mcculloch ringed pistons

[steveohara]

Regarding the porting question, it is all about time/area characteristics of the port layout and not so much about the total port area. From a practical standpoint the bottom half of the exhaust port(s) in a Mac engine contribute very little to the overall performance. If you look closely, most Mac engines end up with the piston close to flush with the bottom of the exhaust port when the piston is at BDC which would seem to suggest that the entire port is needed to let the burnt charge out but the reality is that the vast majority of the exhaust gases are already out by the time the transfers begin to open and the piston still has roughly another quarter inch of travel to go before it is at BDC.
When the exhaust is widened with the eyebrows the gain in performance is the result of the gasses getting out faster or another way of saying it is the exhaust blows down over fewer degrees of crank rotation so that the residual pressure in the cylinder is lower when the transfers open. You could fill in the bottom third of the exhaust ports and the engine would not lose any power if the top of the ports have been widened.
There have been many variations of the headers over the years with different size tubing and even some "big mouth" headers that have a much larger flange and opening but they never caught on because the "standard" size was the correct area to handle the needs of the engines.
Regards,
Steve O'Hara

[steve miller]

ok thanks steve .. hey on my chainsaw piston i machined off the skirt to match the skirt on the 91 series piston ..is it gonna matter that its not a blind wristpin piston .. ?? the block cleaned up nice at .052 over to bad i couldnt find a model 890 mcculloch thin ring saw piston ... i had to tare appart 10 saws till i found a good enough piston to stuff in the dang thing ..

[steveohara]

Steve,

If you machined away the bottom of the skirt except for the exhaust side you cannot use it with the third port passage still in play. Even if you left enough material to maintain a little bit of skirt below the inboard end of the wrist pin you will have dramatically changed the opening and closing timing of the piston port openings and it will have a ton of blowback with the trimmed piston. As for the wrist pin mounting arrangement, just make sure the engine can't vent to atmosphere through the pin.
Steve

[steve miller]

thanks steve ..the motor im putting together is a 91 of some sort ..i might be a little confused ..i didnt think 91 series motors had 3rd port passage .?? just the two finger ports ..?? or the square hole in jug .?

[david a luciani]

hi Steve,
as usual you've reinvented the wheel.
rather than making yourself nuts and building a custom piston why don't you just get a blocksaver from terry ives.
he has all sorts of new mcculloch pistons for about 100$.
as tearing down ten saws probably took hours forget about all the machining i'd say you haven't saved anything except actual cash.
nice to see ya trying to get over your mac phobia though.
yer buddy
dave

[steve miller]

thanks dave and everyone else ..i have to admitt for once i been having fun .!! its something different and i seem to have my own way about goin at things ..ill get it sorted out ..yeah dave it took awhile to take apart all them saws but man my pile of shrouds, cranks, heads,cheater 49 blocks ,and recoils is lookin alot better .. thanks again everyone ..!!

[steveohara]

Steve,

I missed where you said which block you were setting up. With any of the 91 series blocks the modified piston will work.
Steve O'Hara

[steve miller]

hey guys .. i here lotta storys about not enough lube to exhaust side of old style mcculloch piston wrist pin bearing . ?? would it help and can this be done... i want to machine a grove in piston next to the trust washer on exhaust side of piston , make like a 1/16 " hole in wrist pin on exhaust end ..or both ..!!