Morris Minor Owners Club

Moving off from a set of lights I lost a cylinder and discovered a nasty rattle. I'm thinking a big end has gone but I haven't taken the sump of yet to look. What am I looking for and where else should I look if nothing seems to be out of place there?
I had to drive about 400m until I could pull over and got a tow home so hopefully I haven't stuffed anything too badly. In younger, more foolish days I would've tried to drive it home.
Any advice is welcome

Most likely to be head gasket failure or possibly even just a plug that has failed on one cylinder. A quick compression test will tell you for sure...

I changed the plugs first thing, seems to idle alright but give it a bit and it misses in No. 4 and the rattle comes back.

Certainly sounds like HG - But, do a compression test to confirm before pulling the head off.

What engine is fitted and any mods? Have you done anything or changed anything recently? Anything at all? Has the head been converted to unleaded with bronze guides?

Definitely do a compression test before anything else, but if all 4 cylinders are around what they should be, then need to look elswhere.

Does it idle and run on all 4 quite normally on light throttle openings, and then under load straight down 3 and the rattle at the same time? I had this happen after fitting the first set of bronze valve guides and if the engine was working a bit more than light loads.

I tried a number of things and each time it seemed better, and only happening at higher and higher loads. After changing the whole ignition and fuel system, the cause was found to be that the No. 1 exhaust valve was being nipped by the guide and sticking = 3 cylinders and a rattle from the excess tappet clearance.

Engine is a stock standard 948. I don't know what size cylinders are in it but I guess the only way to find that out is to take the head off (or is it? Is there a secret way to find out?).
Idles OK, but not as smooth as it was. A bit of load and straight away I definitely lose a cylinder. After pulling the leads off the plugs it seems to be No. 4.
I'll splurge on a compression tester tomorrow (Public holiday here for Australia Day so everything is shut, talk about great timing).
What sort of values should I be expecting from the gauge if the cylinder is healthy and do I turn the engine by hand or use the starter motor sans ignition?

Its best to turn the engine over on the starter and crank it until the gauge stabilisers. Expect to see 120psi upwards for a healthy engine but what you are looking for is a big difference in readings. So, say you have 120 / 125 / 115 / 80 then you have a problem....

For a compression test:

Warm engine, plugs out, ignition off or disconnected, then spin over on the starter until the reading stabilises. As PSL184 says, it's differences that you are looking. This is the standard 'dry' test.

You could also try a 'wet' test as well. Squirt some oil through the plug hole being checked and run the test as above. If the readings are 10/20 lb/sq in higher, it would suggest that the rings are not sealing as well as they should.

This could also be a valve or valve guide problem - the valve could be stuck partially open and the rattle is the rocker hitting the valve (a 12 thou clearance would be taken up during normal running, but with a stuck valve it could be over 1/4 inch of travel for the rocker, plus the rocker would not push back to be in contact with the push rod - plenty of source of noise).

First things remains a compression test to see what's happening. Head gaskets normally go between 3 & 4, so expect both to be down; and you get a banging sound rather than a rattle. If only 4 is down then look for a valve. Worst case, the piston could have failed and you can hear the con rod rattling up and down the bore, but you'd expect clouds of smoke from the exhaust.

Hopefully it won't be anything too serious.

Could anyone recommend a particular make of compression tester,as I am thinking of buying one?

I'd buy one according to your budget ... in most cases you arent looking for exact figures but a difference between each cylinder (or rather little difference between each cylinder) ... so long as it comes out with basically the same number each time you try it on the same cylinder then whether it is actually accurate or not doesnt matter so much IMO.

Did the oil pressure light come on?
Is the oil in the sump?
Big end failure is quite rare and normally indicated by the answers being yes and no.

Is there a conrod sticking out of the side of the block?

Just kidding, although it's happened to me before! It idled very smoothly!

Andrew

MarkyB wrote:Did the oil pressure light come on?
Is the oil in the sump?
Big end failure is quite rare and normally indicated by the answers being yes and no.

No light and plenty of oil. Also no apparent bubbles/oil slick in the radiator and plenty of coolant.

No conrod sticking out of the block

as a basic compression test try turning it over on the handle, when my gasket went, there was an obvious lack of compression on one cylinder.

Andrew

OK, the compression test results are in:
No.1 = 120 psi
No.2 = 120 psi
No.3 = 75 psi
No.4 = 75 psi

On that basis, looks like a blown head gasket between 3 and 4.
You guys are awesome!

Time to get the tools out

ALL these compressions are desperately low - you should be looking at 160 psi for a good condition engine - 140/150 minimum for something that will run ok. It is however the head gasket that has gone- but either the rings/bores are worn or the valves are all needing attention. Certainly while the head is off you should grind in all the valves - and renew any that are pitted.

Cheers BMC. I was having similar thoughts. This will be the first time I've had the engine apart since I bought the car a year and a half ago so who knows what I'll find. I have a tube of grinding paste and a lapping tool on standby and once I discover what size the pistons are I will be looking at a new set of rings.
I have to say I greatly appreciate everyone's advice, the forum is worth it's weight in gold.

If the engine was not excessively smokey and didn't use much oil then it will probably be valves that need seating. You could try a wet compression test on the 2 good cylinders (before you take the head off) and see if it brings the compressions up. If you get a significant improvement you could look at new rings whilst you're doing the head.