Needing competent machinist in Kampot...

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Freightdog
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Re: Needing competent machinist in Kampot...

Post by Freightdog »

You say it’s already been bored out, once, but in the earlier write up it sounded like the cylinder was the coated type, like Nikasil.

If It’s Nikasil, then any rebore will have taken that out. If it’s not Nikasil, but a plain steel bore, then surely an oversize bore, and matching piston and rings would suffice? Or even a decent honing and appropriate rings.
If it was all very tight before reassembly, chinesium or otherwise, that sounds like the rings are the wrong size. Even dry fit, with no lube, the piston would/should slide fairly freely.

As for decking the gasket face, why would he do that unless there’s significant damage? After reassembly of the cylinder and piston/rod, was the crank turned over to establish that all was right?

If the pistons are in question about their origin, then it could easily be that the gudgeon pin to crown height is wrong. It’s sounding like a mismatch of parts, as much as a bad machining job.

Not insurmountable, but you need to know what you’ve got, before any further fettling. Otherwise you might simply be scrapping one or two otherwise serviceable cylinders.


Just a few additional thoughts..
The cylinder has been sleeved? Has it subsequently been bored to match the piston size, or to a standard bore size with no oversize? Honed?
Are the rings going to be the same as for the original standard bore? I know on other engine types, the ring profile for honed steel vs coated bores can be different.
Ring gaps, and if the pistons are dubious, are the ring grooves allowing sufficient free movement?
SlowJoe
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Re: Needing competent machinist in Kampot...

Post by SlowJoe »

Freightdog wrote: Mon Jul 04, 2022 2:54 am You say it’s already been bored out, once, but in the earlier write up it sounded like the cylinder was the coated type, like Nikasil.
The cylinder I had work done on was bored out, but had no liner. I have three bad spare pistons (got in a pile of parts for free) and I did the "go no go testing" and found the 0.25 over piston fit "snug" (what you'd expect a piston to feel like when matched to a bore without rings), and the 0.00 piston slid straight through without resistance.

So the bore was likely 0.25 over but not recoated because if you tried a 0.00 piston in a 0.25 over it'd make an enormous racket.

Freightdog wrote: Mon Jul 04, 2022 2:54 am
If it was all very tight before reassembly, chinesium or otherwise, that sounds like the rings are the wrong size. Even dry fit, with no lube, the piston would/should slide fairly freely.
I meant "tight" as in tight for a piston. I test fit the piston first, and then do the rings to check. Usually a Chinesium piston you want to fit it somewhat "loose" because they expand so unevenly, but with an OEM you can run it easily to OEM tolerances and be fine. I've had Chinesium pistons in the past (on Honda motors) where we tried stock tolerances and compression and the motor didn't even last through break in period.

So at least for us, we'd fit the pistons "loose" and derate the power, usually with a cylinder spacer at the bottom (because it's cheaper than swapping in a new cam).

Freightdog wrote: Mon Jul 04, 2022 2:54 am
If the pistons are in question about their origin, then it could easily be that the gudgeon pin to crown height is wrong. It’s sounding like a mismatch of parts, as much as a bad machining job.

Not insurmountable, but you need to know what you’ve got, before any further fettling. Otherwise you might simply be scrapping one or two otherwise serviceable cylinders.
I can only say that after having added a significant spacer at the bottom, there shouldn't be much of an issue even if the crown is a "high compression" version but I'm still getting the severely worn skirts now. I pulled the cylinder once to add the spacer and the skirts were noticeably worn at the time, perhaps allowing the piston to rock back and forth in the bore and touch the head (which it looked to be doing, had a bright spot in the carbon build up).

When I added the spacer it quieted down, but only for a short time and the rattling came back, so I think I was only delaying the inevitable.

Freightdog wrote: Mon Jul 04, 2022 2:54 am Just a few additional thoughts..
The cylinder has been sleeved? Has it subsequently been bored to match the piston size, or to a standard bore size with no oversize? Honed?
Are the rings going to be the same as for the original standard bore? I know on other engine types, the ring profile for honed steel vs coated bores can be different.
Ring gaps, and if the pistons are dubious, are the ring grooves allowing sufficient free movement?
The cylinder has now been sleeved and that sleeve was bored to match the "original" piston (the one that was with it). Hone looked good, seemed like a very uniform finish so either he's really good with the hand hone or he had some kind of automated machine for the job.

Rings I would assume were matched...I don't have any of my tools but I could eyeball test the gap and see they were there gapped to some degree (scratches on the inner ring gap face).

Rings were nice and free in the piston's grooves (standard check in rebuilds, at least for me).
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pissontheroof
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Re: Needing competent machinist in Kampot...

Post by pissontheroof »

I don’t know if you got myface , neither do i , but saw this comment to you on facebook ….


Metalwork shop on the PP Road between the market and the Durian. Coming from the the Durian, its on the left hand side, maybe on the 2nd or 3rd block, no sign outside, beige brickwork

:hattip:
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SlowJoe
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Re: Needing competent machinist in Kampot...

Post by SlowJoe »

^^^Thank you

I don't have a Book with a Face on it either, but I'll take a look. Might be in need of some small structural work on a frame too.

I did go back and talked to the guy, just explained I wanted a recut but would do the work, and we worked out a deal where we're both happy. I think it all ended up being fair, and so we'll see how the rework ends up, but I'm happy so far with how he's dealt with me so I still have positive things to say about them.
SlowJoe
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Re: Needing competent machinist in Kampot...

Post by SlowJoe »

Rework went well, timely turnaround, new piston in and the motor feels so much better. Even though it's in the break in stage, it has a ton more power than it used to.

Rode around half an hour, bike easily made it to 70 kph without a struggle (before wouldn't pass 45), so I'm happy with the result, and happy with the shop too.

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Freightdog
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Re: Needing competent machinist in Kampot...

Post by Freightdog »

The engineer in me wants to see the before shots of the old piston. Morbid curiosity.

I miss being able to tinker with machines.
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Re: Needing competent machinist in Kampot...

Post by SlowJoe »

Intake side first, then exhaust. Not sure how well it conveys in the picture in terms of skirt wear, but it's deep. Not much gouging or galling though.

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