Thrust Faces

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davidblythe
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Sun Jan 11, 2009 4:34 pm

Can understand why the exhaust and inlet sides of the barell are the thrust faces, but can not get my head around which would be the primary thrust face when the piston is rising or falling

reason is am trying to work out if using a longer piston will help longevity of the barrell and piston once windows or a V Shape is cut into a piston for a reed valve

ie will a longer skirt on the exhaust side help

davidblythe
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Sun Jan 11, 2009 7:45 pm

someone must know, its driving me mad trying to think it out :oops:

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RinB
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Sun Jan 11, 2009 9:15 pm

David are sitting down m8

Ill try to explain

Right lets look at a lamies engine from the Flywheel side looking into engine, the crank rotates clockwise
So as you quite rightly say there are 2 thrusts, the major one is on the power stroke.

Basically the side of the bore that the rod is pushed into on the power stroke.

The power stroke being the stroke from piston tdc to exhaust open.

So on a lammie its the exhaust side.

Now the question you ask about longer skirt IMO I dont believe it will do much as the forces are around the horizontal of the gudgion pin and the angle there of. Also you are restricted by lenght of skirt when the piston reaches BDC etc

What helps is the Rod stroke ratio ;) But thats another thread

Smiffy
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davidblythe
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Sun Jan 11, 2009 9:26 pm

God im thick :?

but here goes, on the power stroke surely the rod is moving towards the inlet side

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RinB
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Sun Jan 11, 2009 9:32 pm

David

get a crank & point the flywheel side towards u
remember the flywheel spins clockwise.

So from TDC spin crank clockwise and the small end rolls over and the conrod points down to exh side.

Hope this explains ok

Smiffy

Now got image of David rumaging around like a headless chicken looking for cank :lol: :lol:
If its made of Metal Fettle it !!

davidblythe
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Sun Jan 11, 2009 9:39 pm

Got that :)

but here goes again and where i really started to confuse myself

as the conrod descends is the thrust on one side totally ie the exhaust side or does the piston tilt slightly so rings exhaust side and skirt on inlet

sorry to waste your evening smiffy explaining the basics to a numpty :lol:

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sean brady scooters
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Sun Jan 11, 2009 9:41 pm

smiffy is right dave.................
con rod on power stroke always points to major thrust face.........
your just looking at it the wrong way round..................following conrod from piston to crank ,instead of from crank to piston............................. :lol:
Sean Brady Scooters - 01765 690 698

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RinB
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Sun Jan 11, 2009 9:59 pm

Come on David u can get it :lol: :lol:
If its made of Metal Fettle it !!

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sean brady scooters
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Sun Jan 11, 2009 10:08 pm

basically dave.........a longer con rod will reduce side thrust..........major and minor......
and for eg ..........a short stroke/short con rod set up will suffer most..........

also i wonder why your interested in this question......and hope your concerns are not confused with excessive bore wear and piston rocking..........?
Sean Brady Scooters - 01765 690 698

davidblythe
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Sun Jan 11, 2009 10:14 pm

sean brady scooters wrote:basically dave.........a longer con rod will reduce side thrust..........major and minor......
and for eg ..........a short stroke/short con rod set up will suffer most..........

also i wonder why your interested in this question......and hope your concerns are not confused with excessive bore wear and piston rocking..........?
intellectual (or otherwise in my case) curiousity and trying to determine why so many barrells i have seen have heavy wear under the inlet port

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