450 scrambler to r/t desmo head

[themoudie]

Thank you George.

I was too busy giving the 450 an oil change and a new chain and set of sprockets, followed by a canter to bed it all in.

Congratulations, to your team beating the 'Boks tonight!

Good health, Bill

[Jordan]

A cylinder 'muff' is the aluminium cylinder jacket the surrounds and supports the iron liner.

Thanks George.
That's not all it does. It also supports the cylinder head!

[themoudie]

Aye Jordon,

That's not all it does. It also supports the cylinder head!

It shouldn't do directly, unless you have a muff machined out of true by a rough old MotoTrans production worker, as I have!

The cylinder head should sit square on the top of the cylinder liner that in turn should sit square in the cylinder muff and the whole assembly should sit square on top of the crankcases and be parallel to the crank shaft. I have an air gap of at least 0.010" (0.25mm) between the cylinder head casting and the muff casting on the 450 engine. This is why lapping the new liner to the cylinder head to form a gas tight seal and the correct "O" ring to seal the oil feed to the head is important. If the clearance that I have given above, between the cylinder head casting and cylinder muff is incorrect, I should be pleased to learn of the correct one including the tolerances (= or -).

I have yet to stick a large mandrel through the crank cases main bearing housings to measure if indeed they are truly horizontal and parallel to the mating surface of the crankcase mouth. If not, I then have a "Pup!" and would have to decide whether to break the remaining items for spares or become like the Honda MotoGP team and keep pouring cash into the abyss!

Good health, Bill

[Jordan]

It's easy to remove and install liners to our Ducati cylinders, using a kitchen oven. They just fall in or out as required.
There have been reports of liners creeping out during the cooling phase, so placing a heavy object on it is recommended.
What else could cause your air gap?

[themoudie]

Aye Jordan,

It's easy to remove and install liners to our Ducati cylinders, using a kitchen oven. They just fall in or out as required.
There have been reports of liners creeping out during the cooling phase, so placing a heavy object on it is recommended.

They may well do, but when the cylinder liner flange on one side sits tightly against the cylinder muff casting and on the opposite side there is a 0.0055" gap between the underside of the new liner and the cylinder muff, you have (I have) a problem! The whole of the underside of the liner flange should be sitting tight and square against the cylinder muff casting.

My liner is being held in place using steel plates and a heavy threaded rod to prevent any such creep, as the liner and muff cool. The lack of an interference fit and some appalling machining can not only allow the liner to creep as it cools in the muff, but also not provide external support to the whole of the liner walls. This can lead to unwanted movement and increased piston/ring wear when the engine is running.

What else could cause your air gap?

The clearance gap that I was describing, is that left between the the underside of the cylinder head casting and the top of the cylinder muff casting, when the cylinder head is bolted securely (28ft/lbs torque) to the cylinder muff and resting on the upper face of the cylinder liner flange alone. This allows for potential differences in expansion and contraction of the two aluminium castings and the iron cylinder liner.

I hope this answers your question and clarifies which gap I am refering to.

Good health, Bill

[Jordan]

Thanks Bill, I understand now.
Do you think you can identify the faulty component and get it machined to make good?

[themoudie]

Aye Jordan,

It would appear that the 350's muff is the faulty component, as it appears to have been bored out of square (at a diagonal angle), to both the upper and lower machined mating surfaces for the head and crankcases. This appears to be by 0.0055" and requires the liner to be partially inserted in the muff, then for mandrels to be made up so that the whole cylinder assembly is held square in a lathe. Then using a long boring tool, to clear the barrel fins, the upper and lower muff mating faces can be machined at 90° to the bore of the liner. After this operation, the whole assembly has to be re-heated, the liner pushed home, so that the flange contacts the muff around its entire circumference, clamped and left to cool. Then we get to see if this has solved the problem!

Having spent approximately 3hours heating, checking and fettling the components, so far and with engineers costing £65 per hour, you will appreciate that sorting out a roughly machined casting is not a quick, easy or cheap job.

Just be canny when purchasing old MotoTrans Ducati components, from bikes that apparently have had a low mileage recorded, they may have self-destructed and be better consigned to the smelting pot!

Good health, Bill

[blethermaskite]

Bill I had a 250 that had exactly this issue, to such a degree (about 6thou of a gap) that it had actually broken off 1/4 of the liner top flange, I removed the damaged liner and trial fitted a new one (6thou gap) but the complete assemble set on a surface table with a digital height gauge showed the muff base was perfectly true to the liner it was the top face of the 'muff' that was out......so as you suggest I partially pushed the liner out by about 10mm,chucked the whole assembly in the lathe by the liner and skimmed 7thou of the top of the muff. When it was all pushed back together properly the problem was gone.......however its also worth checking that the cylinder liner is actually perfectly central in the crankcase mouth and that the rod is also central (back to the dreaded shimming again) and don't forget the gasket thickness when torqued up, once you think its all ok fit the piston you will use, wipe it with engineers blue (sorry don't fit the rings) fit the cylinder barrel and bolt it down with 10mm threaded rod. Then spin the crank over about 10 maybe 15 times and care fully remove the cylinder, if everything is 'true the blue should have been rubbed randomly of the piston skirt sides (ignore the front and back that's the thrust and drag faces) if the blue is coming off one side much more than the other you will need to have a look at it all again to find out why. If the blue has come of evenly top to bottom of the skirt on one side the crank is not central, or the cylinder is not central to the crank, if the blue is coming of on the top area of one side of the skirt and the bottom side of the other the cylinder is not sitting square to the crank, or the rod is slightly bent.
Of course this all takes a bit of patience and you might need to do the blue test bit a few times to get a repeatable result, but like the shimming marathon this sort of thing is what gets you a sweet and long lasting engine.
Cheers,
George

[themoudie]

Aye George,

Thank you for the words of wisdom and advise, much appreciated. I have another engine builder friend who has advised " "Plasticine" the rod" so as to check the liner skirt cut-out for clearance from the conrod. Of course, with all this machining going on, the squish and the piston valve clearances are going to require the same treatment, in conjunction with the blueing.

There is going to be a lot of patience and persistance required, I hope that I can do the advice justice and build a sweet engine.

Good health, Bill

[LaceyDucati]

A common problem, some pictures I took for explanation.

PXL_20230621_092134794.MP.jpg

PXL_20230621_093618870.jpg

Nigel