do I have a mac 5 motor?

[Mel Ruffner]

Hi everyone I,am a little confused with my motor. I was told my mac was a mc5 when I bought it off ebay, but not completely sure. The block says mc5 but reading the forums. it seems the sparkplug is on the exhaust side on a mc5 and mine is on  the other side. Could a different head be fitted on a mc5? Also does it look like I have the large rings?I know  alot of you know a whole lot more about these motors than I do. thanks for the help Mel

[ted johnson]

Could be an Mc5 with the Mc6 conversion factory kit. It had a stroker crank and a thin ring piston. It had instructions for rotating the head 180 degrees, a template for cutting the shrouds for the plug cover boot that was also in the kit. You then had a 1.5" stroke, thin ring piston, thin head basket, and an engine that would no longer be beaten by Power Products AH58's and Homelites. T^ake off the head and measure the stroke. The Mc5 had a 1.375" stroke, and the piston left the intake ports almost half covered at bottom dead center. I have an Mc5 here in the house with the stock crank still in it.. Ted

[Terry Sullivan]

No doubt the block is MC-5.
Everything else could have been modified, or replaced, but in my mind it is still an MC-5.
It's a keeper.

[David Luciani]

mel,
external parts interchange pretty freely throughout the standard series motors.
internally there are some differences but you aren't planning on being an engine builder so for your purposes i'll skip that part if you don't mind.
as ted said you might have a mc6 crank and piston. you can also have just a thin ring piston.
neither hurts your motor except in duriability of rings but since we rarely run them that shouldn't be an issue.

the history is this,
mc5 was an ok motor. then the modifiers started in on them and added thin piston rings =faster motor. then stroker cranks were created. mcculloch was fast off the line and made thier own "stroker" kit.
then they released the mc6.
this all happened so fast that a ton of mc5's were in production when they redid specs. mccullochs solution was to cross out the mc5 and re-stamp them mc6.
then a new issue cropped up as the cheap mc6's were as fast as the super motor mc10. so mcculloch then stroked the ten eventually changing to mc20.
history over.
you have a fine motor.
easiest way to determine if you have a mc6 crank is look at counter wt' if there a line cut along the curve of the counter wt you have a mcculloch mc6 crank. if not it may be an old reweld so check the stroke as ter said also to see.

[ted johnson]

The REAL reason Mac produced the Mc5 stroker Mc6 conversion was that a few early Power Products AH58 engines appeared, and began tearing the Mc5's up in Wisconsin, Florida and California. Bob Palmini was terrorizing the Mc5's in CA. Mark Allen's pop, Ralph, was a retired PP exec, and got a pre-production '58 in Eau Gallie, FL and ate every Mc5 in Brevard County with it. my Pop called Mac and told Bob McC. what was happening. He already knew! We had a pre-release Mc6 kit within a week and a half. If you look at my avatar, the third kart back is Lanny Rothery's Bug Standard converted to live axle, and sporting that same Mc5/Mc6 conversion. It was so early the paper template to cut the plug boot hole, nor the rubber boot were included in the kit. The head on this engine never did get turned around, and it ran that way for over a year. The second kit we received had the rubber boot and the paper template. I ran that engine myself until I got my first Mc20, nearly a year later. Nobody knew how to break in a thin ring engine, and we must've stuck them ten times between the two 5/6 conversions. Between me running the '5 and installing the conversion kit, I managed to get hold of a very early PP58. It vibrated so badly that I removed it after one weekend and put the Mc5 back on my Wasp. I had my Mc5 converted to a '6 and installed within a couple of weeks. Ted

[doug tenney]

Mel, I'd say you came to the right place.

Teddy, you have one helluva memory. The people from the Smithsonian should be recording this stuff.

(By the way, I've been meaning to ask you, which brother was more fun to hang out with, Wilbur or Orville?)

[ted johnson]

I always liked Wilbur best. He used to fix my bike when it went sour. Of course, it was a bear getting it from East Central Florida to Dayton! Ted

[doug tenney]

Teddy, seriously, what's the story with this?

"Nobody knew how to break in a thin ring engine, and we must've stuck them ten times between the two 5/6 conversions."

How did the breakin processes differ?? Would the "thin ring" process be applicable today to, say, a Mc91x with a new piston & rings?

Also, "stuck" seems to mean different things to different people... To some, it means "spins freely but no compression; rings overheated & lost tension." To others, it means aluminum skid marks on cyl and locked up. What do you consider to be "stuck"?

[ted johnson]

In those days, Mac rings had chrome faces, and with stock factory clearances, needed a slow speed, oil-rich breakin for at least an hour. We were used to the thick, iron rings on Mac 5's and 10's, as well as the cross-scavenged engines. Take 'em out of the box, mount 'em, run 'em slow for ten minutes and have at it. What usually happened with the chrome thin rings, is that they had a bit of a rounded face to the chrome surface, and thus had a line contact with the cylinder wall until they seated. With the line contact, the heat wasn't well transferred to the iron liner, and the piston got hot enough to melt a small area which got dragged down over the rings. This pinned the rings, ruined the piston and cost a few hours and a few more dollars to fix. The cylinders always cleaned up fine with a light hone. The honing increased the skirt clearance as well as deglazing the bore. An hour with a new piston and ring set and you were ready to go. I must say that Pop built a pin pusher, and he and I got real good at replacing the bad pistons! When the all steel rings appeared, the issues went away.
P.S. You gotta remember, we were all direct drive, so when an engine stuck, the rear wheels slid for a LOOOONG ways! When the engine cooled, it'd turn with no compression. Ted

[doug tenney]

As if on queue, check this out on E-Bay: 222658033359

I never saw a Mac with the spark plug on the opposite side!  Learn something new every day.