seperating enginge

WHAT IS THE BEST WAY TO SEPERATE THE BOTTOM END FROM THE TOP END OF THE ENGINE. I HAVE A 75 COMMANDO ENGINE THAT IS RUSTED TOGETHER. I DON'T WANT TO BREAK THE FINS. ANY SUGGESTIONS?

Can you get the head off? If you can, then get it off and soak the cylinders and cylinder/crankcase joint with some penetrating oil. Also, take out all the cylinder-to-crankcase bolts and fill the holes with penetrating oil.

Time and gentle persuasion should do the trick.

If you can't get the head off, try above with the head/cylinder joint.

I worked in a car shop many years ago, where we had an engine with the head fused to the block. It took a week or so to free it - first by soaking a couple of days in the parts cleaner drum, then penetrating oil for a couple of days, then gentle heat and driving a wedge between the head and block.

Take the head off first and put lots of penetrating oil in the bores and around the cylinder base flange. If this is the first one you have done don't miss the centre head bolt way down in the casting between the two exhaust rocker mountings. I would bet that missing this one is responsible for more broken fins than anything else. There are a total of 5 bolts and seperately 5 nuts holding the cylinder head to the barrel.

I cannot seperate the head at all.

I am not able to seperate the head, but I was able to get all the bolts out. I will try what all of you have suggested. Thanks

Diesel is one of the best penetrants, if the engine was mine I would put it in a drum of diesel for a month or so (this is not an exageration).

I have never damaged a fin but the amount of Norton heads and barrels around with sometimes quite serious damage suggests it's not that hard to do. I wouldn't hit it with anything harder than an old soft hide mallet.

Have you got any movement at the crank ?

You don't have to remove the head on a 750 to lift the cylinder.

If nothing moves, you may have to decide which components you would prefer to sacrifice. I think that I would try to save the head and crankcases.

No movement in the crank.

I doubled and triple checked to make sure all the bolts are out and I will pick up some oil to give everything a try. Some of the bolts are completely unscrewed but not enough clearance to pull them out past the fins, but I can lift them all the way up so I know they are not stuck. Any suggestions are helpful. This is my first street bike ever. Frustrated but having fun working on my Norton. Thanks for all the help guys.

Are you absolutely sure you have got all the bolts and nuts out?

There are three at the front on top, down in the fins
four around the sides on top,
one at the back underneath
and two at the front underneath, hidden in the fins.

I have all of them out.

I did check it all out. I have penetrating oil sitting in it overnight so I will try again tomorrow. Thanks Pommie John

79x100 said:

Diesel is one of the best penetrants, if the engine was mine I would put it in a drum of diesel for a month or so (this is not an exageration).

I have never damaged a fin but the amount of Norton heads and barrels around with sometimes quite serious damage suggests it's not that hard to do. I wouldn't hit it with anything harder than an old soft hide mallet.

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Yup, you can fill up the bores with diesel/degreaser through the plug holes. Leave it for a couple weeks.
You could also fill the crackcase with the engine upside down through the drain plug to soak the bottoms of the pistons. Leave it for a couple weeks.
If you pry up using fins they break - right?
Well, with a couple of chunks of hardwood, you can carefully smack the head or barrels SIDEWAYS on the ends of the fins (if needed). This can break the joint loose without breaking the fins.
I have a couple old pieces of rock maple that look like they have been through a war - they never 'wear out' or get broken but have enough weight to do the job (or crush your fingers).

Just a reminder of the obvious. Since you have an 850 your MUST remove the head before you can remove the cylinders from the crankcase. There are four bolts passing from the top of the cylinders to the crankcase that are only accessible with the head removed.

I usually can remove a stuck head with a little heat from a heat gun and careful use of a short length of wood 2X2 inserted into the top of the exhaust port and sharp raps with a mallet. Then additional raps on the same piece of wood placed at the intake ports.