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Compression

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crewless Avatar
crewless Silver Member Kevin Duffy
Raleigh, NC, USA   USA
1951 Morris Minor MM Tourer "Millie"
1965 Lambretta Li 150 Series 3 "Lucy"
1976 Triumph TR6 "Tina"
Just working through some running(or not) issues on the 51 MM , I did a compression check 98-100 across the 4,
Any other data points out there ?
Also I have seen some vapors around the oil filler cap when running for a while , was trying to understand things a little better
so i did the check

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carl.mcclain Carl Mcclain
Elmwood Park, IL, USA   USA
You might want to do your compression test both cold and hot cylinder walls can expand while hot more so than when cold. You can also do a hot and cold leak down test to see if you have excessive blowby. And if that fails you can do a running compression test. if you or someone you know has a blowby meter or gauge you can place it in the oil fill hole and see what your engine is doing. if your compression tester is equipped with a removable schrader in the end that screws into the cylinder you can also monitor the cylinder and see if you have sever fluctuations in the cylinder indicating poor valve seating which can also be done during the leak down test if you can hold the crankshaft while applying pressure to the cylinder long enough. Doing the running compression test can tell you if your cylinders are bulging during high pressure operation .

0123 Mike D
Biddulp, Staffs, UK   GBR
In reply to # 26055 by crewless Just working through some running(or not) issues on the 51 MM , I did a compression check 98-100 across the 4,
Any other data points out there ?
Plug temperature range
Plug colour after a plug chop.

Also I have seen some vapors around the oil filler cap when running for a while , was trying to understand things a little better
Connect your crankcase breather to your
filter box between the filter and the carb.
Block, or filter the air at the other vent.

so i did the check

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John in Eugene Avatar
John in Eugene Gold Member John Quilter
Eugene, Oregon, USA   USA
Keep in mind the 918cc side valve engine fitted to the 1951 Minors only had a compression ratio of 6.5/6.7 per the workshop manual. Not sure what this translates to in pounds per square inch though. Do a dry and wet test and see if there is a great difference. The good news is you will not need high octane fuel.smiling smiley

John F. Quilter
Eugene, Oregon USA

emjay Jim English
Marietta, PA, USA   USA
Using an atmospheric pressure of 14.7psia and 6.5 compression ratio yield a maximum theoretical value of 96psia or perhaps 81.3psig. psia is absolute pressure and psig is gauge pressure that starts at zero in the atmosphere.

0123 Mike D
Biddulp, Staffs, UK   GBR
Assuming 100% efficiency.

Assuming the inducted air
doesn't heat up when it's
under compression in the engine.

Compressions tests are
pretty much witch docter
mumbo jumbo Kevin :-(

The best you can get from
a compression test is to
compare the individual
cylinders.with each other.

If all the cylinders are about the
same, all cylinders are equally worn.

If one cylinder is down a long way
compared with the others you
probably need to sort the head.

Comparing dry test results with wet
test results tells you about the rings.

If you get a big difference between wet
and dry tests you need to sort the rings.

There are at least 2 ways to test compression.

1) Turn the engine over once.

2) Turn the engine over on the starter loads of times,

2) usually shows about twice the pressure 1) shows.

Either way, just compare your results as above.

If each your cylinders read 98-100 with test 1
your engine is in good order.

If they read 98 to 100 on test 2,
your engine is probably knacked :-(

If so, just run it until the winter, then sort it.

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