Two stroke experts seizure advice.

[stevo as b4]

Hi.

I finally stripped down the top end on my KMX today that's done 500miles from new. It had a cylinder that had been bored and fitted with a steel liner, and was running a new std size Japanese piston (TKR I think). However I noticed that the piston has had a very light seizure on 4corners, but otherwise it and the bore looked like new with both the rings intact and the end gaps well within spec too. The hone marks are all visible and you can't feel any scoring in the bore even though you can see where the piston has made contact.

Im not overly worried about it as it was running very well and felt like it had good compression too, despite there being slight signs of blowby past the rings.

Im just after some advice on what usually causes 4-point seizure of a piston and how to hopefully avoid it happening again?

I'm thinking it's most likely to be a clearance issue and maybe the cylinder wasn't quite bored to give enough clearance? I doubt it's lubrication as it was running about 36:1 and it was jetted very rich on the main jet, but maybe was a bit weak on the pilot jet as the pilot air screw had to be screwed 3/4turn in to get it to quickly drop to tickover when hot.

Im not using the linered cylinder again anyway as im going back to a Nicasil plated bore as it should be, and I'm fitting a forged piston not a cast piston this time.

Should I fit a bigger pilot jet and raise the needle to the top position for running in? I really baby'ed it running in last time and it also had a dyno run at 350miles.

Im tempted to fit my new flat slide carb now too, but I wanted to run it in for 100-200miles on the old carb first as the new carb has a power jet and I've got no idea of what jetting it'll want other than guessing.

[redeem ouzzer]

Piston clearance too tight or unsympathetic running in. I’ve recently stripped my X5 cause it was a bit noisy (but running fine for two years), found multiple seize points on both cylinders. It had a very gentle nip not long after rebuild but never did it again, I never thought anything more of it as my YR5 did the same on fresh bores and was fine. The newly rebored block is tight as hell so I have been taking extra care running it in.
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[stevo as b4]

Thanks Blue.

That was what I was thinking, as because it had a re-lined cylinder that would have had to be bored to suit the piston, it opens up the possibility of incorrect piston/bore clearance.

You can't really get it that wrong with new parts on a Nicasil plated cylinder, assuming the cylinder is new or re-plated to std dimensions.

As you know I run it on pre-mix, as the oil pump had been removed by the previous owner and I use just under 40:1 with fully synthetic oil. Its been up-jetted and on the dyno it was running very rich from 6000rpm up.

As for running in I thought I'd been gentle on it, it never went over 6000-6500 for 250miles, and not laboured. It felt like it had loosened up nicely by 300miles. And on the dyno the guy was gentle with it warming it up and doing several part throttle warm up runs before a power run. If he hadn't fitted the extractor over the exhaust it would have filled the shop with smoke as it was probably a bit clogged up.

I'll measure the new cylinder and piston before fitting to make sure they are both in spec, but as I'm fitting an aftermarket forged piston I expect it won't expand like a std cast piston would.

Im more annoyed with the old cylinder as I was hoping to sell it as a good used top end, but with 4 seizure marks I can't pass it off as a good cylinder to bolt straight on. The matching of the liner wasn't perfect either, so I'm hoping that's where I was down on power too.

[Tankie]

If you are or have used synthetic oil during the running in period then it's probably not run in because the synthetic oil lubrication qualities are that good the engine has not run in at all.

Change over to a mineral oil for a short period to allow the cyl and piston to lap in a bit before reverting back to synthetic

If your going back to a plated bore none of the above applies, this diagnosis is based on the fact that you've gone to a ferrous liner which is non standard
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[Sister Sledge]

It's been years since I had a 2T bike but some things are coming to mind.
The first being synthetic oil - back in the day I hated the stuff because it would gum up things. Rings would be free but slightly gummed into their grooves. Have synthetic oils moved on from those earlier days? Just a thought really!

How did you measure tolerances? Do you have full access to many types of micrometer? Have you measured the piston on every outer surface top to bottom and all the way around? Same with the bore??
The fact the piston has several points of contact tells me something is either off round (doubt it) or it's more likely something expanded in a particular place - ideas for that being surfaces between ports where there's less metal and it's more able to expand and slightly distort allowing it to squeeze in a little.
Obviously they all do that but if things are a little too close to each other anyway then the very small expansion of material is going to grab a moving part.

I always stayed with forged pistons. Cheaper ones would be damaged far more easily.
I always stayed with mineral oils but for me my bikes were tuned to the t*ts and rebuilds on the top ends was a monthly thing. These were bikes up to 125 cc and used for general road use and getting to work.

Is there any chance of getting a quick photo for us? I'm more nosy than anything!
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When I raced in the YPM, we had a batch of Mitaka pistons which were slightly over the stated size, making clearance a bit tight and leading to a lot of seizures. It seems Mitaka started making pistons in Japan and somewhere else, the dodgy batch came from their non Japanese factory.

You really could do with checking clearance accurately or maybe whilst it's in bits, but a better quality piston in, if the bore is definitely in spec. Someone like Grampian Engineering will inspect the bore size, correct if needed and supply a piston and ring kit for not a lot of money.

That'd be my guess anyway.
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[bikenut]

Post a picture of the barrel cylinder and ports as well as the piston and rings.

Its a 2t so parts of the cylinder wall will be "missing" as the missing bits form the ports.
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[Old Git Racing]

Non parallel bore, less at the bottom than the top.

OGR.
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[rs700]

four corner seizure is a sign of to quick warming up, unequal warming up. Did you thrash it from cold.
The other thing could be an air leak causing a lean mixture. Crank seals could be letting air in , or it could be whoever rebuilt the engine didnt seal all the leaking points (ie reed valve, power valve, crank to cylinder head bolts etc)

Modern 2 stroke oils are fantastic, so much better. Use a cheap oil for the first few ours to bed the rings, then use something like motul 710 or castor oil mixes like castrol 747.

Forged pistons are known to have 4 corner seizure because of the difference between cast barrels and forged pistons.

[steve the grease]

4 point seizure, absolute classic, happens in just the same places on pistons heavy enough to kill someone when they fall off a bench......

Its the gudgeon pin bosses, there is so much meat there that it expands at a different rate to the rest of the relatively thin walled piston.
A lot of pistons are made oval to compensate for this differential expansion , maybe this one wasn't made like that.....

[stevo as b4]

https://i.postimg.cc/fJVmJ6Nv/DSC-1512.jpg

https://i.postimg.cc/0M5Jx4nD/DSC-1511.jpg

https://i.postimg.cc/18xn4XM9/DSC-1517.jpg

Here's the pictures, they don't show up the seizure marks that well, but yeah it's a classic 4 corner seizure, inlet size worse than the hotter exhaust side.

Ive never had a bike show this kind of seizure before, so that's why I was curious as to what's causing it?

In my mind it had to be a clearance issue, or a lubrication issue. As I said it was run in on pre-mix at 36-40:1, and it was babied by myself, kept under 1/2 throttle and under 6000rpm for 250-300 miles, followed immediately by a dyno run.

The oil used was Silkolene Comp2 plus. Its a fully synthetic oil, but not a full on race or MX oil like Pro2 or MX2 etc. I changed the plug from a BR8 ES to a BR9 ES and I ran it on 97 Ron fuel.

Its all immaterial now as I've removed the nasty linered cylinder and I've already fitted my new Nicasil cylinder and forged Wossner piston. The seized piston is a cast TKR piston probably OEM or a pattern one.

Apart from the seizure marks there no visible wear or damage to the skirt and you can see the machined finish, and the rings are free in the lands and look like new. Therefore I don't think it overheated or was running weak? But my fueling was actually weak on the pilot jet as far as I can see from my dyno graph.

I don't buy the running it in on cheap oil, as race bikes and MX bikes don't use crap oil for running in, and they are run in after an hour or two from first start up generally.

Im changing loads of things all at once on the re-build and the compression is going to be a bit higher too, so I just want it not to happen again. My optimimism says they new parts are better clearanced and getting rid of the shitty liner will make everything ok?

[stevo as b4]

Oh and I never felt any sign of it seizing or any power loss either at any time which is puzzling. It also ran far better and wanted to rev out much more readily after the dyno run which seemed to clear all the crap out of it, and it's probably the first time the power valve opened up in 300miles judging by how much sticky goo they were covered in.

Gotta love two stroke power valves, they must from an engineers perspective the worst thing to have anywhere near a two stroke cylinder full of unburnt sticky oil and crap.

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[stevo as b4]

I'd agree with OGR's diagnosis too, as like you said all the scoring is at the lower part of the bore, and there's no carnage above the exhaust port at all like all the blown up strokers I've had experience of.

The inlet side of the piston is far worse and the corner scuffs in the bore are halfway across the auxiliary transfer windows, not at a port edge as I'd have expected.

I think I'll put it down to a shite re-bore the PO had done and try again. Regards to measuring the new piston I've checked it's diameter with a Micrometer matches the stated piston size, and I've gap checked my new rings in the bore at 0.2mm which is in the middle of the factory tolerance so thats all good.

I've got no equipment for measuring the diameter of the new Nicasil coated cylinder, so that's why I've just fitted it anyway and hopefully it's in tolerance from the plater too.

Got to put the head on this weekend, and having done a squish clearance check with an old head gasket fitted, I'm expecting squish to be about 0.93-0.95mm which sounds reasonable.

Will see how it goes. I've got a new flat slide carb that I want to put on with some baseline jetting taken from a 125cc bike that uses the same carb. Im upping the pilot and main jet slightly to account for the pre-mix fuel too. I'd use the old carb initially but the float bowl gasket keeps leaking and I think it's the carb body that's at fault.

[Tankie]

I'd use the old carb initially but the float bowl gasket keeps leaking and I think it's the carb body that's at fault.


Take the bowl off and check it is flat across the mating face, use a piece of glass or a good straight edge, if it is out, put very fine piece of wet and dry paper onto the glass and using figure of eight pattern flatten off the faces of the bowl
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