Morris Minor Owners Club

Hi, can anyone tell me what the compression pressures should be on 1098cc engine please?

My engine is showing 125. 137, 90, 130 psi dry and 168, 170, 130, 175 psi wet. So looks like it needs piston rings.

My mechanic mate has asked me to try and find out what the dry compression pressure is for a fully warmed up rebuilt bedded in engine.

Thanks
Jon

140+ is fine, to my knowledge. More importantly, they should be within a few psi of eachother. Your readings for cylinder 3 look particularly bad. It seems likely that there's something going on with the valves on that cylinder. Have you set the clearances?

Hi, yes we did the valve clearances.

Someone more knowledgeable than me will probably be along soon but I think a leak down test would be a good idea.

That 90 psi reading seems too low to just be ring related and it's a long way out from the others, hence why I suspect valve trouble on that cylinder.

Thank you

I would suggest that the problem on No:3 cylinder is not down to a valve. If it was the valve the pressure would remain relatively the same when in fact the wet reading has gone from 90 psi to 130 psi.
I would be looking to remove the head and examine the valves and valve seats - if nothing obvious is found then look at replacing the pistons rings after de-glazing the bores.

My mechanic mate has asked me to try and find out what the dry compression pressure is for a fully warmed up rebuilt bedded in engine.

Does this engine fit that description? Without further details, I might suspect valve guides were changed?

As per James, a leak down test at 150 psi should identify a leaky valve with the manifolds removed. Otherwise, the engine has not been successfully run in. But ‘rebuil’ does not mean overhauled/reconditioned.

Head needs to come off anyway, if the valve clearances have been properly adjusted.

Sorry I should have better explained.
The engine is unknown. It's in a 61 minor I recently bought.
The reason for the compression test is that it is blowing oil mist out the breather pipes and oil filler cap (when removed).

My mechanic mate wants a comparison figure so that we know what the compression figures should be.

In addition to my comments above - I would be thinking along the lines 'how much oil if any does the engine use' before thinking too far ahead.
The 'A' Series is good a throwing oil out of any aperture when opened especially when the rocker cover oil filler cap is removed with the engine running.
Do you have the oil separator canister fitted to the front cam follower cover?

Unfortunately it doesn't have an oil canister fitted and I don't know how much oil it uses as it's not on the road yet.

This video shows the problem, ignore the leaking radiator it now has a new one.

https://youtu.be/zqG9TItQCI8

To answer your originasl question Bob, A good engine should have at least 150, dry that is.. My engine just had new rings and a valve job, and is showing 160 to 170.

Having watched the video - rings it is - but saying that I would not do anything until the car has been run for about 500 miles so as to get a better understanding of the car and its if any problems.

Now we are getting some decent information regarding the problem, I agree, it looks like the engine may be in need of an overhaul.

Checking the plugs after a good long run will provide more info on the engine condition - if pot number three is drinking oil, it should be evident, for instance.

Thank you everyone for the info and advice, it's very much appreciated. I need to do some thinking about how to proceed.

If it's been standing for a long time it may be a stuck ring, soaking it in something like redex for a few days may help.

Well this might just be the cause, amazingly it hasn't damaged the bore but I think i'll get it re-bored anyway.

Well this might just be the cause

I tend to agree, but one of our experts will be along soon to give a firm diagnosis.

I'm amazed it had 90psi !

amazingly it hasn't damaged the bore but I think i'll get it re-bored anyway.

No point, at all, of reboring if the bores are perfect. However, somehow I think this one may be in need of a rebore.

Checking the wear is the first thing to do..

Your mechanic mate should at least know how to check for wear in the bore diameter.