Piston

[robboadd]

Any ideas what may have caused my bike to seize? It seized about 500kms after the rebuild. The barrell was rebored not replaced. Also I lost a head gasket at around 200kms.

Picture of piston is attached at the bottom.

I have an idea but am not sure so would like others to use their wisdom.

Thanks Adam.

[satans_BIG_helper]

Lack of 2T oil possibly did you run the bike in ie run it gently for a few hundred miles or just thrash it straight off

Most likely lack of 2 stroke oil tho.... maybe the pump is not adjusted properly or its not been bled properly after the rebuild...
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(Yeah i forgot some too :S )

[robboadd]

Nope I ran it in properly only took it over 7500rpm a couple of times.

I thought it may have been because the piston was hitting the head gasket, I thought that due to it being rebored with the bigger than standard piston that the gasket needed to be bigger also.
Stan Stevens who did the rebore told me to put two bottom gaskets to prevent it knocking. As at the time I had nowhere to do the rebuild my garage did it and when I took the top end off there was no second bottom gasket.
To be honest I'm a bit confused

Thanks Adam

[alains]

the 2 strokes engines do not like a lean mixture . think about an air leak or underjetting
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[robboadd]

I'm still unsure as to it being lack of oil as I checked the plug and if anything it was too oily. I think the best thing is to have another look tomorrow at the oil pump. I don't want to rebuild the top end then have it seize again.

[Kickstart]

Hi

Firstly, thought the Sachs used a TZR engine with a plated barrel (so cannot be rebored).

However that looks like it is on its way to burning a hole in the piston which suggests detonation. Compression too high, crap fuel, wrong plug or ignition timing wrong (possibly made worse by running lean).

All the best

Keith
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[stinkwheel]

If you were told to put two base gaskets in a tuned engine and there was only one, the squish is probably way too small which could cause all manner of problems assosciated with overly high compression.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[bidman]

It also looks like the facing edge on that photo has been catching on something, going by the small chips on the edge of the piston, it may have been catching the lip of the smaller head gasket?
Dan
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[robboadd]

[Robby]

New barrel and piston, with the assorted gaskets, clips, and gudgeon pin.

Did you put it back together with the aid of a manual? I'd be doing it exactly as the manual says.

As for the two base gaskets, I assume there was a reason behind this - something done to the engine to warrant it, such as as having the head skimmed maybe.

That piston looks light has fairly lightly seized all the way round to me, so getting too hot too fast and/or insufficient lube. Make sure the oil pump is set correctly on the next rebuild.

[Kickstart]

[stinkwheel]

The two base gasket thing coming from a 2-stroke tuning company would suggest to me that the barrel has been altered to adjust the port timing and decrease the squish as a means to gain more power.

Either that or the barrel sealing surface was damaged and had to be skimmed.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[DucatiEVO]

[robboadd]

Well the only thing as far as I know stan stevens did was rebore the barrell, which meant an increase in capacity but only a tiny amount. When it seized the time before I thought it was just due to me not changing the top end quick enough, the piston was only scored on the outside. This was due to me just buying the bike and as the previous owner told me it had a new top end it wouldn't need one for a while.

The bike was defiently running too rich at some stage as my local garage adjusted the oil pump due to it fowling plugs after a while but this was before the first seizure.

It is really pissing me off I've had it since january and had it on the road for about 2 months. I really don't want to sell it to somebody and have it break down on them, I want to make sure they will get some use out of it. I need to sell it to buy a bigger bike and progress

Edit: When I get a new top end as it will be standard size will I need to worry, will it still suffer from 'detonation'? Do the slight differences in bore and how high the barrell is really make that much difference?

[Kickstart]

Hi

A normal rebore would only increase the capacity by a tiny amount (normal .25mm oversize piston would increase the capacity by about 1.1cc, a tiny amount. Unlikely to affect the compression.

However the TZR barrel cannot be rebored, so god knows what has been done to it. Possibly bored out to a fair bit larger capacity and then replated. You need to find out exactly what was done to it.

For a start, measure the diameter of the bore. Also check the top of the piston and see if you can read a size stamped on the piston crown.

All the best

Keith
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[Cog Head]

Looks to me that your out of Stella mate
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[smokie]

There is no sign of detonation on that pic its likely a lean mix from a bad airleak somewhere. Id look at the inlet manifold for a ripped gasket if thats ok then check the lefthand crankseal. Is the rear of the piston the same or not as bad? If its the same could have been a bit tight bored too. If its only bad on the exhaust side then Id say def a lean mixture problem.

[robboadd]

Just had a chance to measure the cyclinders' dyameter it is 6cm and the number on top of the piston is 2.00. Also the damage to the top was on the exhaust side.
I've attached a pic of the head because of the spark plug and the damage to the dome bit. I thought when a spark plug went like that it is piston slap?

Thanks Adam

[Kickstart]

Hi

Hell of a lot of pitting around the edge of the head. Looks like either something bouncing around there (eg, bit of circlip) or more likely detonation.

The 2 tends to suggest a 2mm oversize piston, or maybe 2nd oversize (which I would guess would be 0.5mm oversize) so either 58.5mm (or 57mm), but you have measured it at 60mm (and that would suggest 140cc or so).

All the best

Keith
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[smokie]

As there is no pop marks on the piston Id say this is detonation in the head but it was done previously and not from this siezure. Also the pop marks in the head are full of carbon so this to tells me its from an older blowup. If the piston does say 2.00 then its 2 mm oversize and therefore 58.4mm not 60mm. The next piston available is a 140cc from somewhere like NIKKON as I dont think there is an oversize over 2mm for a TZR engine.