Calling All G1 Experts

[sho305]

Compression is more important in a 2 stroke as you only have half the stroke to make it. You can toss rings in a worn 4 stroke its good for a while, but 2 stroke really needs the seal and wears faster....a fresh bore is where its at. Used to be able to buy a bored cylinder and fitted piston pretty cheap but I've not looked in years. Then you have a brand new top end, it will run right for some time and make max power.

[OgreTX]

Ok, had thirty minutes between football games, so I thought I'd get it broken down to work on this next week.

PICS below are:

1. Head - good shape, but needs to be de-carboned
2. Piston in cylinder - carboned up , but no visible damage. No scratches on cylinder walls.
3. Cylinder and attempt to get a good shot. Walls are very clean and no gouges, scratches, etc.
4. Piston without cylinder. Note the bottom piston ring on the left side.
5. Yes the bottom piston ring is almost gone, and the top one is EXACTLY that thin, so it would be breaking shortly IF it could hold any compression.

The good news is that it appears that a good engine cleaning, and a re-ring job will git'r'done. Too optimistic? As Dave recommended, now that it's out of the cart and accessible, crank seals and anything else that can be done while it's out also.

I still need to get out my calipers and do the 8 cylinder measurements in the manual to see if the wear is beyond the recommended limits. I'll update when I get some more time.

As always any and all input is welcome and thanks for the assistance so far.

Ogre

[1989Marathon]

That's good news. Measure it up and see what it needs.
I would recommend a ring/piston kit, and crank seals at a minimum.

[slonomo]

If the cylinder is within spec it should be good to go. Especially if the cylinder is a genuine Yamaha part, those things are durable. The china made ones are not as good and the cylinder liners wear. See if there is a Yamaha stamp on it.

[OgreTX]

OK, quick question guys. What kind of material am I working with?

I was going to soak the piston in some solvent to loosen up the carbon, and then carefully scrape with wooden sticks. However, as I usually do, I placed a small magnet to everything (head, cylinder, piston, et al) and except for the cylinder lining, NO ferrous metal.

I think the piston may be toast. I'll post pics after I get it cleaned up, but it appears to have a small defect (damage) in the piston ring area where it originally let loose the missing ring parts. I need to closely inspect the cylinder wall in this area. I'm also gonna clean out the crank area just to make sure there are no pieces of the piston ring.

I'll check for Yamaha markings, but it may be necessary to replace it anyway, Slo.

Now where's Dave with info on the best (read cheapest) OEM parts source ... piston, piston rings, etc.?

Keep educating me guys. Maybe I can use my gained knowledge to help others out. Reminds me of the time I changed out the differential to a posi unit, and I needed all kinds of help (Pontiac forum) with setting the pinion backlash ... wow!

Ogre

[OgreTX]

Ok, guys next update. I don't think I have to worry about decarbonizing the piston. Unless you experts disagree, it looks like I need to just toss it. Pics are below:

1. Piston with no obvious severe scratches, but
2. Looking closer, it is damaged where both piston rings fit (note: with the arrow pointing forward, this is the side by the exhaust output in the cylinder)
3. Looking down the cylinder by the exhaust port shows no signs of scratches or damage. (note: feeling with finger does not feel like any thing is damaged)
4. Couldn't find any Yamaha marking, Slo. This is the cast markings on the side. Looks original.
5. Need help here. I am uncertain whether the cylinder liner has been replaced or not. This pic shows a "brazing" sort of finish. Never had a G1 two stroke apart so maybe this is factory work when installing the liner. Input?

I could probably take a small file (from one of my other hobbies) and clean up the piston ring slots. If it goes bad 3-6 months down the road, I will not be a happy camper. Is it fixable?

All else (needle bearing in crank rod, piston pin, circlips, etc.) looks good.

Bottom line to me is I need to do a piston/rings job w/gaskets, and crank seals as recommended by Marathon.

Ogre

[Field6565]

Throw that piston in the scrap pile. Get a new one, these motors are cheap to rebuild.

[1989Marathon]

Originally Posted by OgreTX

If it goes bad 3-6 months down the road, I will not be a happy camper.

With that statement alone, here is what I would replace.

1. Piston/Pin
2. Rings
3. Upper connecting rod bearing
4. Crank seals

The cylinder liner is virtually impossible to replace. The bronze color looks to be from improper oil burning onto the casting. Mic-up the bore before doing anything.

[smallblock450sl]

Do you have a machine shop locally that could bore the cylinder for an oversized piston and rings...at a fair price?

[OgreTX]

Thanks for all of the input guys. I agree that the piston is junk, so now I'm looking at piston, rings, seals, upper rod arm bearing. Easy enough to do, but not so easy $$$ ... especially at Christmas time.

Dave, you got me here:

Originally Posted by smallblock450sl

Do you have a machine shop locally that could bore the cylinder for an oversized piston and rings...at a fair price?

Can I assume that with all of your years of experience that you recommend going ahead and getting the cylinder bored and use a next size piston as shown on page 11 of the G1A Service Manual? (pic below, with red outline highlighting). If so, I don't really need to do the cylinder measurements. BTW, I did a cursory measurement yesterday in a couple of areas of the cylinder, and just initial look, the cylinder has very little wear.

The machine shop that did my Pontiac 350 heads (wow $$$) is gone, but there is another close by. I'll call them this morning and see if they can do the job for me, without having to take out a second mortgage on my house. I'll update this after I call.

Ogre