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OUPower.com • View topic - Polishing pistons

Polishing pistons

Do you have a project you're working on that doesn't fit into any of the forums above? Please post about it here.

Polishing pistons

Postby Danberman » Wed Aug 17, 2005 12:53 am

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Postby pertyfly » Thu Aug 18, 2005 6:41 pm

Is it definitely low on compression? Have you done a compression test or anything? I used to do this all the time with old lawn mowers and things. I do work on small engines as a hobby. Why would you want to polish the piston? Is it scored or anything? If it's low on compression, it would be a ring issue, not a piston. As long as it is physically, not scored or damamed then you could probably get away with rings.

Have you taken it aparts? If you haven't, I have had much success in taking them apart, and freeing the rings, as there are often seized after sitting for a long time. Then I make sure everything is really clean, and free of gum, and sludge. It usually works fine. Mine want to hone the cylinders a bit, as well.

Also, for a small engine, I have actually "deglazed" the cylinder with some extra fine wet sand paper and oil, and making a cross hatch pattern. Just don't work in one spot too long, and it's not a problem. It helps the rings "reseat" nicely. (I know this is a "no no" but it's not a race car, just a small engine) I have had quite a few lawn mowers (and actually a couple dirtbikes as well) work perfectly fine for years with this. I am still using my main dirtbike, and have been for years. It was seized when I got it, and I did this (along with some other things, of course) and it's been working for about 6 or 7 years now, racing and all.

Just my advice :)
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Postby Dart » Fri Aug 19, 2005 7:54 am

Hi Danberman.

Here in Denmark you can buy a piston polishing set.
It is made of a round wodden stick, with a rubber suction cup in each end, to diferent sizes. In addition you buy a can of grinding pasta. The grinding pasta is made off carborundum in two diferent sizes and oil.

If you dispart pistons and the top pice, you put a small amount of pasta betveen the piston and the ring in top pice.
Next you put the suction cup on the piston and rotate the wodden stick betveen your hands 5 times, lift piston, rotate again, lift piston, rotate again etc. etc. until it is tight.

I am sure that you kan find the same in a store for automotive spare parts in your country.

Regards Steen
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Postby Dart » Fri Aug 19, 2005 8:19 am

Hi Danberman

I answerd the wrong question. Sorry.

It is always possible to pollish a piston on a rotating cloth(mashine), but it could be the piston rings.

If you take one of the piston rings of, and place it in the cylinder, you kan messure the gap in the ring.

If it is to big, you can try to messure the gap on a new standard ring.
If the gap still is to big, you'l have to find some oversize rings, or you will have to drill the cylinder and a new size of piston.

What about bearings, are they ok?

Regards Steen
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Postby Dart » Fri Aug 19, 2005 8:24 am

:oops:
My first answer was about valve's, not pistons.

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Postby pertyfly » Fri Aug 19, 2005 3:58 pm

Polishing the pistons in a small engine like this, as far as I'm conerned (just my opinion), is pretty muh totally useless. Obviously, as I said, if you find some flaws or something in it, it should be taken out (like light scoring or something) Again, I use an extremely fine grit sand paper for this, as it is not precision in a small engine. However, you will notice no difference at all by polishing a piston in a small engine just because. It won't increase power or help efficiency or anything (based on practical experience). Let me know what you are trying to accomplish by polishing
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so what do i use

Postby Danberman » Fri Aug 19, 2005 7:35 pm

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Postby pertyfly » Fri Aug 19, 2005 7:49 pm

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Compression

Postby OgreOwner » Wed Dec 28, 2005 1:43 pm

Check your local schools to see if they have a night class (cheap) in small engine repair.

pardon me if this discussion offends, but the thread so far is not understandable to me.

There are two types of pistons - Big and tiny.

Big pistons seal the cylinder to give compression using rings. There are grooves in the piston that the rings sit inside. They press outward against the cylinder wall. They are what actually seal the cylinder so that you can have compression in a small engine. Rings do not totally close off the cylinder - each one must have what is called a ring gap. This gap is the space between the ends fo the material when it is inserted into the cylinder without the piston. Gas can escape through the ring gap, so we use multiple rings and space the gaps during assembly. The higher the compression you want, the moe rings you have to use. Since the time you need compression in an engine is so short, this arraingement works well. If you need to produce the compression, then hold it, this will not work since the gas will slowly seep out of the ring gaps no matter how many there are.

In a "tiny" engine where there are no rings, the procedure of polishing the cyinder and piston to each ohter is called lapping. There are instructions how to do this on sites devoted to the stirling engine. The objective is a piston that seals perfectly in the cylinder. It takes time and patience, but can be achieved. The power piston of a stirlig engine must seal perfectly.

Before you start working on engines, you really should understand how they work. There are three types that you should spend some time to understand. First is the standard internal combustion engine, next is the diesel, then comes the stirling. The hardest to understand is the Stirling - since it is actually two cylinders - one hot all the time and one cold all the time. No overunity there. but getting one producing power off of sunlight is fun.

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weed wacker piston

Postby spinning-magnets » Wed Jun 20, 2007 7:52 pm

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Postby AlaskaStar » Thu Jun 21, 2007 4:28 am

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reviving discarded small engines

Postby spinning-magnets » Fri Jun 29, 2007 1:38 pm

If you start looking, its not hard to find discarded lawn mowers, microwaves, washing machine motors, etc. Sometimes the old owner just wanted a new one. Once I took a microwave from the top of a trash can and took it to work for the lunch room, looked old but worked fine.

You'd be suprised how many 3 year old lawn mowers are discarded because they "just dont start as easy, or cut as good as it used to". Forget about cleaning the air filter, which when clogged makes the mower run rich, gunking up the spark plug tip. Or sharpening the blade. When they start cold and run immediately at high RPM it wears out the cheap rings fast, leading to low compression, then they are so cheap, people just buy a whole new one. Few people would rebuild a mower engine, or pay to have it done either.

Buy a new air filter of the proper type (the precise amount of air resistance affects how rich/lean the A/F will be that the simple carb provides), clean the spark plug tip, give the intake a tiny spritz of Ether, and 90% of them will start right up. The mower blade acts as the flywheel, so if you have a mower engine on your work bench you may need to attach a wooden disc or some similar device to get it to start and keep running. I've seen a mower engine on a cart turning two alternators that worked as a welder/generator. A heavier flywheel would allow it to run at a lower RPM (after modifying the carb), but then it might last longer, and you wouldnt buy as many.

If you have to go so far as to rough hone the cylinder and install new rings to restore compression, make sure the rings are broken in with non-detergent 30 wt oil. Once it has enough running time to fully seat the rings onto the cylinder walls, and is running smoothly, switch to the appropriate grade of synthetic oil so the rings will last longer from the abuse of revving at full RPM when cold.

Meybe not so important with free mower engines, but useful for any engine you want to last. -Ron
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