2 stroke crank seals.

[stevo as b4]

I had a quick look over my KMX200 engine yesterday in the garage, just to see how much work is required, and how badly the bore was damaged that need's welding+replating.

Now i took the engine out and stripped the top end over 4years ago now, before boxing/wrapping everything up in bags and putting them in a plastic box. I was suprised to find how easily the crank turned and i barely felt any play in the con rod either, so i thought bonus! Just the mains and oil seals might need replacing.

But after 4-5revolutions of the crankshaft, it suddenly got saturated in black oil, which was the gearbox oil that i'd left in there 4+years ago. I guess turning the crank must have disturbed the perished oil seals and allow the gearbox oil to come flooding into the crankcase.

I know i left the engine with the top end off for 4 maybe nearly 5years which is a bit of a fail i agree, but it's finally proved to me that 2stroke crank seals don't like long periods of inactivity and will be ruined if a motor is stood still, especially with parts removed for years.

I was skeptical before TBH, and thought it was a bit of a myth that all 2strokes will be knackered if they are sat still for years without sealing all the inlet and exhaust ports airtight, but now i'll always treat long standing 2stroke engines as (in need of a re-build) before trying to start them.

I did think that one possible way to protect from and prevent crank oil seal failure, could be to pour in clean 2stroke or engine oil before storage, so that the whole crank case is full of oil, which should stop any degredation of the crank seals?

[_Iain_]

Workshop
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Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[HD]

Are you asking a question or just thinking out loud? Either way, its good as I never knew that
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Rusty '02 Vito Camper + CBR600F3

[_Iain_]

Tis. Might pay just to chuck the whole engine minus electronics in a bucket of cheap 10/40? At least that way the seals wouldnt perish from the outside in if you see what i'm on about?
____________________
Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[Pete.]

Seals exposed to moisture can weld themselves to a bearing surface (i.e. the crank journal) and when you turn it the lip is torn off, leaving a thin black line of rubber on the journal. It seems to be peculiar to 2-strokes though, because there are millions of cars out there which have the crank/gearbox/diff seals exposed 24/7 without spewing their guts. Maybe it's the vapourised petrol in the crank cases that affects them.

In any case, filling the engine with oil for long-term storage is a well-known trick for preventing the seal from perishing.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Im-a-Ridah]

Replacing said seals is a massive pain thats for sure.

[stevo as b4]

Cheers Pete.
I though that filling the crank case with oil on any nice stroker that you want to store long term would be a good way to go.

I'm going to send away my crank for a full re-build, seeing as the rest of the engine needs so much work too. I suppose i best replace every bearing and oil seal in the gearbox as well to do a proper job.

I have another question for you though if you don't mind?
What do you think to having the damaged bore bored out 2/2.5mm before it's re-plated? Im almost certain that re-boring by this much would go way past the damage that the broken piston rings caused.

I was thinking it might be as cheap as getting the bore welded up, and i can get an oversize piston up to +2.5mm. My only worry is would the head gasket be likely to cope ok with a 2.5mm bigger piston, or would it need a custom head gasket, as i presume trying to trim a head gasket is a massive fail and wouldn't work?

[Blue_SV650S]

Seals do go off over time, rubber perishes AND hardens with age, simples. But I do suspect your problem was indeed exacerbated where the seal was not kept 'wet' both sides.
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The purpose of life is to fight maturity.

[Compo]

stevo just rebuilt a kmx200 engine fully every single bearing and seal. I used koyo stuff. I also rebuilt a kmx125 bottom end too but that did not need a replate or crank build just bearings and seals.

The 200 did need a replate as it had a score which went all the way through the transfer ports about a mil deep, when i sent this in to langcroft they just welded line bored and replated . The welding cost £30

Can you get a piston 2.5mm larger ? seen them upto + 2mm for use with steel liners .

As for the seals mate they fail because they go dry unlike in 4 stokes where the seals are in a bath of oil in a bath of oil, the fill the crank with oil is what ive done with the KMX200 engine while i wait for my lad to gain a full license on the stator plate side where the seal does not sit in oil i oiled it then seals the oil onto it with grease. Best i could think to do.

[Vincent]