Author Topic: Swapping Cranks and Cylinders  (Read 2363 times)

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Offline martyinmi

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Swapping Cranks and Cylinders
« on: November 04, 2012, 09:55:00 pm »
I want more cubes from my new to me Mac 250. Will the crank from a 1-75 (1.625 stroke vs 1.375) fit into the 250 crank housing? Is the bolt pattern for the cylinders the same? They both share the same bore @ 2.125, as do many of the older Macs. Those components would jump me from 80 to 95 cc's. If the internals from a 1-75 won't fit, will the internals from a 1-70 fit? The crank and cylinder from it would take me up to 87 cc's.

I'm aware that the 250 is not considered a hot saw, but I love the looks of it and the way it sounds. Also, about 5 years back I watched one out cut an 046 at a farm show up north. I made up my mind that someday I'd have one and I'd get it running well enough to out run my brothers John Deere (82 cc Efco).

Al is already aware of my dilemma, but I was hoping a few of you other experts might be able to help me out too.

Thanks
"Everybody Lies"

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Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #1 on: November 05, 2012, 04:24:27 am »
Probabley if it would work you'd jamb the piston through the top of the head deck .

You can get into all kinds of problems .Like for instance clearance on the stuffer .On a Mac 125 for example they  use a special six splined screw on the rod because believe it or not a standard grade 8 #10-32 socket head will hit the stuffer .That just one of many problems you might encounter .

Although I'm not exactly certain I'd imagine talking a standard 250 at 80 cc as to a super 250 at 87 cc ,they most likely used a shorter rod .I'd have to wade through a ton of IPL's to be certain .

Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #2 on: November 05, 2012, 04:43:01 am »
Couple more points .You obviously have not really looked at the design because this is not a removable cylinder type engine like a Stihl .

By and large all McCullochs are cast as a block .The reed valve models it's all one assembly and the 10 series and Mini Macs it's a clam shell design .There is no swapping cylinders although on the cast iron liner types they can be over bored but you'd have 6 times as much as the saw is worth to have it done .

Facts : A 250 was made basically in the early 60's when the transition was from heavy old gear drives that weighed a ton to lighter direct drive saws .Now they will cut no doubt about that and can be modified .However is the cost and frustration worth it ?

An 82 CC J-D branded saw will run at RPM's that would shake that old reed valver apart .It produces it's power like a Chevy 350 at RPM while a reeder reachs peak ,like a Ford 390 at lower RPM .

If you  want a Mac that could stand up to the BIL's saw for speed get an 850,805, SP 81 .I'm not trying to be pessimistic  just the facts as I see them .

Offline martyinmi

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Re: Swapping Cranks and Cylinders
« Reply #3 on: November 05, 2012, 06:34:41 am »
I am going to mod this one! I've actually got another 250 powerhead on the way that is supposed to run well. I'll also find a 1-70 or a 1-75 and do some swapping...if it will work ??? I have a mill and the necessary tooling to bore the cylinder. I just don't care to start welding on the crank to stroke it like we do on our antique pulling tractors if a crank swap will work.

Remember Al, you got myself and many others in to this souped-up-saw thing. Now the way I see things, you need to help me out along the way. ;D

Time for work. Thanks
"Everybody Lies"

Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #4 on: November 05, 2012, 08:43:26 am »
Maybe so on Stihls .However other than a piped mini Mac and a finely tuned and slightly altered 6-10 with a larger carb I've never done a thing other than rebuild and restore them .

I wish you a lot of luck,go for it.

Oh I'll do that super 44 one of these days which is just an intact  pile of parts that Dan Henry sent me .It's kind of low on the priority list though .

Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #5 on: November 06, 2012, 02:51:52 pm »
A little fly in the ointment .According to the IPL's the super 250 uses a ball bearing on the clutch side while a standard 250 uses a needle bearing .Which is not to say you can't use a needle with a super crank providing the shaft size is the same ,dunno .

An I-70,Super 44a is a right hand start saw which uses a right thread nut so you don't spin the clutch off starting it ,keyed clutch which has a tendency to tear out the keyway which is why in later years Mac used just the taper to hold the clutch,no key .A standard 250 uses a left hand thread screwed on clutch .

Now then  the rods on a 250 and a super are the same which I assumed them not to be  but they list a different piston so I assume if you use a standard 250 piston with a long throw crank  you'd  chunk it through the head deck .

I-75 have no idea .It's not on my micro film nor in the big black book that weighs 20 pounds .According to Mike Acres site they only made it a year .So reading between the lines it's rare as a hens tooth .

Now before you get the idea you can take a crank from a 650 gear drive here's that deal .They are also rare as a hens tooth .That I know of they were made for 10 months .I have one,Mike Acres has one and there are only two others I've heard of .Besides being a geardrive that crankshaft is not designed to run a direct drive clutch .

Offline martyinmi

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Re: Swapping Cranks and Cylinders
« Reply #6 on: November 06, 2012, 06:56:15 pm »
Thanks for the legwork Al. I really appreciate it!
If push comes to shove I'll just weld up the crank myself and have it offset ground. If I add a quarter inch stroke I'd need to shorten the rod an eighth inch or so. I've done both(crank welding and rod lengthening/shortening) many times.
An old trucker (late 60's) came in my office and commented on the 250 saw in the back of my pickup. He seemed to know a lot about them. He said that he thought a reed  engine would tolerate the extra stroke much better than a piston ported one. What do you think about that?
You wouldn't know where I might find a set of rings for it, would you? I'm calling chainswr tomorrow to see what they might have.
Thanks again!
"Everybody Lies"

Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #7 on: November 07, 2012, 07:43:33 am »
They pop up on flea bay every so often but are often oversized ,left over new oldstock .

FWIW I turned a set of rings out of tool steel for a Mac 125 but never finish them because I found a set for 14 bucks .It can be done but it takes hours and hours of tedious machine work .

Now if you are serious about all this why don't you just bolt that block up in a face plate and cut the head deck off .Make another head and punch the iron liner out or make a new one .

If you really want to go whole hog find a kart piston .Wiseco made them for the 101's that were 100 thou over sized .Of course it would just be easier to find a 101 rather than all that fiddling around but they are very proud of those things as far as price .

Offline Al Smith

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Re: Swapping Cranks and Cylinders
« Reply #8 on: November 07, 2012, 07:52:20 am »
More ramblings .If you go to Mike Acres site and look up 125 Mac ,that all yellow saw is mine .

Rebuilt from a chrome liner worn out 125 .New iron cylinder sleeve ported pretty much stock with slightly wider transfers .Some people have hours in rebuilding saws ,some days .That one was literally weeks .Ya know as of yet I haven't even ran it long enough to seat the rings .

So what I'm trying to say is a project such as you are undertaking probabley takes on the challange of proving you can do it .However instead of the weeks I had in the 125 project you are going to have months .

Offline martyinmi

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Re: Swapping Cranks and Cylinders
« Reply #9 on: November 07, 2012, 07:16:58 pm »
Stroking a crank is very easy, as is shortening a rod. I've taken WC Allis Chalmers cranks (4.0" stroke stock) and added two inches to the stroke. That usually takes about a full day of welding. The compression height is so tall on those old tractors that we can usually add stroke, go to an automotive style piston, and then actually make the rods longer than they were stock. I usually buy 8 SB Chevy rods(5 bucks each), strategically cut them, weld them back together, box them in for strength, then balance each end.
As far as the welding of the crank and shortening of the rod goes, I can't see that taking even a day. A machine shop a few towns over can grind it(the crank) for me for 50-80 bucks. So far the biggest hold ups right now appear to finding a set of rings and reeds. Chainswr can't help me, and I'm not finding anything on fleabey.
And speaking of reeds, if I get a fog of gas near the inlet of the carb at WOT, thats usually an indication of reeds not sealing properly, isn't it?
"Everybody Lies"

 

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