Mac engine info!!

mransleben

Member
Joined
Sep 24, 2011
Messages
128
Location
Millican texas
Aircraft
Bensen
Total Flight Time
3000 hours fixed wing and helo
I have a Mac 72 model number 4318-A I had a Mac attack Sept 3rd, lost number 2 piston and cyl. I have acquired another engine for parts, 4318-G model number. I have found the pistons and rings are different between them. The A model has wider rings that are positioned lower on the piston that the G model piston, on the G model the rings are thinner and positioned closer to the piston top. ( more like in used to seeing in automotive engines). Now the question is will the G model piston and jug work to replace the one I lost on my A model engine, if so will that cyl perform as well as the other three or will it cause an imbalance in the engine. I,m a master ford tech for 42 years but I have never worked on a Mac engine and I know very little about them. I have tried to contact a person who is suppose to be a Mac Guru but cant get a call back. Any help or advise would be greatly appreciated!!!! If you want to call me or email response's 979-229-7034 [email protected]
 

Rick Whittridge

Gold Member
Joined
Dec 30, 2003
Messages
1,762
Location
Ohio
Aircraft
Bensen B8-M / Twinstarr
Mac Info

Mac Info

The A model is 72 HP The G model is 90 HP You can`t interchange one 90 HP cly & piston onto a 72 HP with out doing all the cyl`s (port timing is different) If all the 90 HP cly`s are good shape then use them all on your crankcase. Increase carb jet for more fuel & set timing at .100 BTDC
 

Gyro28866

David McCutchen
Joined
May 8, 2009
Messages
1,881
Location
Dickson, Tn. USA
Aircraft
Benson B7m & B8m, Dominator II; Kolb Mark III Classic
Total Flight Time
750 combined FW & Gyro
Question #1:
What caused it to melt #2?
A leaned air fuel mixture?
The #2 jug is the most common one for a lean mixture. I would be looking for the cause of the lean mixture. crankcase end seals, cylinder base gasket, carburetor base gasket, etc.
or did you change the prop to a less pitched one and the increase rpm cause the lean.
Just thinking outloud here; but I would definitely diagnose the cause.
or IT WILL HAPPEN AGAIN!!!
.
pay attention to the wrist pin when you reinstall. The capped end always faces the exhaust port.
 

Mark Spies

Newbie
Joined
Apr 9, 2008
Messages
282
Location
LaPlace,La & Houston ,Tx
Aircraft
1979 Bensen B8M 90HP Mac TFG E81
Total Flight Time
200+
Mike,

Which way did you have the cooling cans on ? I have see them on backwards ( opening to the front not back and cause this.
 
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