First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 11:31AM

I recently got these replacement rings from Treats for my Hobbit:

https://www.treatland.tv/OEM-honda-NC50-NA50-PA50-ring-SET-p/oem-ring-set-13010-gf8-306.htm

I had never done this before, but it seemed pretty straight forward:

-I removed the old rings, carefully slid the new ones on

-Put on new base and head gaskets

-Oiled everything up, carefully compressed the rings and slid the cylinder back on

-Swapped the old bolts for studs and torqued them down to spec (the bolts were stripped out or something and wouldn't torque down enough when I first put the cylinder back on)

I felt pretty good about everything, but when I went to start it back up I had NO compression (gauge reads only 25 PSI). I checked for leaks around the cylinder (head and case) by spraying soapy water and didn't find anything.

So I'm thinking that it must be that I didn't install the the piston rings correctly - I did leave out the "helper ring" that came with the set, so I'm hoping that it's. Are the other things I should be checking for when I pull the cylinder back off, though? (edited)

Re: First time replacing piston rings

Bob Naske /

bobqt50

Aug 10, 2020 02:03PM

Never had one of those apart but wondering if the piston has those tiny pins in the ring groove that you put the ring gap on each side of it. Sounds like you did everything right though ??

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 02:16PM

Hey Bob-

I think you're talking about this part when you say pin, yeah?

I assumed since they were OEM rings they would be gapped correctly so I didn't check the spacing I just slid them in there so that the pins were between each end.

Am I correct that the best way to check the gap would be by taking the rings off the piston, sliding them into the cylinder bore, and using a feel gauge to measure between the two ends? (edited)

Re: First time replacing piston rings

Mike Boyd /

Go to branch

boyd

Aug 10, 2020 02:34PM

Yup, that's correct. I try to place them in the cylinder where the pin locates the gap and use the piston to push them up evenly to the top of the cylinder to take the measurement, one at a time and careful not to get caught up in the ports.

Re: First time replacing piston rings

P D /

pdcomm

Aug 10, 2020 04:55PM

> Mike Boyd Wrote:

> -------------------------------------------------------

> Yup, that's correct. I try to place them in the cylinder where the pin

> locates the gap and use the piston to push them up evenly to the top of

> the cylinder to take the measurement, one at a time and careful not to

> get caught up in the ports.

I think Mike meant to say to the top of their normal travel .

Any distance above that would be unworn territory and would likely present a different measurement . (edited)

Re: First time replacing piston rings

Dirty30 Dillon /

hydra

Aug 10, 2020 05:42PM

If you had a stripped stud hole, I bet it's loosened up again.

Re: First time replacing piston rings

pat splat /

ggspeed

Aug 10, 2020 07:18PM

whats wrong with og rings, why did u get new ones?

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 07:21PM

> pat splat Wrote:

> -------------------------------------------------------

> whats wrong with og rings, why did u get new ones?

Broke them when I pulled the cylinder off to remove the bolt I snapped off in the engine case (edited)

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 07:23PM

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> If you had a stripped stud hole, I bet it's loosened up again.

I was thinking about that, but if it was loose shouldn't I be able to find it leaking on that side with the soapy water?

Re: First time replacing piston rings

Richard Eberline /

rockettman

Aug 10, 2020 07:44PM

What about checking your ring's end gap in the cylinder's bore. 3 spots, top, middle and bottom.

Any time you replace piston rings, you need to crosshatch the cylinder's bore.

Re: First time replacing piston rings

P D /

pdcomm

Aug 10, 2020 07:52PM

> Richard Eberline Wrote:

> -------------------------------------------------------

> What about checking your ring's end gap in the cylinder's bore. 3 spots,

> top, middle and bottom.

>

> Any time you replace piston rings, you need to crosshatch the cylinder's

> bore.

Good practice ^ .

Considering that some bores are wider at the bottom than top , I'd expect those measurements to slightly different . But , not always , if we're lucky .

Re: First time replacing piston rings

Dirty30 Dillon /

hydra

Aug 10, 2020 08:31PM

> Jessie Paull Wrote:

> -------------------------------------------------------

> > Dirty30 Dillon Wrote:

>

> > -------------------------------------------------------

>

> > If you had a stripped stud hole, I bet it's loosened up again.

>

> I was thinking about that, but if it was loose shouldn't I be able to

> find it leaking on that side with the soapy water?

Are you doing a leakdown test? If youre just spraying water when it runs, that won't do anything.

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 10:00PM

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> > Jessie Paull Wrote:

>

> > -------------------------------------------------------

>

> > > Dirty30 Dillon Wrote:

>

> >

>

> > > -------------------------------------------------------

>

> >

>

> > > If you had a stripped stud hole, I bet it's loosened up again.

>

> >

>

> > I was thinking about that, but if it was loose shouldn't I be able to

>

> > find it leaking on that side with the soapy water?

>

> Are you doing a leakdown test? If youre just spraying water when it

> runs, that won't do anything.

Well it's not running, so I'm just spraying soapy water while someone pedals for me. I've never heard of a leak down test - I'm still learning here, so I'll look into that for sure.

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 10, 2020 10:09PM

> P D Wrote:

> -------------------------------------------------------

> > Richard Eberline Wrote:

>

> > -------------------------------------------------------

>

> > What about checking your ring's end gap in the cylinder's bore. 3

> spots,

>

> > top, middle and bottom.

>

> >

>

> > Any time you replace piston rings, you need to crosshatch the

> cylinder's

>

> > bore.

>

> Good practice ^ .

>

> Considering that some bores are wider at the bottom than top , I'd

> expect those measurements to slightly different . But , not always , if

> we're lucky .

Newbie question here: what's the difference between cross hatching and honing?

Re: First time replacing piston rings

P D /

pdcomm

Aug 11, 2020 07:22AM

> Jessie Paull Wrote:

> -------------------------------------------------------

> Newbie question here: what's the difference between cross hatching and

> honing?

Honing produces cross hatch when done properly .

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 11, 2020 07:47AM

> P D Wrote:

> -------------------------------------------------------

> > Jessie Paull Wrote:

>

> > -------------------------------------------------------

>

> > Newbie question here: what's the difference between cross hatching and

>

> > honing?

>

> Honing produces cross hatch when done properly .

Got it - thanks for the clarification

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 15, 2020 12:17PM

So I got myself a hone, pulled the cylinder and removed the piston rings again. After I honed the cylinder I measured the ring gaps and came up with .024mm gap for the top ring and .022mm for the bottom ring.

According to the wiki I should have a .016mm gap since it's a 40mm bore (I also measured my cylinder to make sure this was what size I had) and that too big a gap can cause blow by, which could explain my low compression issue. But the wiki also says (when measuring gap), "Larger is OK, smaller has to be corrected"

So is there a certain tolerance that's its acceptable, and is my gap within that range? Or are these rings just too small? They were listed as OEM hobbit rings on Treats so I'm surprised that they aren't right. I guess follow up question if this is the case is can anyone recommend rings that are definitely the right size?

Re: First time replacing piston rings

Papa _ /

honkycat

Aug 15, 2020 01:19PM

I doubt that .008mm discrepancy is causing your lack of compression.

The gap allows the rings to expand when hot w/o closing the gap completely and swelling out against the cylinder too tight. Compression should still be good when cold and best at operating temp when the ring ends touch (ever so delicately) at the gap and make a near perfect seal.

If the new rings are installed and compression still sucks, I’d look real hard at the head to cylinder seal and try your compression tester on a known good engine to see if it’s giving accurate pressure measurements. Maybe it’s messed up and your compression is fine? And the engine has a totally different reason for not starting. Weirder things have happened.

Re: First time replacing piston rings

Jessie Paull /

jessikwa

Aug 15, 2020 02:01PM

Well I guess now that I've honed the cylinder I'll reinstall with new gaskets, I just wanted to make sure I wouldn't be wasting my time with the rings gapped that amount.

MOPED ARMY

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