Hitchcocks Forum

My new compression tester says warm bike up then do compression test ? i had always done mine cold before now .

my two classic bullets ranged between 90-110 lbs pressure cold , which seems about right for one and other is starting to require more effort to start.

i feel cold is right IMHO

cheers 73a

I always understood that compression tests should be carried out when the engine is hot; the rings will seal properly when in this state and an accurate reading obtained.

My engine gets quite hot when it’s running. Surely running condition is more relevant...

I've always done them hot.
Normal running conditions.
Apart from anything else there won't be as much oil on the cylinder wall/rings in a cold engine. The oil does make a significant difference to the pressure reading.
If you have to do it cold for any reason, squirt a bit of oil down the bores and you'll get a more realistic reading.

Hot, cold.......If there is any real doubt about the compression, why not do both ?
That way, you will also have a worthwhile comparison between both conditions. Dare I say that if the "cold" compression is that low, you won't get the bike started to do a "hot" test, anyway ?.........

It should be determined by the purpose of the test.

Is the test to determine a minimum compression ratio [to determine if there is a ‘problem’] or a maximum compression ratio [as in tuning for speed]?

If the former is the purpose – test cold. If the latter – test hot.

I would have thought that squirting oil down the bore is a separate issue to whether compression is tested hot or cold. If compression is found to be lower than expected, adding some oil will help determine the reason for the low compression ie if adding oil results in a higher reading, the rings/piston are suspect, if not it’s the valves. Or am I wrong?