the early 750SS) and I don't think those long inlet tubes would line up to a twin choke Weber?

I noticed that there are oil cooler hose spigots which might make it less likely to be from an SS as the 750 was not fitted with one. (In the early days.)

Are we going to start a book on this?..
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Old 01-07-2022, 02:14 PM   #8
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Having spent half an hour with the digital calipers I think I can confirm this engine did not come from a Ducati.

The swinging arm hole on the chain side measures (average of 3 measurements in each axis) 20.7 mm North/South and 22.47 mm East/West (so 1.77 mm oval) on the clutch side the hole is 20.27 mm N/S and 22.00 mm E/W (so 1.73 mm oval).
A Ducati SA spindle is 20 mm diameter and the smallest needle roller bearing with a 20 mm ID is 26 mm OD so these cases have never had a SA bearing.

As it stands the engine is scrap, it cannot be used in a Cagiva without the crankcases being bored and sleeved and it cannot be used in a Ducati without the crankcases being bored to accept needle roller SA bearings. I have no desire to acquire a Cagiva Elephant but I do have lots of Monster spares that could almost make a complete bike.

I have a cunning plan to enable the crankcases to be taken out to accept Ducati SA needle roller bearings involving some 10 mm aluminium plate (250 mm x 80 mm) and a steel bar (45 mm diameter x 70 mm long) both of which are sitting in my scrap bin.
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Old 01-07-2022, 02:41 PM   #9
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Quote:
Originally Posted by 350TSS View Post

As it stands the engine is scrap, it cannot be used in a Cagiva without the crankcases being bored and sleeved and it cannot be used in a Ducati without the crankcases being bored to accept needle roller SA bearings.
Apologies but I disagree with both statements

1: If goinig into an Elefant frame then that oval wear is no issue as the pivot pin doesn't need to be and is not supported by any part of the case and unless something is out of alignment should'nt touch so - not scrap

2: If you didn't want to bore out those cases to accept the SA bearings, which actually is not that tricky a job (easier with the cases seperated but done in the factory with the engine bolted together), you could put bearing in the swingarm like the 916,996,998 etc etc,

I think calling it scrap is a little bit defeatist
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Old 01-07-2022, 03:04 PM   #10
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Elefant swing arm

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Old 03-07-2022, 12:12 AM   #11
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wait a minute My 1991 750ss does NOT have needle roller bearings in It has original nice big bonze bushes , much more civilised than those horrible fiddly roller bearings in my 2000 750 M or S4 ,, horrrible things to remove or install .

Maybe your s could have Bushes intalled as I have seen other early 750ss with the bronze bushes
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Old 13-07-2022, 06:13 PM   #12
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I spent a hot, dirty and dusty couple of hours with my battery drill and a nylon abrasive brush and it now looks like this:

The nylon brushes are quite aggressive and can score the aluminium if you lean on them too hard but they do get the crud off. The original Ducati paint, where it was still evident, virtually came off of its own accord. The paint applied by a previous owner with a bog brush/trowel was a little more troublesome. Lack of additional battery power stopped play, nothing below the oil sight level has been touched yet and above it detailed removal is still required with a dremel and a small brass brush, so at least two more hours work just to remove the old paint /crud.
I am awaiting delivery of M12 x 1.25 bolts so that I can make a jig to bore the crankcase SA hole 26mm to accept either the standard needle rollers or bronze bushes (not decided yet) either way a grease nipple will be added.
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Old 13-07-2022, 07:44 PM   #13
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Looking good . Not everyone enjoys doing this but I do for sure. Polishing turns is good for the mind.
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Old 16-07-2022, 07:33 PM   #14
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Grabbed half an hour in the garage this pm and thought I would check out the gearbox sprocket and output shaft. The sprocket was decidedly sloppy with about 6 mm rock along the axis of the gearbox shaft, I was hopeful that the gearbox shaft would be OK as there was very little circumferential movement and I was not disappointed. Whist there is evidence of movement the gearbox splines are perfectly serviceable with no lifting of any case hardening or chunks missing.

The gearbox sprocket is an AFAM and I have never noticed this before when changing sprockets that this one whilst 14 mm thick only has splines that are 8.9 mm deep and what is worse is that they are located not on the toothed edge of the sprocket but on the spacer edge. This would have the advantage of putting them closer to the output bearing but under the sprocket there is c 5 mm unsupported. Are all Ducati gearbox sprockets like this or is it just AFAM ones?
If it is just AFAM ones I shall not be buying from them in future
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