As nobody else has had a go I’ll put my twopenn’orth in. You must understand that I don’t have the deep knowledge of the minutæ of Scott variations that many will have, what I do have is the experience of making a living in the engineering game. That said I’m guessing that this engine is a blind head, (non-detachable) type? I’ve looked at some pictures of the period but cannot tell for sure.

Presumably the problem is corrosion in the bores and probably some chemical reaction between the aluminium piston and the iron block. The trouble with oxides is that they are larger in volume than the original metal they replace and chemical reactions can effectively unite the surfaces. They are also gritty all of which leads to your problem.

Attempting to use hydraulic pressure, grease/oil in the combustion space runs the risk of putting undue strain on the connecting rod and/or crankpin so if this were to be attempted I would be inclined to remove the big end rollers at least until some movement is noted. Removing the rollers also has the advantage that it separates the cylinders to the extent of two roller diameters, which immediately halves the force needed to get something to move.

Heat and cold are useful allies, cold is in some ways easier to direct. To that end can you find a source of liquid nitrogen? A lot of university departments use it. It is used in a lot of processes, there are companies that use the stuff too but finding them is another matter. If you can source some then it can be poured into the piston through the crankcase doors. That will shrink the pistons like it is going out of style and get things moving with a bit of your hydraulics. “Cardice” solid CO2 is the next best thing and probably easier to obtain, a bit slower in action so there is more time for the block to cool too but the operation is the same.

Heating the whole block might be worth a try as a last resort; quietly smoking is about as far as you should go, say about 300ºC any more will create a serious risk distortion. As it cools there is a reasonable chance that a thin oil will creep down the sides of the piston and the differential expansion rates will help break the mechanical/chemical “lock”.

Another possibility but one that has its risks is that with the big-end rollers out would it not be possible to lift the block far enough to get some steel plate in the gap between the block and the crankcase? If it is then you have the basis of a puller with some studs and more substantial section steel with provision to use the spark plug holes as an attachment point. The danger is that you are putting a strain on the big-end pin that was not intended for it.

Of one thing I am sure and that is you are surely not the first with this problem and hopefully someone will be along with a proven method.

All I can add is “The Best of British Luck!”

Hi Robert
Can’t advise on moving pistons, but the big-end roller plate screws are only removable in one position on each side of the crank. With the crank case doors removed, you will see a half moon cut-out in the crankcase opening flanges and this is the position that the big-end screws must be removed in order for them to clear the crank-case without damaging it.
Without removing the screws you cannot remove the roller plates and therefore cannot extract the roller bearings.
Dave

Dohh!

Dave Bushell is entirely correct, well spotted that man! I’d forgotten that important point despite having been tinkering inside my engine’s big-ends within the last month.

Wots a goldfish? Told you I knew nuffink! I’ve just trotted out to the shed to have another look at my DPY engine after that faux pas! Looking a twit just once a day is quite enough thank you . . . !

On the premise that the screws are less expensive and certainly easier to make than a cylinder block the answer would be to cut/grind enough of the screw heads away to get them past the crankcase door lips having first made sure that they are loosened in order that the remains can be removed without further grief! There is enough room for at least one full turn so the full circumference of the heads can be made accessible and that should also be enough to gain the room tuck some protection for the big-end side plates under the heads. Even if the side plates were brand new they too are more easily replaced than the block and should be considered “collateral damage” in this instance, if not new they quite probably will need to be replaced anyway.

Despite the above seeming to be a classic case of one step forward and three back I still consider that gaining some, even if limited, potential independence of the cylinders to be worth it. The ability to mount an assault on one bore at a time will surely pay in the end as well as eliminating the big-ends as a source of stiffness.

Preparation is all as Dave Bushell’s remark proves only too well. The spirit of Professor Sodd is alive and well and is lurking close by! Do some dry runs and remember the old toolmaker’s addage; “Measure twice, cut once!”

Hi Robert. What an interesting problem you have. I have been very reluctant to make a suggestion as one man’s interpretation of what constitutes excessive force can vary widely from another person.
We do not have easy access to low temperature liquids, so if we were faced with this problem, I would first be sure that I had tried all traditional methods. I would have soaked the whole thing in a bucket of diesel for about two weeks.
I am assuming that you have removed transfer port covers and have examined those areas of the pistons that you can see for evidence of corrosion.
If you have access to suitable machinery or tooling, I suggest that you make two recesses in the door seating face that are co incident with the position of the crank screws. Copy the original Scott features and be careful to leave some sealing face for the door.
Remove both screws, outer plates and rollers.
If the stiffness is due to corrosion of the pistons in the barrel, then you should be able to rotate the flywheel a little, as we need to isolate exactly where the problem is
Check condition of rollers and big end bearing surfaces, this may give some useful indication as to the extent of any corrosion
If rollers and bearing surfaces are in good condition, replace rollers on one side only and try to move piston by rotating flywheel.
Now repeat with the opposite side,
Do we have movement one side but not the other
If both pistons are solid but the big end bearing tracks are in good condition,
Turn up a dummy ring in aluminium or brass to take the place of the rollers, to avoid brinelling of the track by the rollers when extra force has to be applied
Place in one big end
Clamp the crankcase firmly to a bench
Turn an aluminium bar to pass through the spark plug hole
Get a friend to pull the flywheel in the direction that is correct for piston on the downstroke
Give the aluminium bar a hit with a medium lump hammer
Repeat the process on the other piston
Consider that if the pistons are corroded in badly, then they are hardly likely to be useable and if necessary could be considered sacrificial.
I have found that if a steady force is applied and then a blow from a medium lump hammer with reasonable mass is applied in addition, then this will often solve the problem in hand.
Best of luck and kind regards Roger Moss

Nice pictures and there is always Bablefish for the writing!

Glad to hear there is progress, sooner or later the brute will learn manners!

Can you not fish out most of the grease and use diesel as a fluid? With your pressure device it should force it down the sides of the piston and help free things up a bit more. This is the one time when you hope the rings are stuck!

Also have you tried the cold treatment yet?

You are doing the right thing, the best tool you have with a job like this is endless patience.