mercruiser 470 compression

[Dehn]

Thanks to all that responded to my question about hard turnover on my 1978 470. My question this time is what should the compression PSI be on this engine, I checked it and it had 125 psi on 3 and 127 on the 4th, is this within specs for this engine? Thaks for your help!

 

[beckerdg]

Re: mercruiser 470 compression

I am not a mechanic so don't take this as gospel. I am guessing your compression is a little low but probably acceptable. Even compression across all four cylinders is good. I have an 86 version that was rebuilt a few years back. The cylinders range 150-160. I think a compression leak down test, which I have never done, tells you more.

 

[beckerdg]

Re: mercruiser 470 compression

Per the SELOC manual:<br /><br />Manufacturers spec 150-170. However they add that it is possible to get a reading as low and 120 on an engine in good mechanical condition. Variarition of 20-30psi between cylinders is acceptable.<br /><br />You can do a simple leak test by putting a small amount of oil (teaspoon) in each cylinder. Crank a few times to distribute the oil. Then check the compression. If the reading goes up then the oil is making a temporary seal on possibly worn rings. (Also per SELOC manual)

 

[Trent]

Re: mercruiser 470 compression

Its good. No worries

 

[chiefmike58]

Re: mercruiser 470 compression

Dehn, although those compression readings that you reported are a tad low, the more important thing is that they are similiar and consistant, which they are. I would think you could get a few more years service out of this motor the way it is, all other things being ok.