260 Compression

[dubmoney]

I am looking a purchasing a Mercury 2.5 260 that has had some "port work done to it". All six cylinders have between 5-7% leak down. The tech says all cylinders are between 118 and 122 # compression. The compression numbers seem low to me, but the owner says that ported engines don't produce cranking compression like stock motors. Could this be true?

 

[XB03AllyProud]

That doesn't sound right to me, I'm not versed on 260's but was around a lot of built up fishing blocks and BP/ FP motors years ago that had extensive port work, they still had compression around 145 for those aiming to stay on pump gas, I don't remember though how many cc's the heads were. What cc are the heads on this motor you are looking at? Hopefully some of the family here will chime in that run ported 260's to help you out. What type(Lake cut, race, etc) port work and who did it, does this motor supposedly have? In my simple mind once the piston passes the top of ports on way to firing/ compression that motor should have another 25# or so of pressure no matter what was done to the porting.

Guy's, Whitney here is a great guy. He bought my XB03 (in my avatar) last month and is looking for power now to put on it. He would like a 15". We had numerous conversations and he committed to buying it. Between my lack of schedule working out of state and his it took several weeks to work out but he flew in to St Louis from Washington state and spent a Saturday hanging out. Really nice guy that like a lot of us has been working towards the day he could own one of our great boats. If we could help him out it would be a big plus to him in way of getting educated and not taken on a bad motor deal. This motor he is looking at may be a great motor, I don't know what it is or where or if that is truly BS they are telling him. Just doesn't sound right to me, but again I'm not educated enough myself to fully call BS. Thanks all for any input you can offer, I'm curious on answers for my own knowledge as well.

 

[Bobalouie]

Seems low to me as well. I thought most of the 260's were in the 140-145 range for a healthy motor. Plus, with 5-7% leakdown, it may be close to needing a re-ring. I would be re-ringing at 8% leakdown.

 

[Waterboy334]

I personally wouldn't buy that motor without pulling the heads and inspecting it. I'd even supply head gaskets to the owner, incase I didn't buy it. Compression sounds low to me. Stock they are around 135ish. If they are that low, might be time for a rering, but like I said. I'd pull heads to inspect first.

 

[dubmoney]

Thanks for the input fellas! Owner says it has about 3-4 hours break in and he expects the compression to come up. What is a good leak down percentage?

 

[nort]

If it has 3-4 hrs of break in time it should not have compression that low and leak down that high.
Something is NOT right.
Not saying run away, just saying do your due diligence.

 

[Dave Hensley]

A rebuild can mean many different things!!! I would want to know everything that was done to it, what parts were replaced and on a nik motor I would definitely want to see the condition of the plating. Especially around ring gaps and right at the top of the ring travel. I would be pulling heads like was suggested earlier. If you don't know what to look for and don't know the seller I would walk.

 

[BGohr]

the leakdown numbers are correct, so, if they raised the exhaust port, and didn't cut the heads, the compression would be lower, although I've never seen anyone want to do that, but, it is possible, I wouldn't buy any of these engines without pulling the heads..............................