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 another dumb question

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mikerwt



Number of posts : 13
Age : 67
Localisation : bristol
Registration date : 2015-08-28

PostSubject: another dumb question   Mon Sep 14, 2015 6:26 pm

with piston at BDC should the inlet port be open exposing piston crown? Is this position what you call draw down?It is only exposed for 5/10 degrees/ or is this port window too high? Any pointers great fully received thank you. Mike
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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Re: another dumb question   Mon Sep 14, 2015 7:19 pm

Mike,
Absolutely not!
Get some quality epoxy filler, score the surface well for an effective key, de-grease and fill it in. Short term fix but learn using this barrel and then go for a new one! There can`t be many Bantam race engines that don`t have filler in them somewhere so you are in good company?

Mike, there are no dumb questions, just questions, and we all ask those.

Cheers, Trevor
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john bass

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Number of posts : 1712
Age : 88
Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: dumb observation   Tue Sep 15, 2015 10:03 pm

This is a dumb observation --

-- because at my unnecessary and boring longetivity, why should I need to know anything about tuning a Bantam engine?? Too late -- they cried!!

OAPs at my stage of fading-away should shut-up and curl-up but I cannot when popping in onto this site.

I left it to Derek Neil to do Icarus-1 engine and I made a slap-happy job of Icarus-2, although the Icarus-2 motor did rev on quite a bit yet I never did anything clever with my so-called Tuning certainly too windy to try Devcon -- or whatever -- filling.

-- what about a picture Trevor? haven´t you already got some with young Edward? Like a picture -- or two -- would show exactly where and how the filling is done?

Cheers!

John-Boy .... with wheat-stalk sticking out of the mouth and yokel-cap sitting upon the head.....
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dansofield550

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Number of posts : 362
Localisation : gravesend kent
Registration date : 2011-10-23

PostSubject: Re: another dumb question   Wed Sep 16, 2015 5:41 am

Not too sure epoxy or devcon type fillers with be of any use if the hole expose's the crown as that area will be subject to high heat and pressure, those type of repairs work well in transfers and inlet under the ring area,
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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Re: another dumb question   Wed Sep 16, 2015 9:19 am

At bdc with the exhaust and transfers fully open and the exhaust pipe diffuser doing it`s job there will be virtually no cylinder pressure. Heat won`t be a problem as the coolest the piston crown will be is also at bdc; and it will be additionally cooled by the incoming transfer gas streams. Piston temps continue to fall by conduction and radiation until commencement of the next combustion event. Further, the soon to be flowing, cold inlet charge will cool the piston skirt, underside and the port/duct filler simultaneously. All together not a significant problem then, at least it isn`t in my water cooled engine?

I know of numerous race engines that have had Devcon, and similar fillers, moulded into transfer ducts for timing and directional control of mixture flow with little ill effect and with reliable longevity.Transfer ducts are subject to much higher thermal loading than an inlet could ever be by virtue of their proximity to the exhaust port and flow reversal of hot exhaust gas!


Trevor
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dansofield550

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Number of posts : 362
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Registration date : 2011-10-23

PostSubject: Re: another dumb question   Wed Sep 16, 2015 8:39 pm

hummm , not convinced on that one, id be unable to stop myself from constantly checking if its still there and not fallen out /in or under something, Exclamation ,

mike, whats your goals with the bike? something to build for looking at or hoping to ride it at a track or parade?
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john bass

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Number of posts : 1712
Age : 88
Localisation : Bensberg, Germany
Registration date : 2006-12-06

PostSubject: Draw down...   Thu Sep 17, 2015 5:03 am

There was no answere to Mike´s query on "Draw Down"  -- maybe what was meant was "Blow-Down" --

and that´s the degrees of difference between transfers opening and exhaust opening---


--- AIN`t IT...???

Schnarsch!
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Trevor Amos



Number of posts : 860
Registration date : 2010-08-13

PostSubject: Re: another dumb question   Thu Sep 17, 2015 6:45 am

Like you John, I am not familiar with "draw-down", at least within the context of a function within a two stroke race engine; could be that blow-down was intended and your definition is spot on?
Mike, can you maybe expand on this a bit to help clarify things to enable a more accurate answer!

Trevor
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john bass

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Age : 88
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PostSubject: Dumb Obs again   Fri Sep 18, 2015 4:06 am

Dumb observation again -- Mike said at BDC,  5° to 10° [of crank rotation (?)]  of piston crown exposed which had me thinking -- wrong barrel, or wrong crankcase ... and as Trevor said that bit of the port  could be blocked off and the port opened up at the bottom --

-- could it not?

-- or machine the crankcase cylinder-base area until the the piston crown can no longer be seen thro´the inlet port -- from calculations, long-long ago there´s only a small amount of piston movement per degree of crank rotation at TDC and BDC... at 5° it is only in thous and at 10° still quite a small amount.....

Is that observation  with a gasket on the barrel base?

... in any case, Mike  it does seem a bit odd and not quite right....

In any case Mike!  you woke up some of my ancient grey-matter -- brain, I think it is called, thank-you!

Cheers!

JayBee-- forJohn-Boy.
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