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#Post#: 84018--------------------------------------------------
372,372 Bigbore piston swap/mod
By: trappermike Date: January 4, 2019, 11:49 am
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I came up with this mod back in 1998 for modded work saws or
race saws. It involves installing a different piston in the
372(Use Jons 670 piston) in 372 BB (use 272 Husky piston),then
the squish area on the top of the piston is simply machined
leaving a comp. dome, and the stock cyl.gasket and squish gap
are correct after. This mod is made to work with a
raised(modified) ex. port, or compression will be very high with
stock ex. port.
The biggest benefit of this mod is that the fuel charge in the
crankcase and under the piston will now have a FAR better path
into the transfer ports with the windowed piston! This you must
see with your own eyes- put a stock piston on the rod and rotate
crank to BDC with no cyl.on,at this point the upper transfer
ports in the cyl.are wide open,BUT the stock Husky full-skirt
piston with no side windows almost completely blocks off all of
the fuel charge under it and in the crankcase from escaping past
the piston to the transfer ports! I consider this a colossal
blunder in design,but with the windowed piston in place you can
see that the fuel charge in the crankcase has a direct fast shot
straight into the 372's big modern transfers.
This mod really adds some serious power to these saws,I have
done very many since '98.
Included is the machining specs,sorry for my crude diagram.
#Post#: 84019--------------------------------------------------
Re: 372,372 Bigbore piston swap/mod
By: trappermike Date: January 4, 2019, 12:08 pm
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Machining specs:
For some reason this thread says my diagram file is too big!? so
I can't post it...
So to machine the piston top you must simply-
Machine the squish area on top the piston 10mm wide,040"(1mm)
deep,then machine at 45 degree angle the small step where
machined squish meets unmachined dome. While the piston is
still spinning in the lathe smooth the entire piston crown
with,400,then 600 grit emery,then finish with metal polish and a
cloth,try to get a mirror finish,piston will run much cooler.
The intake skirt is narrower than stock,so do not widen the
intake port before you install this piston,then measure how much
is allowable.
This piston assembly is far lighter than stock,taper-boring the
wrist pin will lighten it further.
#Post#: 84021--------------------------------------------------
Re: 372,372 Bigbore piston swap/mod
By: Chainsawrepair Date: January 4, 2019, 5:02 pm
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Send it to me I will resize it to fit hopefully. Will PM you
email
#Post#: 84024--------------------------------------------------
Re: 372,372 Bigbore piston swap/mod
By: trappermike Date: January 4, 2019, 6:46 pm
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OK I sent it to you,please post it,crude as it is... thanks.
#Post#: 84030--------------------------------------------------
Re: 372,372 Bigbore piston swap/mod
By: Chainsawrepair Date: January 5, 2019, 7:21 am
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[quote author=trappermike link=topic=7141.msg84024#msg84024
date=1546649182]
OK I sent it to you,please post it,crude as it is... thanks.
[/quote]
Didnt need resized. I changed the pix from 90 to 72 and made it
bigger. I hope so can read easier.
#Post#: 84037--------------------------------------------------
Re: 372,372 Bigbore piston swap/mod
By: trappermike Date: January 5, 2019, 8:39 am
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I recommend for a simple exhaust port mod for work saws with the
piston mod is to raise the ex. port 2mm,then widen 1mm each
side,then carefully put a good chamfer on the ex. port
again,very important for longevity! This is not a lot for the
saw and compression will be in a more ideal range.
The intake skirt of the piston will be 1mm shorter than stock
when installed,which is a perfect increase in port timing for a
work saw. :)
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