proper compression for 470

[fmalott]

i will be testing the compression on a 1983 470 170hp serial number 6283569 i could not find what the compression is suppose to be in the manual what compression range should i look for to be great compression and at what compression range should i rebuild piston rings

 

[stonyloam]

Re: proper compression for 470

IMHO, I would be pretty comfortable with 135-150 (warm engine) as long as they are consistent. I would be concerned however if #4 is low even 5 pounds, could be indications of trouble to come. #4 seems to be the canary in the coal mine for a 470.

 

[fmalott]

Re: proper compression for 470

i was in another forum which i stumbled across (i'm still a iboat fan ) this dude was saying 150-170 he did not mention warm engine though what do you think

 

[stonyloam]

Re: proper compression for 470

I believe the proper procedure is to run the engine to get it up to temperature before doing the test. 150-170 seems a little high. Last time I did mine my "good" cylinders (1&2) were 150-155 warm, with no significant change with oil added. My "bad" cylinders (3&4) were 130 and 35, no change "wet". A new head gasket and valve job (with hardened exhaust seats) fixed it. The problem with giving a "hard" number is that there a ton of factors that influence the reading: air temp, humidity, warm engine vs cold even the particular gauge that you are using. If you get "decent" numbers that are consistent across all 4 cylinders and no significant change dry or after oil added, you should be OK. If you get a significant jump in compression after adding oil, would be an indication of worn rings. Again IMHO