MRC Forum- brought to you by www.morini-riders-club.com Member: g7AW3f

Pulled the 350 apart this afternoon to try and work out why it has such low compression and burns a little oil.

It is not the guides or valve seats as there is no real slop in the guides, and the seats look perfect, so no compression leaks there, and no oil going past the guides.

I have managed to source a supplier of rings so I am going to re ring it as there is no step in the cylinders, so assume that they are still withiin serviceable limits, does anyone know why the front piston has a spring behind the oil ring but the rear one does not?

When I took mine apart the spring behind the oil ring on the front pot was missing - perhaps the cause of the oil burning ? The rear pot hardly burns any oil.

When I took it apart I noticed that who ever rebuilt it the last time used a lot of sealant on the gaskets, should they be assembled with this or no sealant? I tend to use a little "Wellseal" or Hylomar if you need to use jointing compound.

It also looks like both head gaskets had been leaking judging by the top fins of the cylinders, that would not have helped compression.

I look forward to replies from the experts as I am still a Morini novice when it comes to rebuilding one of these.

The older type piston rings did have a spring behind the oil scraper on one piston only. I never really understood why but the later types did not have this feature, but the rings were of course updated and more like most other manufacturers did. Neither type gave any trouble I am aware of.
Wellseal or hylomar are both fine but the factory used a silicone sealant (sparingly of course) and the nearest today is from 3M, Permabond. Yamabond is another version of the same thing and where I first used it in a Yamaha dealers. Great stuff and less likely to fail than anything else.
If your head gaskets were leaking they may also have been blowing into the pushrod tunnels and pressurising the crankcase? That could account for oil being lost via the breathers or burnt after bypassing the rings.
A Morini in half decent state doesn't usually blow any oil out of the breathers, maybe some condensation but not oil. The breathers do need the labyrinth in them or at least some form of PCV to prevent symptoms.
No need to overtighten the head studs/nuts, but worth checking all the studs and threads are clean and free running.
These are pretty simple to work on as you've seen and normal good workshop practice is all that is needed.

Steve

Thanks for the replies - is there any way I know if I have an older type piston? Perhaps mine has the later ones, hence no spring.

I have some "3 Bond" which I used on the KH rebuild, which is virtually the same as Yamabond, so I will use that on the gaskets.

As you say the gaskets may have been causing some of my problems, I am going to check the ring end gaps later on today and see if they are within spec, if they are then no need to replace.

The breathers do have the little conical labyrinth breathers.

I was pleasantly surprised to see how good the valve seats were, they are totally pit free and look just about perfect.

As you say nice and simple to work on only took me a couple of hours to get both top ends off.

Plan on using the guidance re torqueing down detailed in the "blue" workshop manual.

My later Sport (K1) does not use the springs. The inside of the piston may say Vertex or Sciam.
I have been using Threebond 1215 for a few years now on various applications( including plumbing!) .
This motor never uses oil at all. It has been apart but all I did was lightly hone the bores after acquiring some new pistons.
Incidentally, clean the exhaust threads of any carbon deposits etc before refitting the nuts. I’ve been using some Optimol TA paste ( as per my old beemer) on the threads.

I sure the pistons remained the same. When you get the new rings check that they are the correct depths in the grooves in the piston, a nice fit up and down in the grooves too. A comparison ring gap check is a good idea too. The old rings might also be lacking a bit of spring tension against the bores and I think they too can become glazed. So a gentle hone of the bores along with decent rings and you should be on song again.
Good advice about the exhaust threads too from 3potjohn. I'd also add to be sure you don't over torque the rocker trunnion studs/nuts. They can easily pull out of the head, especially if using one of the very old manuals where the conversion from metric to imperial was mis-calculated! The first versions had a 7mm stud into the head, the later type was 8mm into the head with 7mm above for the trunnions-so a stepped stud. There is some useful stuff in this forum about taking the endfloat out of the rocker shafts too. Not absolutely necessary but a nice little touch that can help them stay quiet.

John / Steve.

Thanks for all the sage advice.

Pistons are marked SCIAM cast into the inside of the skirt, there is also a number 785 cast in.

Head stud nuts are a nice running fit on the threads, although one had a nut of the wrong pitch on it and screwed the cylinder stud out when I tried to undo it, running a die down the stud restored the thread, and the wrong pitch nut is now in the bin!

Checking with a vernier - yes I know not the right way to measure bores, but it seems to be on standard bore.

Ring gaps are rather large 0.8 for the compression rings, and 1.3 on the oil ring which is way outside spec.

Does not seem to be any reason to fit springs behind the oil ring.

As I said earlier there are no steps in the bores and if I measure the skirt clearance in the bores it is 0.1mm measured with a feeler gauge, crown clearance is slightly more, but believe the crowns do have a bit more clearance than the skirts - correct me if I am wrong.

0.1 is the limit according to the blue book.

Ring groves were pretty dirty, so cleaned them up very carefully with a small brass wire brush.

What torque do you recommend for the trunions?

Thanks for the advice re the exhaust threads, would not copper grease do the job?

Looking like new rings, and new gaskets properly sealed will fix the problems, to be honest it was not burning that much oil, and the motor was pretty quiet mechanically.

I think folk use copper grease, it’s just I use the Optimol TA grease as I use it on the beemer exhaust nuts.

Thanks John only reason for asking it that use copper grease for this sort of stuff, thought perhaps the Optimol was better suited or something.

Was doing a bit more to the Morini this afternoon, I said that the valve seats looked perfect - in the head yes, without doubt, but closer inspection of the Exhaust valves, not so.

Both have some very deep pits in them, and they are both cracked, just a small crack from the edge of each valve accross the valve seat in one place, so they are toast. Certainly would not have been good for compression or power!!

Dont know how the seats in the head can be so good yet the Ex valves not.

Valve clearances were Ok by the way - I checked them when I bought the bike.

So where do I get a pair of Ex Valves? Looks like Mdina might have some, but the part Nos dont seem to match - any advice / recommendations?

Valvemaster after the rebuild ?