mcculloch ringed pistons

[steve miller]

i just tore apart my mc7 that siezed at avon ..one of the piston ring pins broke ruined head and piston jug is still perfect ..now im concerened about all my other mcculloch motors ..with pinned pistons ..any opinions.?? worry ..dont worry ..uncommon ..common.thanks all ..

[Sonny Gerber]

After building and running McCullochs for 15 years or so in my first karting life and 6 years in vintage karting, I've seen very few ring pins come out and cause the damage you describe. The only time in memory was a customer engine and my guess was that he did not set the ring end gap properly. I've used .060 as end gap and had no problems. My personal experience is that the item most likely to break is the rod, particularly modified engines.

[steve miller]

thanks sonny ..i guess i shouldnt worry about it ..the motor is 5 over and ive had it 35 yrs and on about ten different karts .i never ever touched it till yesyerday ..i was really twisting it when it happened ..i was keeping up with pete on his methed 101 ... i was shocked it didnt even mark the bore.. now i need 5 over piston and ill be all set ..i might be a little more encouraged to run some of my other motors ..thanks again

[steveohara]

Steve,

Are you sure the damage was caused by a ring pin coming out? The OEM Mc 7 pistons did not have pins to locate the rings. Maybe you had a later model piston in it designed for one of the models that had the larger exhaust ports ???
Losing a ring pin is not real common but does happen once in a while but usually the failure is prompted by something out of the ordinarly like a ring land failure. I have never had one come out of a McCulloch or Burris piston but I have had the problem with Wiseco pistons. The problem lies in the way they manufactured the pistons... Mac and Burris pistons have the pins slightly recessed from the surface of the piston while many of the Wiseco pistons are made with the outboard end flush with the OD of the piston... in fact they often appear as though the pins are installed long and the ends are ground off as part of the finish sizing of the piston.
Does your piston still have one pin in place? If so, does it look like the tip was rubbing on the cylinder?
Steve O'Hara

[steve miller]

hey steve ,it must be a 005 wisco piston .. the pin that aint broke is flush with piston ..must be a later piston too as i said ive had the motor 35 yrs never had a proplem ..i guess its just one of those things ..i feel a little better now ..jeeez i didnt really wanna tear 30 plus motors apart and start buyin pistons ..thanks again ..

[Chris Hamm]

The piston ring pins should not be a problem as long as the pins are mounted, ala late Wiseco's, so that they will be under the rings which are notched to cover the pins while in the cylinder. I also ran many earlier Wiseco pistons with the ring pins flush with the outside of the piston and never lost a pin. Were you running McCulloch pistons?

[Chris Hamm]

Sorry, I missed the other replies. Did not see them until I had posted my reply. I am surprised that a pin did come out of your Wiseco piston, though. In many years of competition with alky-Macs, I never lost a pin nor did know anyone who did, and that's a lot of karters. Did you seize the piston when the pin broke?

[steve miller]

yes sir ..after further reveiw .i noticed i think i did squeak it a tiny bit .ill bet it was on verge of seizer and was so hot the pin kinda just fell out melted out ...i was at end of short straight and it stumbled a little i pulled off track ..check plug that was fine but it just didnt seem right when i rolled it over i fired it couple times didnt trust it so swapped it out and finished day ..i still got it after more than yr. and a half ..tryin to post pictures .

[Chris Hamm]

Still, shouldn't have happened, but even lightly seizing a bucket doesn't help. Just a 1/16 turn richer on your high speed needle would most likely have prevented that problem. And watch your temp and humidity. Cooler, richer; hotter, leaner; more humid, richer; less humid, leaner. Don't forget, Macs are air cooled and lean oft times means too hot.

[steve miller]

i agree chris it shouldna happened .. i was brought up by an old saying you can run a motor 30% rich with noooooo power loss ..i got to retire from pro motocross when i was 27 ..ive raced about everything you can name in the past 30yrs .. i have pretty good idea whats going on ..i was really twisting the poor ol motor it stumbled a tiny bit i pulled off lotta guys mighta kept on goin and ruined the rest of motor ..im pretty fussy on how i run my stuff ..not a lotta guys can retire at 27 that dont no much about motors and tuneing .. ive done pretty good ..but i do not hesitate to listen and learn ..i learn new stuff almost everyday .!! thanks for your reply ..hey by the way im still looking for a 5 over mc7 piston i dont really wanna use a 91 piston ..