OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Replace PISTON on my Shindaiwa trimmer?

Bad news. Took my Shindaiwa T230x apart and found this:
Exhaust Side of Piston

Intake Side of Piston

Exhaust Side of Cylinder

Exhaust Side of Piston

Piston was scored throughout. The cylinder was extremely smooth except for a single small spot where you could feel a little bit scratched if you touched with the fingernail.

QUESTIONS: Do I need to replace piston rings AND piston AND cylinder? Or can I just get away with buying piston rings? The trimmer works!! It just has trouble idling so you have to keep pulling the throttle to keep the engine running. Will it cause more damage in the long term if I don't replace the entire piston? I'd like to know your opinions...

That piston is significantly damaged - it looks as if someone has run it without oil in the petrol. There seem to be score marks running across the piston rings as well, but it's hard to tell from the photo. The score in the cylinder is probably more important though - piston scoring is most important when it reaches all the way to the skirt and the induction process is port controlled. If there is any scoring or roughness on the outside surface of the piston rings, that is a critical fault and they must be replaced.

It would be possible to remove the rings, polish the piston exterior with very fine wet-and-dry or something similar, fit new rings, and give it a try. How well it worked out would depend mostly on that cylinder score you mentioned. That will bypass the rings causing blow-by, and the blow-by will erode the new rings and ruin them much sooner than wear would have. So, how much do you intend to use this trimmer? If it just sits in the shed and you occasionally cut the edges at home, it may last several years. If you are a contractor, forget it.

Grumpy,

What I find interesting is the PATTERN of scoring/damage of the piston. On the intake side, you only see two major scoring streaks right along where the little pins that keep the rings from rotating, and the scoring goes right to the skirt of the piston. The center is pristine clear with no marks.

At the other end (exhaust side) however, it is scored throughout (the exhaust port had a bit of carbon deposits which might have fallen inside and caused damage).

The piston rings are definitely damaged on the exhaust side; I will replace them.

The cylinder score would better be described as a barely noticeable scratch/roughness felt with your fingernail. It does reach the bottom of the cylinder skirt. However, I don't see any blow-by below the top piston ring...

I use this trimmer 3 times a month for 15mins at home. So I guess I'll go with your advice and polish the piston, but I wonder what product should I polish it with?

Dario, I think a variety of things would work. I might spin it in the lathe and polish it with some extremely fine emery cloth, but I think wet-and-dry paper of 250 to 400 grit rating would work quite well for the small amount of metal removal required here. Remember, the idea is not to remove, or even reduce, the scores, but to remove only the raised-up parts, so what is left will bear properly on the bore.

Remember to check the piston ring locating pins - do not let them stand out past the aluminium surrounding them.

Valued advise. Thanks. I'll keep posted when I finish the job!

UPDATE: So after a few months I bought all I needed to repair the trimmer.

But I still have another question: Do I need to sand the cylinder as well or only the piston?

I will sand the piston with 400 grit wet sand paper and engine oil in a cross-hatch pattern. I will only sand/smooth the raised-up filaments like you said.

So would you reccommend to sand the cylinder too (it has very light scratches, the worst one you can barely feel with a fingernail but it does reach the cylinder skirt)?

There are two separate issues here: dealing with damage to the bore, and dealing with normal glazing of a well-used bore.

So far as the damage is concerned, the main problem is that it bypasses the piston rings, providing leakage paths for hot gas moving at supersonic speeds and carrying chunks of burning carbon with it. This is a recipe for eroding your new rings immediately, and ruining them in probably well under an hour's running. You need to completely eliminate vertical marks in the cylinder bore, by honing it properly. Proper honing does not make circumferential marks either. I suggest you look at these threads:

https://www.outdoorking-forum.com.au/forum/u...=5705&Words=honing&Search=true#Post23859

https://www.outdoorking-forum.com.au/forum/u...in=6860&Words=hone&Search=true#Post34409

The el cheapo hones you can buy for a few bucks are pretty much useless, they are better at scratching bores than at polishing out score marks. You might be able to get some help from a garage or mower shop with the right equipment. Remember, your repair can only work if you completely remove the longitudinal scores, but if you increase the bore diameter more than say two or three thousandths of an inch, it will be oversize which will result in a loose piston and an excessive piston ring gap. A loose piston in a port-controlled 2 stroke will result in poor performance.

On the subject of removing glaze from the bore, this is a controversial subject. Glazed bores can, under some circumstances, cause new rings to fail to bed in and seal against the cylinder, which results in after-overhaul blow-by that just gets worse with time. However most attempts to break the glaze in a bore do more harm than good. If I recall correctly Briggs and Stratton recommend that the glaze in aluminium bores should not be disturbed. From what I've been able to find, Honda makes no mention of removing glaze in their small engine workshop manuals or their Common Service Manual. Note that Honda small engines have steel cylinder liners, not cast iron. I do not normally break the glaze in engines I overhaul unless they are diesels. In the case of diesels, running the engines for long periods without load causes severe bore glazing, followed by blow-by and ring erosion. If the diesel engine has glazed bores prior to overhaul, glaze-breaking is essential. If it does not have glazed bores, I would probably not perform a glaze-breaking operation.

Grumpy,

I was hoping you did not suggest boring the cylinder.

I trust you when you say that most times it's just better to leave them alone beacuse it could cause more damage. Besides, spending time and money on that it's not worth it. If it fails I'll just buy a new piston and cylinder.

But thanks for all the info anyway.

Actually I started to put the trimmer back together but I'm stuck at installing the piston circlips. So I'll create another post and upload a video on that.

I'll keep updating here too though!

Dario, I've tried to explain how to install and remove the gudgeon pin circlips in your other thread. With regard to the bore of your trimmer, I do not think you can avoid doing something to polish it, because it is damaged. I do not advocate honing to remove glaze, but you have a scratched bore, which is a different situation.

Yes, As grumpy said...

When you are installing new circlips and or rings on a scored cylinder you MUST do something to remove the scoring or you will destroy you're new parts.....



Kori

In another thread I talked about what I ended up doing with this scored cylinder and piston. I will summarize it in this thread as well:

I sanded both the cylinder and piston with 400 grit wet and dry sand paper and 2-stroke oil. I did this very thinly, just smoothing out the raised peaks only. I did this in a cross-hatch pattern.

I also applied muriatic (hydrochloric) acid to the cylinder with the idea of removing the aluminium transfer but in my opinion it was not necessary in this case- just the sanding was enough. I also replaced the piston rings.

I did a lot of research online about doing this (sanding + acid) to scored pistons and cylinders and it's very common practice. A lot of people end up salvaging cylinders/pistons this way. If the scoring is not too deep then it's most likely going to work as long as you don't sand off too much surface.

I reassembled the engine and also replaced the clutch (which was seized) and the machine now works very well. It starts right away and doesn't have idling problems

Since this thread is complete I will close it. If anyone needs to post to it, just PM a moderator.