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Dec 01, 2021, 05:45 AM
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Discussion

engine rebuild, modifications sub-project.


Toughening up the piston a bit.

- My Torqpro 61cc two-stroke.
It has slightly overheated and damaged the piston and ring- It doesn't like running over 7500 rpm!

I have sanded back the piston and ring, and the piston surface now seems very soft.
It had some partial seize marks, and was wearing off-center- slightly oval bore? - it looks about 0.1mm out-of-round
when I use a vernier caliper.
I need to settle the ring and piston back in.

The thrust side of the piston and barrel is wearing OK.

The barrel looks pretty OK, apart from a worn spot down at the front, where the piston was wearing as well.

I have burnished in some pencil lead into the piston skirt, with the back of a sandpaper sheet, to reduce initial seizing tendencies.
This seems to toughen up the surface a bit, too.

Can I dissolve a little Tamiya moly grease in the oil for the first tank? - about 6 minutes running.
This is a sort of runny anti-seize compound, in a tube. You can use it on bushes as well.

I will bump the oil mix up to 50 ml/lt at well (20:1)
Recommended is 25:1
If I use Stihl Ultra HP, does that have a different mix ratio?

My present oil is Gulf Western, but I have the Stihl oil on order.

What else can I do? - the engine air-set moly spray from Dupont is a bit expensive here.
What run-in routine should I use?

It will be bench run in.
For the first tank, I can idle for a minute, run up to 5000, then gradually run back to off-idle, then finish the tank.

I have a 23x10 prop now, so that should load it down to the correct rpm.

How many runs do I need before I go full power with this prop?

It should run between 6,000 and 6500 rpm when run in.
Last edited by Owen_bern; Dec 27, 2021 at 10:38 PM.
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Dec 01, 2021, 12:03 PM
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turbonut's Avatar
Get new parts and start over...That much damage will not be going away..it will run but never as good as it should
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Dec 01, 2021, 01:55 PM
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Just because the piston is not round does not mean it is faulty or the bore oval. Modern pistons are manufactured with an oval profile on purpose.
Last edited by tubalan; Dec 01, 2021 at 06:09 PM. Reason: comprehension corrected
Dec 01, 2021, 02:32 PM
No more wasted spark 😉
ClubFlyer's Avatar
I once had an engine with a square piston in a round cylinder bore. I almost got it started, but the blow-by was horrible. Had to spin it rather fast though, and hand starting was almost impossible.
Dec 01, 2021, 02:47 PM
Registered User
if you can get over 6K rpm with a 23x10 prop on your cc and have it idle and transition reliably, your golden IMO...

since you altered the internals, id run it minimum 30:1...maybe even fatter, but id hate to give you bad advice
Dec 01, 2021, 03:51 PM
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1QwkSport2.5r's Avatar
...
Last edited by 1QwkSport2.5r; Yesterday at 06:45 PM.
Dec 01, 2021, 04:29 PM
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Quote:
Originally Posted by 1QwkSport2.5r
Or better yet - get the right engine to do the job in the first place. Less fuss and much more worry free IMVHO.
1) The right engine would be a rotary/wankel.- not available.
2) A nice Saito 75cc or so 4-stroke radial triple.
A little overweight at 2500g base weight, and expensive.

3) I have sanded and re-assembled RD 350 Yamaha bike pistons before.
It was slightly more rattly, but It ran well and lasted for ages.

4) I am just finessing the piston a little. Rotary burnish with leather would be nice.
I will try Dremel buffing with graphite- It can't hurt, plus assemble the piston into the bore with some Tamiya moly "grease".
It is not a real grease, but it has a lot of moly disulfide in it. It shouldn't be to sticky to start.
I could also spin it up with the plug out with a drill, and squirt a little premix in the carb.
That may help bed the sanded compression ring.

- the Yamaha was different in that it had new chrome-faced rings, and honed iron liners.
Dec 01, 2021, 04:52 PM
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turbonut's Avatar
The real question is do you just want it to run or run well and make good power..
You ask for lots of help and have lots of questions but keep giving your own answers.
Don't get me wrong..I am all for making things work that most would toss in the bin..But I am fully aware of the application I plan to use them on or in
Last edited by turbonut; Dec 01, 2021 at 05:16 PM.
Dec 01, 2021, 04:54 PM
Still gassin' it.
Quote:
Originally Posted by Owen_bern
1) The right engine would be a rotary/wankel.- not available.
Yes they are... There were a number of wankel powered utilities on the market, like chainsaws and such.

All you got to do is find one, and tear out the engine and convert it.

https://www.bing.com/videos/search?q...%26FORM%3DVDRE
Dec 01, 2021, 05:17 PM
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turbonut's Avatar

Yes Some do


Quote:
Originally Posted by tubalan
Just because the piston is not round does not mean it is faulty or the bore is oval. Modern pistons are manufactured with an oval profile on purpose.
But not all and they dont do oval cylinders to match
Latest blog entry: In flight
Dec 01, 2021, 05:32 PM
Still gassin' it.
Honda did...
Dec 01, 2021, 06:05 PM
Registered User
Quote:
Originally Posted by turbonut
But not all and they dont do oval cylinders to match
Never meant to say the bore might be oval to match have corrected my post.
Last edited by tubalan; Dec 01, 2021 at 06:16 PM.
Dec 01, 2021, 06:09 PM
Registered User
Quote:
Originally Posted by tubalan
Never said the bore might be oval to match.
You didn't, and you seem to be saying it hasn't been done, even with Bert's hint. Honda did.
Dec 01, 2021, 06:23 PM
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Gary Cee's Avatar
Yessir
The rod twister, sudden disintegrators of the late 1970s


I understood that he was referring to the very well used practice of fitting cam ground, intentionally oval pistons into a round bore.
Latest blog entry: Responsible fliers.
Dec 01, 2021, 06:25 PM
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Quote:
Originally Posted by Gary Cee
Yessir
The rod twister, sudden disintegrators of the late 1970s
1) foreshadowing
2) it still happened.


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