Power Head Compression

[Seahawk]

In my Evinrude service manual there is a Compression specification which I would like to know how the test is preformed. Since I am a fan of the 35 hp Larks I will use that as an example. It states Compression (fully compressed) 8-12 lbs. How is this test done and with what measuring device?

Thanks
Howard

[frankr]

You are reading something wrong. No way is that the compression spec. Sounds like a spring test. Compression relief valve springs??? If so, I can’t say I’ve ever bothered to test them.

[Seahawk]

I agree it does sound spring test. I am reading it correctly, the specification is listed under Powerhead Data > Piston Rings > Compression (fully compressed) 8-12 lbs

You may be correct, however another engine list which has same compression relief part has a lower value. It state a valve of 6-9 lbs. How can the same part of different values when fully compressed?

[frankr]

Ah, now I get it, they are speaking of the springiness of the piston rings or pressure against the cylinder walls. I must confess that in all my years of engine work, I have never made such a test or even know how, other than “looks good” or “feels good”. I’ve seen a wide variation of “normal” though. The skinny pressure-back rings are very weak. On the other hand, I’ve seen rings for industrial four-stroke engines that had so much tension that they were very difficult to get into the cylinders.

• This reply was modified 5 years ago by frankr.

[Seahawk]

Thanks for the clarification and explanation. This must be some sort of quality control test during the manufacturing process of the piston rings with a specialized jig. Would be interesting to hear from someone who has experience in this type of measurement and what tool was used to obtain the measurement. In the manual the lower hp models have lower values while the higher hp models have greater values. In the manual there is a 35 hp rated motor which has two different ranges for different models. Might be an engineering change since the follow year models kept the higher rating.

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