Weed Trimmer Cylinder Bore

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Weed Trimmer Cylinder Bore

Post by upupandaway » Mon Sep 19, 2016 11:09 pm

I have a 78 trimmer cyl that is scored from the owner running without an air filter.
Just for fun I wonder if anybody here has taken the cyl and bored these. stihl\echo have a piston 1mm and 2mm larger so maybe I could go the oversize route.
Being this motor is not like a 4 cycle with a solid cyl wall the full length, instead with notches on the side, Anyone try boring one of these?
I wonder if a 4jaw chuck lathe will work...

I know- "cheaper to buy one" if was still available. The ignition works so this is the only thing stopping it from running.

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Re: Weed Trimmer Cylinder Bore

Post by SteveM » Tue Sep 20, 2016 8:03 am

I think the deciding factor might be if oversize pistons are even available.

They are available for a 327 Chevy or a Model A engine because the cost of the piston and the work is less than the value of the engine. Not sure that is the case in your situation.

Of course, you could always MAKE a new piston.

I wonder if it would be possible to fill the scoring with some weld/braze material and then bore to the original size, or would you be looking at distorting the cylinder too much? Minor distortion might be tolerable, as you would make it round again when you bore, but everything will still need to bolt up.


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Re: Weed Trimmer Cylinder Bore

Post by Harold_V » Tue Sep 20, 2016 3:46 pm

upupandaway wrote: stihl\echo have a piston 1mm and 2mm larger so maybe I could go the oversize route.
Depends on the head of the piston, the distance from the head to the wrist pin, and the diameter of the wrist pin. They may, or may not, fit a different engine, depending on design.

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Re: Weed Trimmer Cylinder Bore

Post by Richard_W » Sat Sep 24, 2016 10:33 pm

You could always bore and sleeve it back to fit the old piston.

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Re: Weed Trimmer Cylinder Bore

Post by steamin10 » Wed Sep 28, 2016 3:43 pm

In my admitted limeted experience with these 2 cycle engines, they are finicky all around. Boring the sleeve with an interupted cut due to porting is a challenge I could not do. However a grinding setup, with a mini stone did it for me, A new piston was used as the score mark was around 3/8 wide, and deep. It was restored, and functioned well for many hours after.

If a small score is a problem on the piston, It can be heated and welded with common aluminum wire. starting at about 450 degrees will help keep the distortion at bay. Careful machining to remove the bulk of the weld material, and then smoothing the area with course emery will get you by. Smooth finish is not desirable, as some roughness will hold oil, and help the seal.
If you rework the bore, you must gently break the corners, so there is no snag on the rings during movement past the porting.

In water craft, we have redone engines with less success. After boring, they tend to run lean, and since you run hard on the throttle, they will either seize, or hole a piston from excessive heat. Usually this is a carb problem where build up will lean them out , and is the original problem to begin with, and re-appears when the engine is rehabbed.

Its fun to tinker with these things, as the cost on whackers is low, and the engines can be used in a lot of fun things. A freind collected a few Toro units, and built a C-47 model, with about a 5 foot wing span. The body and wings were ugly, made of 1/2 inch foil foam board. It was crashed regularly, but cheap to build new parts and silver tape together. No retracts, it was flown with wheels down. It had incresed dihedral, and was stable and fun to fly once trimmed out, it would fly itself commming back level.

Any way, they are fun to play with.
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Re: Weed Trimmer Cylinder Bore

Post by spro » Thu Sep 29, 2016 3:00 pm

I hear what you are saying about the same situation going back on a rehabbed engine. Stuff needs to be tuned up. I think we may be talking about the same engines as a Ryobi, some Deere Toro and others. I think they were IBC and very light per hp and were used for small aircraft also.
Minor story: I blew out the engine of my Ryobi some years ago. At that time, Stabil and a certain 2-cycle brand oil had a similar bottle with a ratio mixture section at the spout. At the same time, the labels floated off the bottles due to heat pouring in where they were stored. So I carefully measured the amount of oil to mix with the gasoline for correct ratio and the trimmer ran like a scalded buzzard.
Until it locked up. :) I bought a used short block at a small engine repair shop for $40.00 and it runs to this day :)

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