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Old 05 July 2014, 15:07   #1
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2 stroke Crankshaft oil seal orientation.

Hi all.

Canít believe Iím asking this as Iíve built quite as few 2-stroke powerheads up in the past, but have been thinking about this so much Iím now doubting myself.

Iím in the middle of putting the younger Nashers DT15 back together from the pile of parts I brought and am about to drop the crank assembly into the block.
I donít have a proper manual, just a parts explosion that doesnít show enough detail to help.
I didnít take it apart either so didnít see the old ones in position.

Obviously normally with an oil seal the cup or open ĎUí side points towards the inside of engine where the additional pressure helps the seal open up against the shaft itís sealing.

With a 2-stroke it doesnít work that way when itís sucking in mixture, but on a 2-stroke bike youíd normally still point the open bit inwards.

Iím convinced that other 2-stroke outboards Iíve put together have the top seal with the ĎUí pointing inwards but the bottom one pointing out, ie with the powerhead in position both open cups point downwards.
Probably because the exhaust is at a higher pressure on the outside of the lower one.

Can anyone confirm Iím right?

Thanks

Nasher.
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Old 05 July 2014, 15:32   #2
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Old 05 July 2014, 16:17   #3
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Country: Other
Town: Principalite d'Chaos
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Length: 6m +
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Join Date: Jan 2004
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Thanks Yam Man.

However I've been outside for the last 1/2hr doing some investigation.

I realised the lips of the seal are offset, so I reassembled the old bearings onto the crankshaft with all the spacers then tried the bottom seal both ways round.
The witness marks on the shaft only line up when the seal has the open side pointing down, or to the outside of the engine.

Doing the same to the top seal shows it fits in the conventional way, or open side inwards.

It is normal practice to put seals in with the open side inwards, or as you say writing towards you, but it looks like my initial thoughts were correct and in the case of outboards they go in the other way round at the bottom.

Nasher.
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