Compression test on my motor after rebuild

[Rampage]

I did a compression test on my motor after about 7 hours on the motor. It was 160 on each cyl. Not sure but is it better to do it when its warmed up or cold. This was when it was cold.

 

[BMANN06]

I did a compression test on my motor after about 7 hours on the motor. It was 160 on each cyl. Not sure but is it better to do it when its warmed up or cold. This was when it was cold.

Hi Rampage,

Doing the compression test when warm is what I learned growing up. Also, doing the test on fresh engine may not yield expected results as well. Have you gone through the break in process?

 

[Dr Honda]

I recommend doing it on a cold, dry engine. (no extra fuel or oil in the jugs) and hold the throttle open.

160 is a little high, unless it a 951 engine. If it's one of the smaller Rotax engines... I would double check the squash band. You don't want to melt the piston crown.

 

[BMANN06]

I recommend doing it on a cold, dry engine.

Hi Doc, not questioning your knowledge but I was always taught that its best to do compression test when engine was at operating temp. As this is where all the alloys would be at there expanded and normal position. For my own personal knowledge could you tell me why the compression test should be done on a cold engine.

Many thanks!

 

[Rampage]

I will do both ways LOL!!

 

[Minnetonka4me]

If you dont have a snap on or mac gauge it isnt gonna be exact anyways.

I use it more to get an idea of the compression and that they blow the same.

 

[Dr Honda]

Hi Doc, not questioning your knowledge but I was always taught that its best to do compression test when engine was at operating temp. As this is where all the alloys would be at there expanded and normal position. For my own personal knowledge could you tell me why the compression test should be done on a cold engine.

Many thanks!

There are different ways of doing things... but on a cold engine, you will have the largest ring gap, and it should be the lowest pressure. Also, SeaDoo spec's pressure on a cold engine. (other manufactures spec differently)

If you dont have a snap on or mac gauge it isnt gonna be exact anyways.

I use it more to get an idea of the compression and that they blow the same.

You are right... but it's easy enough to stick a hunk of solder down the spark plug hole to check.

 

[Minnetonka4me]

Good point...check squish to make sure you arent too close.

 

[Rampage]

not sure what the heck the squish is hahah!! Im sure this will just be another thing i learn from this place! I re-tested with my very high dollar Harbor Freight Limited addition compression tester hahaha!! and they are both almost dead even at 155 psi.

 

[Minnetonka4me]

Squish is the clearance between the piston and the outside edge of the head. you want it to be within a certain spec for best performance. Its in the manual.

 

[TheChad]

I just completely rebuilt my 720 engine and have run about 1/2 tank of break in fuel threw it, I'm only getting 140PSI in each cylinder.

I would like to see higher, but I am told that as long as they are even I shouldn't worry about it..

I test mine cold.

-TheChad

 

[Dr Honda]

not sure what the heck the squish is hahah!! Im sure this will just be another thing i learn from this place! I re-tested with my very high dollar Harbor Freight Limited addition compression tester hahaha!! and they are both almost dead even at 155 psi.

155 is better on a fresh engine.

I'm sure you have the manual since you are member here. In the top end section, you will see how to do the test. Basically, you take a length of solder, put it down the spark plug hole, and right to the edge of the piston crown. Then roll the engine over, to crush the solder between the head and the piston. Then, pull it out, and measure the thickness. On an 800 engine, it should be 1.2 to 1.6mm. If it's tighter than that... you could melt a piston crown. (I've seen it happen a bunch of times, when people think that a thin base gasket will make more power)

I just completely rebuilt my 720 engine and have run about 1/2 tank of break in fuel threw it, I'm only getting 140PSI in each cylinder.

I would like to see higher, but I am told that as long as they are even I shouldn't worry about it..

I test mine cold.

-TheChad

Did you have the throttle open? That will make a 5 to 10 PSI difference.