Full Bore


Full Bore

A word about compression.

A word about compression and compression testing a 2 stroke engine.


Every day I am asked what should I get for a compression reading.

Please read and we can go from there.

So many people are wrapped up in some magic number they should see when doing a compression test. It's almost to the point of pissing contests for the highest number.

Here is a question for you. Why doesn't sea doo publish a psi for compression test? First there are to many variables between testers let alone the condition of the starter, battery, and anything else that could alter the reading. So for example if they said your 717 should have 150psi compression what would happen? Joe Blow shows up with his harbor Freight compression tester, worn out starter and a tiered battery and gets a reading of 137 PSI. Now what? This joker will be screaming warranty when everything is fine. This would cost them millions. The point is there is to many variables and the first key is what is the difference between the cylinders.

So why won't engine builders not give you a magic number?

First because of the same variables for one and then when rebuilt there are other variables that come with different brand pistons. What brand pistons absolutely has an effect of what your reading will be. The actual deck height vs the piston dome at TDC is different from one brand to the next. Different style rings such as L rings vs keystone and the location of them will affect the reading.

Time for some controversy how to do a compression test.

If you ask 10 guys how to do the test you will get 9 different answers.

So other spark plug in or not? My way is all plugs out. Some well respected engine builders will disagree so I will explain why I do it this way. If the biggest thing you are looking for is the difference between cylinders then you want to keep the "control" the same. In other words if one side is good and the other is low it will effect the readings differently then the other side. So to not have the opposite side effect the reading we always remove all spark plugs. With that part out of the way lets get to the rest.

So spark plugs out and the caps grounded on the grounding straps. Gas turned off and throttle held wide open crank the engine. Here is the key ! Don't just blindly crank it forever to get the highest number you can. I limit the crank time to no more then 5 seconds. While doing this keep an eye out for what the reading is after the 3rd or 4th revolution verses the reading after the needle stops climbing or about 5 seconds. If the difference between the two readings is quite a bit your top end is probably tiered and on borrowed time. If it is only 5 10 PSI then it is probably pretty good providing it was a consistent solid number. More importantly what is the difference from one side to the other? most engine builders were taught the 10% rule including me. I don't live by that rule. If I see 6 or 7 PSI difference I tear it down and take a look. Guess what? every time I have seen 7 psi or more difference and tore down for inspection. There was definitely an a serous problem such as a stuck ring that oil was masking.

So focus on doing it right, what is the difference between the cylinders, what is the difference between the first few revolutions vs 5 seconds and using a good gauge.

This will help make a well informed decision on what is going on.



Contact us

Email us at info@fullboreonline.com

 Phone 989-340-1321

Mon - Thur 9-5, Friday 9-4

Eastern time

Shipping address (shipping only)

Full Bore
486 S Ripley Blvd
PMB #228
Alpena, MI 49707