Vintage 1986 PGO Sport 150 Piston and Rings
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PGO Sports 150, 1986
Joined: 29 Sep 2019
Posts: 17
Location: Canton, Ohio
Fri Jul 31, 2020 9:37 am quote
I have a PGO Sport 150 that i think needs a ring job. How do i determine what rings to get? I did a compression test and it is 70psi. Scooter was running then stopped and won't restart.
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BV 350
Joined: 22 Nov 2016
Posts: 637
Location: Nebraska
Fri Jul 31, 2020 10:27 am quote
Generally
Generally there are markings on the piston and/or cylinder to indicate bore diameter. If not, borrow a set of digital calipers or a micrometer. Sometimes also useful to slide the piston into the bore (without rings) and see how big a feeler-gauge you can get between the piston and cylinder wall. Can that model be bored oversize? If you want to get really fancy, need a bore gauge to check cylinder taper (they wear more at the top than the bottom). Presumably a shop manual will tell you the wear limits. To do it right, may need to have the cylinder bored oversize, and go to a new piston as well as rings.

For a quick-and dirty, get a brush hone to roughen the cylinder walls, pick up a set of rings in the proper size for the piston, check the end-gaps and adjust as needed, toss it back together and hope for the best. If the bore and/or piston are excessively worn, the new rings will lose seal fairly rapidly.
Member
PGO Sports 150, 1986
Joined: 29 Sep 2019
Posts: 17
Location: Canton, Ohio
Fri Jul 31, 2020 12:08 pm quote
Piston rings
Thanks for the useful info. I plan tearing into it this weekend. Another question. Could bad or dirty valves cause a low compression reading? This scooter only has 3,800 miles on it but had set for several years without being run. I cleaned and rebuilt the carb. It was running good prior to and after I did the carb. I was adjusting the mixing valve when it quit and now will not restart. I know I have good gas supply and a nice spark. I thought it might be a timing issue but after checking the compression, started thinking it was a ring problem.
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Joined: 26 Oct 2015
Posts: 2542

Fri Jul 31, 2020 1:31 pm quote
Re: Piston rings
Bwaltz wrote:
Thanks for the useful info. I plan tearing into it this weekend. Another question. Could bad or dirty valves cause a low compression reading? This scooter only has 3,800 miles on it but had set for several years without being run. I cleaned and rebuilt the carb. It was running good prior to and after I did the carb. I was adjusting the mixing valve when it quit and now will not restart. I know I have good gas supply and a nice spark. I thought it might be a timing issue but after checking the compression, started thinking it was a ring problem.
your "valves" should be fine. If you want to know the current condition of the piston rings, perform a "wet test". Squirt about a spoonful of oil down the sparkplug hole, then test your compression again. If it's a lot higher, then i would take a close look at your rings.
Member
PGO Sports 150, 1986
Joined: 29 Sep 2019
Posts: 17
Location: Canton, Ohio
Sat Aug 01, 2020 1:12 pm quote
Vintage 1986 PGO Sport 150 Piston and Rings
First, let me apologize for the question about the valves. Of course there are no valves like on a car, duh. I did the oil test and the compression did increase about 10 psi but i also noticed that some oil had leaked out between the head and cylinder. When i pulled the cyl. head, it was obvious there was a leak path. i was surprised there was no head gasket. should there be? the piston, bore and rings actually look real good. very smooth, no score marks, no cracked or broken rings, the ring gap measures about .3mm. one of the head nuts came off easier than the rest. now I am wondering if that was the problem. should i put new rings in anyway?
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2007 Stella 150
Joined: 02 Nov 2019
Posts: 746
Location: Rochester, Minnesota
Sat Aug 01, 2020 1:20 pm quote
I would read through this and measure the bore etc. With that small of a gap you should be good. You can also check the head for flatness by sanding your t on a piece of glass using a 400 or 600 grit paper and see how it cleans up.


https://www.scooterhelp.com/tips/engine/piston.install.lf.vespa.html


Ring gap...
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