22 replies to this topic

[propercornish]

hi, i have just bought a clubman with a 1275 engine. i want the engine for my other mini and will check it over before i fit it. i did a dry compression test it last night and and got the following results-

cyl 1= 165psi
cyl 2= 175psi
cyl 3= 165psi
cyl 4= 110psi

approx readings as my gauge goes up in stages of 10psi.

what should the compression be on a good engine? obviously i got a fault with number 4 cylinder and will do a wet compression test later to see if is pistons/rings or valves/ head gasket.

can anyone tell me what the engine originally came out of? the number i found on the tin plate above alternator reads-

12HD09 135905

cheers, steve

[Guess-Works.com]

The PSI reading is only useful to compare cylinders with each other, ie balancing or problem determination.

You can work out the actual compression ratio from the psi reading, but you also need temperature, altitude and also barometric pressure and some long winded maths...

but from your readings you're going to be taking the head off at least so it'll then be a good time to measure volumes etc...

[propercornish]

The PSI reading is only useful to compare cylinders with each other, ie balancing or problem determination.

You can work out the actual compression ratio from the psi reading, but you also need temperature, altitude and also barometric pressure and some long winded maths...

but from your readings you're going to be taking the head off at least so it'll then be a good time to measure volumes etc...

the engine will be stripped down and treated to a rebuild with a kent cam and a big valve head. all i want to know is what compression reading should i get on my gauge when doing a compression test if the engine was in good servicable condition?

[propercornish]

also does anyone know what car my engne came from? the engine number is on the start of this topic.

cheers

steve

[maddogmcg]

hi
i bought a mg metro engine recently and did the same checks when cold i got readings between 145 and 175 ,the engine is now fitted and runs ok it got 78 bhp on the rolling road

i took the head off and reground the valves ,but have never checked the psi values since

the engine does leave a small amount of oil on whatever is behind the exhaust when i start it ,but doesnt seem to burn tons of oil
also bear in mind it is a high compression engine
hope this helps in some way

t

[padnic]

hi, your engine number 12HD09 came out of a metro. Its an A+ engine and is leaded fuel.

[propercornish]

hi, your engine number 12HD09 came out of a metro. Its an A+ engine and is leaded fuel.

thanks mate, thats what i wanted to know. the head on it is a 12g940. is that right for the engine? would it be a standard 1275 or mg 1275?

[propercornish]

hi
i bought a mg metro engine recently and did the same checks when cold i got readings between 145 and 175 ,the engine is now fitted and runs ok it got 78 bhp on the rolling road

i took the head off and reground the valves ,but have never checked the psi values since

the engine does leave a small amount of oil on whatever is behind the exhaust when i start it ,but doesnt seem to burn tons of oil
also bear in mind it is a high compression engine
hope this helps in some way

t

do you know what head you got fitted?

[maddogmcg]

hi
i bought a mg metro engine recently and did the same checks when cold i got readings between 145 and 175 ,the engine is now fitted and runs ok it got 78 bhp on the rolling road

i took the head off and reground the valves ,but have never checked the psi values since

the engine does leave a small amount of oil on whatever is behind the exhaust when i start it ,but doesnt seem to burn tons of oil
also bear in mind it is a high compression engine
hope this helps in some way

t

do you know what head you got fitted?

hi

its a standard mg metro head ,but i have never checked the casting number
sorry

[Sprocket]

The PSI reading is only useful to compare cylinders with each other, ie balancing or problem determination.

You can work out the actual compression ratio from the psi reading, but you also need temperature, altitude and also barometric pressure and some long winded maths...

but from your readings you're going to be taking the head off at least so it'll then be a good time to measure volumes etc...

You can only work backwards if you know what the cylinder volume is when the valves are closed as well as the pressure at this exact point. Any other way will have errors in the calculation and not worth the bother

[miniboo]

12G940 is the correct head and from the engine number the CR should be 9.75:1 leaded head with dished pistons and interference fit gudgeon pins

[Ethel]

All 1275's have used the 12G940 casting since the demise of the original Cooper S. The figures don't sound too bad, the lower figures on 4 could just be down to the battery and starter needing a rest if you tested the lot one after the other.

Metro engines come in 2 basic flavours, high 'n low compression the engine number should tell which yours is. GuessWorks site comes up trumps again for info.

[propercornish]

The PSI reading is only useful to compare cylinders with each other, ie balancing or problem determination.

You can work out the actual compression ratio from the psi reading, but you also need temperature, altitude and also barometric pressure and some long winded maths...

but from your readings you're going to be taking the head off at least so it'll then be a good time to measure volumes etc...

You can only work backwards if you know what the cylinder volume is when the valves are closed as well as the pressure at this exact point. Any other way will have errors in the calculation and not worth the bother

i dont want to know the compression ratio. all i want to know is what psi or bar to expect from a good 1275 on compression test and on a worn one so i can compare to mine. i know you shouldnt have no more than about 2 bar difference between cylinders but nobody seems to know what is an acceptable pressure on a good engine.

[miniboo]

well yours are good on 1 2 and 3 but 4 is too low. maybe the head gasket but could be many things. basically you want all four to be about the same.

[Ethel]

That's because there are too many factors to take in to account. I'd be happy if an engine makes over 100 psi with little variation between the cylinders as long as it runs ok.

The theoretical max for your engine is 136psi heat expansion might add a bit.