Johnson 15 rebuild - pistons too tight?

[wineyarders]

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]

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]

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]

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]

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.