Crankcase mating faces

Previously wrote:

Oooops! Not sure what happened to the original message so lets try again.

I've acquired a pair of matching Commando Crank Cases. When I place the mating faces together there's a slight rock indicating they're not perfectly flat. I can get a feeler gauge of around 0.008 thou in between the faces at the worst point. I've checked for burrs and bruising on the faces that could have caused the problem but all seems fine although, it looks like quite a bit of leverage has been employed in the past to separate the cases going by tell tale screwdriver marks. Light clamping does seem to bring the faces together and I'll use a liquid gasket product on the faces during final assembly. My questions are A) Do these cases often distort like this or settle down after machining at the factory? B) Is the gap I've measured acceptable or are they out of tolerance? C) Can I expect slight oil weeping from the joint in future if these cases are utilised in a motor? All helpful comments gratefully welcome and thanks in anticipation.

What you have here is some one being a butcher!

Have you considered taking the dowels / studs out and on a flat surface (glass / marble or what ever is as flat as you can get) using a very fine grade of wet and dry placing case face down on the wet and dry paper / flat surface, rubbing the case in circles the marks should come out and you will get a even flat amount removed all over the mating faces of the cases; this would not take long to remove the 0.008 you require.

You will know when the offending marks are removed as the surface will be the same grey all over, This is a similar principle to grinding in the valves.

For the size of it you will need a few sheets of wet and dryâ?.

I have done this on a few bits on my commando, the rocker covers as an example because these do distort when people over tighten them.

I hope this can help you.

Rich

Hi Raymondsleigh. You say there are no burrs or obvious reasons why the two cases do not come togethere accurately. It could be that at one time, one case or both were heated severly to remove/replace the main bearings and some distortion has occured. Obvioulsy I do not know if this theory is correct but this is whatI would do.

Make certain the faces are perfectly clean and put the two halves together. Bolt up very tightly using every hole available on it and that will mean you will require some 4 inch length bolts (don't worry about exact bolt diameters or threads) I use M6 coach bolts, I think M8 might fit too and be nearer the complete hole size but either way it does not matter, they just need to be very tight. Heat up your oven to 200 degreesand put in the assembled cases(if the cases are scrupiously clean it won't stink it out) let them cook for about half an hour oreven a bit longer and then turn off the oven. Leave the cases to cool off in the oven overnight. Remove from ovenand then re-examine keeping your breath held and fingers crossed (very tricky, probably impossible, LOL) There is just a chance that this heating cycle will have relaxed the castings for them to come together more accurately. If they don't, please don't send me the electricity bill, you could alwaysput in a cake to bake at the same time to getsome benefit from the hot oven (only joking)

Good luck

Les

Previously wrote:

Hi Raymondsleigh. You say there are no burrs or obvious reasons why the two cases do not come togethere accurately. It could be that at one time, one case or both were heated severly to remove/replace the main bearings and some distortion has occured. Obvioulsy I do not know if this theory is correct but this is whatI would do.

Make certain the faces are perfectly clean and put the two halves together. Bolt up very tightly using every hole available on it and that will mean you will require some 4 inch length bolts (don't worry about exact bolt diameters or threads) I use M6 coach bolts, I think M8 might fit too and be nearer the complete hole size but either way it does not matter, they just need to be very tight. Heat up your oven to 200 degreesand put in the assembled cases(if the cases are scrupiously clean it won't stink it out) let them cook for about half an hour oreven a bit longer and then turn off the oven. Leave the cases to cool off in the oven overnight. Remove from ovenand then re-examine keeping your breath held and fingers crossed (very tricky, probably impossible, LOL) There is just a chance that this heating cycle will have relaxed the castings for them to come together more accurately. If they don't, please don't send me the electricity bill, you could alwaysput in a cake to bake at the same time to getsome benefit from the hot oven (only joking)

Good luck

Les

NOTE THAT THE OVEN TEMPERATURE IS 200 DEGRESS C (CELSIUS)....400 degrees FARENHEIT....or GASMARK 6!

Les