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1984 60 Johnson

guido2

guido2

Regular Contributor
Hi, what is the minimum acceptable compression for this motor? Lite rust in upper cyl and normal wear in lower cyl. Upper cyl had water leaking in through crack in gasket. Had 80 psi then and added some lite oil and it ran up to 110. The lower cyl was 120 psi dry with no extra oil. Customer complains the Johnson was low on power for a 60.Thanks, Guido
 
Head is still off and wondering do I button it back up with the new gasket, or recommend an overhaul? Piston clearance at top of ring travel is about .011 on both cylinders.......ouch! Wouldn't we like to see about .06 to.08 max? I pulled the head suspecting the gasket leak and the customer is wondering if the motor would be useable if I put it back together.....maybe gaining enough compression in the infected cyl. by replacing the gasket. I priced the overhaul out at about $1,000, going .030 over. The motor is really nice otherwise......overall condition......not a "junker". Like you say, the pistons are pretty loose until the engine would be warmed up. I measured the clearance at about 45 degrees F. Compression test was done cold as well. Now I would have to sacrifice the $44 head gasket to provide another compression test.......after warming up the motor. There are no signs of overheating, but he said that it did overheat once on the previous owner and the water pump was replaced. No word on what the water pump really looked like. Thanks, Guido
 
Ok, the old gasket was badly stuck and with corrosion to the compression sealing lip in the suspect (upper) cyl. Can be certain this is the cause of water infection in the #1 cyl. I will lightly dress block/head surface, carefully clean out residual rust in cyl. #1, install head, check thermostat, fire up in the large tub (don't like muffs as can skew test of normal cooling system operation)........then retest compressions and report back to you guys on Monday. Gotta work on an old Merc next. Thanks.....you guys are top notch. Guido
 
The 0.011" clearance is of no concern at all and absolutely normal !!-----Pistons have a " wild taper " on them.----------------Even new pistons have that much clearance near the rings and that is a fact.---------------Pistons are measured at the bottom of the skirt and there you would like to see about 0.005"-----------The aluminum expands as the pistons heat up and it does fit a lot better at operating temperature.-------------Always warm up a motor before full throttle operation.
 
Very good points, both of you. Thank you. We might just end up with a decent engine without the overhaul. 120 psi really wasn't too bad. Thats spinning over pretty fast with the other spark plug out too.
 
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