Thrust Faces

Anything related to Lambrettas... ask tech questions, post helpful info, or just read and learn.
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sean brady scooters
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Sun Jan 11, 2009 11:39 pm

show me some pics dave please..................just not quite sure what you mean........
and after all a picture is worth a thousand words............... :D
Sean Brady Scooters - 01765 690 698

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RinB
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Sun Jan 11, 2009 11:46 pm

Sean

I think he is saying not to put windows in piston but from bottom of inlet skirt cut up to the rings
basically the width of the inlet port and up until the skirt is open into the port @ BDC
But the thing is how far up to the rings do u go.

David I would cut a reverse U not V and I would maybe cut up so that the area open @ BDC is the same as would be on a ts1 window piston at BDC it a start point

I think it would have to be trial and error
If its made of Metal Fettle it !!

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sean brady scooters
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Sun Jan 11, 2009 11:57 pm

lets just wait for the pics............... :D
i,ve a feeling that maybe dave wants on one hand max power ..
but on the other total reliability............. :shock:
sounds good of course ,but in reality is really not poss.........as we know.
if dave is building his own "yob engine" then ultimately dave himself can only make the decisions as regards the compromise between power/performance............and reliability.
dont get me wrong here dave..........i think this is very interesting and a great topic.......as i,m sure smiffy does ,which is why we are trying our best and spending our time with you here........... :D
Sean Brady Scooters - 01765 690 698

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jonny snatchsniffer
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Mon Jan 12, 2009 4:29 pm

i know this is completely different and in the wrong section, it was a polini 130, but one year an hour before i was due to set off to margate my piston skirt decided to disinergrate luckily only at tick over, and there was nothing left on either side below the gudgeon pin, now i got the bits of ally out the crankcase and saw that there was no damage elsewhere so put it back together and went regardless 2 up on a smallie and the funny thing was it ran better, i notice that 4 stroke pistons are very short so why not on a reed motor can they not be short and have less thrust face ? is it excessive wear ?

156 D
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Mon Jan 12, 2009 5:35 pm

Here i go now!........all bore`s whether 2-stroke or 4-stroke wear oval....that is why Dave you see wear on the inlet side as well as the exhaust side....but the power stroke side(exhaust) will be more worn.......
Nostalgia is a thing of the past!....

156 D
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Mon Jan 12, 2009 6:28 pm

Just been reading my old engine books......the longer the piston skirt the less cylinder wear or the shorter the piston = more cylinder wear....as "RinB" said the downstroke (clockwise rotation of crank) exhaust side wears more as it`s the power stroke.....but the inlet side wears on the upstroke due to the continual crankshaft rotation of the crankpin on that particular side......On a 4- stroke the ovality of wear is less due to each cylinder having less power strokes in a cycle, so therefore a four stroke in theory will last miles longer....
Nostalgia is a thing of the past!....

156 D
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Mon Jan 12, 2009 7:20 pm

davidblythe wrote:
have seen pistons that have been cut as a reverse V instead of having windows
Whichever way you decide to go.....if you choose the "V" cut, you should have a small drilled hole at the apex of the two cuts....otherwise a split will definately occur....this is where "newschool" joins "oldschool"....Hope this helps?..
Nostalgia is a thing of the past!....

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RinB
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Mon Jan 12, 2009 7:29 pm

156 D wrote:
davidblythe wrote:
have seen pistons that have been cut as a reverse V instead of having windows
Whichever way you decide to go.....if you choose the "V" cut, you should have a small drilled hole at the apex of the two cuts....otherwise a split will definately occur....this is where "newschool" joins "oldschool"....Hope this helps?..
That was my thoughts as well the apex is a major week point
If its made of Metal Fettle it !!

davidblythe
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Mon Jan 12, 2009 7:32 pm

ta for that 156D

Re the ref to old engine books any isbn No's

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sean brady scooters
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Tue Jan 13, 2009 11:50 pm

just to add as a matter of interest .................
vespas differ to lambos in respect of major thrust face....................
on a lambo ......major thrust face is exhaust side............minor thrust is inlet.
on a vespa ......major thrust face is inlet side...............minor is ex............
Sean Brady Scooters - 01765 690 698

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