Author Topic: 372,372 Bigbore piston swap/mod  (Read 43 times)

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Offline trappermike

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372,372 Bigbore piston swap/mod
« on: January 04, 2019, 12:49:58 pm »
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.
Experience builds real horsepower...
When the hammer drops the BS stops:Loggers sports competition!

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Offline trappermike

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Re: 372,372 Bigbore piston swap/mod
« Reply #1 on: January 04, 2019, 01:08:36 pm »
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.
Experience builds real horsepower...
When the hammer drops the BS stops:Loggers sports competition!
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Offline Cut4fun .

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Re: 372,372 Bigbore piston swap/mod
« Reply #2 on: January 04, 2019, 06:02:00 pm »
Send it to me I will resize it to fit hopefully. Will PM you email
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Offline trappermike

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Re: 372,372 Bigbore piston swap/mod
« Reply #3 on: January 04, 2019, 07:46:22 pm »
OK I sent it to you,please post it,crude as it is... thanks.
Experience builds real horsepower...
When the hammer drops the BS stops:Loggers sports competition!

Offline Cut4fun .

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Re: 372,372 Bigbore piston swap/mod
« Reply #4 on: January 05, 2019, 08:21:54 am »
OK I sent it to you,please post it,crude as it is... thanks.

Didnt need resized. I changed the pix from 90 to 72 and made it bigger. I hope so can read easier.

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Offline trappermike

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Re: 372,372 Bigbore piston swap/mod
« Reply #5 on: January 05, 2019, 09:39:05 am »
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. :)
Experience builds real horsepower...
When the hammer drops the BS stops:Loggers sports competition!

 

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