Johnson 15 rebuild - pistons too tight?

wineyarders

Seaman Apprentice
Joined
Jun 22, 2011
Messages
45
Hi, I've been down the long low compression troubleshooting road on my '85 johnson 15 which ended with 2 new GLM cast pistons standard size. After finally getting them inserted into the cylinders they seem too tight. I can move them but there is definitely friction and it is not a smooth pull and the rings are snug without any play. I lubed the pistons and cylinder with oil before putting them in. I plan on calling GLM in the morning but thought the helpful folks on this forum might have some insight. IU degalzed the cylinders with some borrowed spring hones but went very light as I didn't want to throw anything out of whack. Another point is there doesn't be a single point that is causing a hang up. Is it common for these pistons wo be out of speck? I have a cheap pair of calipers that seems to say the old and new clylinders have very similar diameter, and the pistons fit, they just seem tight. Any thoughts? Many thanks,

Ryan
 

jimbo_jwc

Ship Happens
Joined
Dec 19, 2010
Messages
633
Re: Johnson 15 rebuild - pistons too tight?

Rings bigger or ring groove undersized ? Might need the to use your calipers on them to spot a difference .
 

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
Re: Johnson 15 rebuild - pistons too tight?

Lacking sophisicated measuring instruments, you could have used a common set of feeler gauges to check the clearances before you put it together. Piston to cylinder clearance is .0025" min, .0053" max (roughly 2 to 5 thousandths). Piston ring gap is .005"-.015". Specs are for a 1976---Double check for your model.
 

1946Zephyr

Vice Admiral
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Oct 21, 2008
Messages
5,556
Re: Johnson 15 rebuild - pistons too tight?

Does the piston feel tight without rings on it? Or does it feel tight, after the rings are installed? If the pistons are tight without the rings, then it sounds like a flaw, with the piston to cylinder size, meaning the pistons were made out of spec (too big). If it gets tight, after the rings are installed, then I would say, make sure the ring ends are in the correct place. Some pistons have little pins in the ring groves, to secure the ring ends and to make sure you install the rings, with the ends 180 degrees apart from each other. I have pulled pistons out only to find that the last rebuilder didn't stagger the ring ends and that would likely cause a bit of "blowby" on that portion of the piston.
There is also a possibilty that the ring groves are not deep enough, or there is something in a ring groove, keeping the ring from going deep enough into the groove.
 

wineyarders

Seaman Apprentice
Joined
Jun 22, 2011
Messages
45
Re: Johnson 15 rebuild - pistons too tight?

Thanks for all of the advice. Picked up some digital calipers, bore gauge and feelers and took some of your suggestions. Measurements of bore, piston and ring gaps are good. Pistons without rings are pretty smooth in the bore. When inserting the ring into the bore to measure ring gap there was pretty good tension moving it into and out of the cylinder. So I put it all back together, ring position is good and I still get the same tension. Any other thoughts? I guess my compression will be pretty good.
 
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