Hello Kart Guys

[emfalz123]

Hello all!  My name's Eric, I'm a vintage chainsaw collector, specifically Mccullochs, and am here to learn some engine building tips from you guys!  You got a pretty neat forum here, some good info on the old Macs.  I'm located in southern WI, so I imagine a lot of you have been to the Brodhead racetrack. 
Anyways, I have some projects that will require a fixed head saw engine to be bored over, so I'm looking for machinists who can handle working on them.  Preferably they'd be somewhat near where I'm located to minimize shipping costs. 
Specifically, I'm working on a Mac 650 geardrive and a Super 250, which are very similar:  2.125 bore and 1.500 stroke, third ports are open.  I have a 48890B carb that I want to use, a 5 degree advance flywheel, and want to use the reed components from an MC 49E.  I also have a NOS piston, rings, and rod for a MC 91 that I was thinking about using (2.125 bored .040 over = 2.165) but I have a feeling you guys might have reasons not to do that, which I'd also be interested in hearing.

Thanks in advance!
Eric

[Tom Day]

Hello all!  My name's Eric, I'm a vintage chainsaw collector, specifically Mccullochs, and am here to learn some engine building tips from you guys!  You got a pretty neat forum here, some good info on the old Macs.  I'm located in southern WI, so I imagine a lot of you have been to the Brodhead racetrack. 
Anyways, I have some projects that will require a fixed head saw engine to be bored over, so I'm looking for machinists who can handle working on them.  Preferably they'd be somewhat near where I'm located to minimize shipping costs. 
Specifically, I'm working on a Mac 650 geardrive and a Super 250, which are very similar:  2.125 bore and 1.500 stroke, third ports are open.  I have a 48890B carb that I want to use, a 5 degree advance flywheel, and want to use the reed components from an MC 49E.  I also have a NOS piston, rings, and rod for a MC 91 that I was thinking about using (2.125 bored .040 over = 2.165) but I have a feeling you guys might have reasons not to do that, which I'd also be interested in hearing.

Thanks in advance!
Eric

Eric, I don't know anyone in your neck of the woods that doe's the blind bores. We send all ours to Terry Ives in California who is excellent and a good guy. Maybe someone else knows someone but if you you want someone that does excellent work Terry is your man and it may be worth the little extra cost.

[David Luciani]

just send them to terry and don't waste your time trying to find someone local.
he does them at about 80 bux per matches piston to bore.
he has a dedicated lathe jig set up for the bores and produces work hard to match anywhere.
if you are using a saw with a full piston skirt switching to a partial skirt as found on mc91's usually won't work.
biggest issue is the pin to crown height is different if i remember correctly.
i could be wrong the best way to tell is measure the piston from saw then measure piston you are thinking of using.

[emfalz123]

Thanks guys, sounds like Terry's the man. From what I measured, the piston from the 91 would work as far as crown height is concerned. They differ in the skirt length and the port openings around the pin. I'll try to take a few pics to explain and see if i can figure a way out to post them

[thomas kelley]

there is a vintage/modern race sunday at broadhead
i and a bunch of local racers will be there
tom

[emfalz123]

I'll try to make it out there Sunday. I'm going to attach a couple pics from my phone, see how it works.

---

The 91 piston is on the left.

[David Luciani]

wow used to be i could tell why a mc91 piston has issues in an older motor but seems that memory is stored deep?
anyway i think the 91 piston might work in an old motor but an old piston won't work in a new motor because it kills the extra ports.
if you just want to run the correct piston with thin rings but oversize i probably can help you out.
i have early oversizes that would suit your project better?

i'd be open to a trade as mc91 oversizes are getting low in my pile.

dave l

[emfalz123]

Dave,
From what I gathered, the mc91 piston would work as you stated. I'm also leaning toward just buying a thin ring MC49 piston +.030. It seems like a waste to use the 91 piston where it's not required. Its also standard diameter, not oversize. I'll probably just keep it because I'm pretty much a hoarder
Eric

[emfalz123]

I snagged the two oversize 2.125 pistons on ebay, thanks for not bidding me up too bad guys

So, the question is: Do I go with the +.030 over thick ring version or the +.010 over thin ring? This won't be a daily driver sort of saw, more of a hot rod, show saw. Looking at short run times a couple times of year.

Eric

[David Luciani]

for fast the thin ring is way to go.
for durable the thick ring is best.