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MG Midget and Sprite Technical - Compression Test Results

I have just carried out a compression test on my 1500. The head has been off and I have re lapped all the valve seats. The compression test reads 125 across all 4. This seems a little low to me? A positive that they all read the same though. I am going to carry out a wet test tonight as the engine has covered 105,000 miles so possibility the piston rings are a little worn. What is every ones opinion on 125 is this too low?
C Carter

Consistency is good! The engine does need to spin quite well to build up the pressure readings, and you need to be sure that the engine is able to suck in the maximum amount of air so that it is compressing well filled cylinders.

Do the test with the throttle pressed to the floor. (WOT)
Check first that the carb butterflies ate fully opened when the pedal is down. It is common for the throttle cable and linkage to be wrongly adjusted so that the carbs never actually fully open up.
Remove all the plugs when you do the test - it will spin over faster
Do each test the same - I spin the motor through 10 compression cycles for each measurement. 8 would do. You do need a decent charge in the battery!

Having said all that, 150 might have been expected but 125 is not particularly low for a standard compression engine.
Guy W

Thanks Guy, I did the test with out the exhaust and inlet mounted so I guess you can say the throttle is wide open. All plugs were out. Just hoping tonight the wet test doesnt show a massive increase in compression. Even if it does it wont be getting rebuilt till winter.
C Carter

Yep, I guess you could say that counts as pretty wide open!!
Guy W

Hi Christian.

"as the engine has covered 105,000"

I would say that definitely answers your question. You have to remember that 100K miles wasn't "that" common on these old engines. It was easily possible if the engine was looked after, but back then not many did to sufficient extent, and often most were smoking at 60K and before.

How were the bores when you had the head off? How big were the wear ridges? Any marks down the bores? Did you check for sideways piston movement at the top and half way down the bores?

At 105K miles on the same rings, before I did anything else, I'd sacrifice another head gasket, drop the sump, and if the bores are good I'd whack on another ring set.

You can put it together again now, but you'll likely only get another 20/30k max, before some serious smoke and oil burning.
Lawrence Slater

Bores were very good no marks or scratches. Ridges at the top were very small, no piston movement at the top or half way down. Engine when running has no smoking issues no bottom end noise and is very smooth. I am planning on doing a full rebuild this winter but for now want to just use it for the summer so will be doing less than 1000 miles on it. For now I will put it back together and enjoy it.
C Carter

******At 105K miles on the same rings, before I did anything else, I'd sacrifice another head gasket, drop the sump, and if the bores are good I'd whack on another ring set.******

I have to agree, even a new set of bearing shells and connecting rod hardware being your right there

A 3 day weekend with a cly hone, some new deves rings and some bearing shells, and you may not need to mess with this agian for another 3-7 years depending on driving time per year, plus you dont even have to pull the engine

Normally this isnt the case with most midgets , but it sounds like this car has been well maintained so that the wear down was consistant and even across the life....

Obviously someone not only purchased nigals service manual, and read it, but actually employed it to perfection

I think risking $200 - $300 and a 3 day holiday weekend is a great gamble for your current situation

Good luck


Prop and the Blackhole Midget

You might be right Prop, I am thinking that if I do this properly now then fingers crossed it will be a long time until I have to do it again! Working on the car is easy enough just those bloody manifold / exhaust nuts. If I ever get hold of the guy who designed that I am going to punch him clean in the face!
C Carter

I hear ya...

what I found that helps, is to clean to perfection the stud and nut hardware for the intake/exhauste manifolds

Make sure the nuts go on As easy as can be...that way it makes it easier to get them started....

I also found that those stubby short wrenches do much better then the standard length ... for reaching between the manifold and the head to access those nuts

But the best tool and my favorite tool of all is the ....magnet on a telescoping stick.

Being able to just pick up the nut while hunched over the engine is a god send, compared to standing up, crawling all the way under to the back of the car to pick up the nut and washer

Another tip, bit messy... put a small dab of silicon on the front half of the nut threads...agian NOT alot... this will give it a little extra stick while trying to get the nut threads started, without falling off the moment your finger is needing to be repostioned

Prop and the Blackhole Midget

If I were to hone the cylinders with the engine still in the car how would I get rid of all the metal particles surely they could damage the engine?
C Carter

If you hone it with the sump off, it should just wash down out of the bottom with paraffin or similar. Wrap the crank with layers of well sealed clingfilm to keep it clean and to stop dirt getting washed into any of the oilways.
Guy W

You can do it Christian. You've got the sump off and you just stuff some rag at the bottom of the bores, previously smeared with grease. You don't need to go all the way to the bottom of the bores, and you won't be wanting to take much off anyway. All you need really is a glaze buster, not a hone. Takes seconds in each bore.
Lawrence Slater

This thread was discussed between 16/08/2013 and 17/08/2013

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