Mcculloch sp125/101b kart saw build thread

[jacob j.]

Well, here is the 101 I purchased. Needs a good cleaning. Numbers on the bottom are 66019. Did some exploring and it has a .040 over piston. In one of the pictures you can see ports with grooves that run into them. Hoping that it can be bored out and a piston can be found. Feel free to tell me what you think.

That's definitely earlier than what we were thinking. Someone might have punched the block numbers in later - but Lee and Nathan are right, that's Mc101 era with the bridged exhaust.

I would be surprised if it didn't have a 9/16" wrist pin and rod.

That head chamber is different than I have seen before.

Looks like the head is recessed for the Mc-91 style head gasket instead of the copper ring style.

 

[Bigmac]

That's definitely earlier than what we were thinking. Someone might have punched the block numbers in later - but Lee and Nathan are right, that's Mc101 era with the bridged exhaust.

I would be surprised if it didn't have a 9/16" wrist pin and rod.



Looks like the head is recessed for the Mc-91 style head gasket instead of the copper ring style.

Agree, it definitely an aluminum head gasket block. I haven’t had one of them in my hands to actually know the details! The chamber seems odd too!
I have an odd 101. It had all the casting details to be a 101aa, but the exhaust port looks early. It has a casting mark for an exhaust bridge delete, so I would think it should have been a 101a. Dose not make much difference, it has the lowest exhaust port of all my blocks thou

 

[Scott Kelsey]

I was able to run downstairs before I got out the door. To me it looks like the piston is flush. Also, here is the measurement you asked for. I did it very crude and also believe I measured properly. Top of cylinder where head gasket would sit to the nearest edge of the exhaust port from that point. If I didn't do it right please let me know.

 

[Scott Kelsey]

May I ask what the piston being flush and the measurement to the exhaust port means?

 

[Bigmac]

May I ask what the piston being flush and the measurement to the exhaust port means?

Thanks, have been curious, and have been making some thoughts on what the port timing was, and weather the block could be oring-ed like the later blocks. I have been guessing to the port timing based on pics, this confirms what I visually saw. The mc101 is extremely close to cp and 797 timing, closer to cp timing, so basically a 3mm lower exhaust port than the later 101’s . Basically a saw block with removable head, this isn’t exactly a bad thing. It could allow more of a torque monster, or build a motor somewhere in the middle. I think you could cut a .015 deep oring grove and with a .050 copper gasket would have .035 squish

 

[Scott Kelsey]

Bigmac, thank you for the info. I have some questions to ask, because I would like to learn. When you speak about squish you are referring to the difference between the head and piston correct? What would be the purpose of cutting in an original gasket and copper gasket? If you guys had this motor what would you do with it. I see lots of 101b's in saws. I am assuming it is a bit more of a wild motor?

 

[Bigmac]

Yes, squish is piston to head clearance. The standard gasket is .050 from terry. He used to have .035 but isn’t currently making them. .050 is generous squish, I like between .030-.040 in this size engine. I run big single Hondas 363cc at .040. But they still run fine at .050 so don’t worry. The copper gasket is a lot more common and easier to get, and should seal better too. But it’s not a must or anything. So, from my research, it looks like the mc101 has the lowest port timing, it also has the 9/16 pin and in the rod and the rod bolts need an update. The 101a was originally meant for a single engine class in racing, and has the next lowest exhaust port, the 101aa was meant to be a dual engine kart setup, it has the wildest port timing in general, the 101b is is slightly less aggressive. The 101d was made at the same basic time as the 101aa and as far as I can tell is identical.

The 101b, is the only 101 with a recessed exhaust gasket, I think Mac offered it as a kit, and that’s why it’s the most popular. The differences that I have measured between the 101a, aa, b and d block at basically.020 different, that’s not much, could easily be production variation, my 101a/aa block is .020 lower than a b block so that’s why I think it’s a transition block from a 101a, that should be lower. .020 is almost unnoticeable when running, but .040 is. And your Mc101 .120 lower.
I really think that mc101 would make an exceptional saw motor, especially with an sdc20. The biggest downside to the saw motors is bad squish and lower compression. That is something that is much better in the kart motors, and can also be tuned, to what you want. The cp125 for example, has a horrible combustion chamber and massive squish, the squish on my cp is over .120, that is killing power! The sp blocks are much better

So in short, the mc101 can be built to do whatever you would want, it’s a blank canvas, you could build a torque monster, lift the exhaust port and make it run like any of the other kart 101’s, or lower and have something in the middle.

 

[Bigmac]

Tried some different tunes and put a tank through the cp

 

[jacob j.]

That last video sounds great - sounds like you're right in the high torque curve.

My grandpa loved the sound of the Macs from that era and he hated the sound of the comparable Homelites. He cut timber with a guy who ran Homelites (Charlie) and one
day he told Charlie they were getting "divorced" because my granpda couldn't listen to his Homelites anymore.

 

[Bigmac]

That last video sounds great - sounds like you're right in the high torque curve.

My grandpa loved the sound of the Macs from that era and he hated the sound of the comparable Homelites. He cut timber with a guy who ran Homelites (Charlie) and one
day he told Charlie they were getting "divorced" because my granpda couldn't listen to his Homelites anymore.

It’s starting to feel right, it was pretty fat. I like to be conservative first! In person she is really screaming, I can load it pretty good in the rpm’s. I had to pull the duckbill out, it wanted to stave for fuel in a long cut, but that fixed it

thats a great story! I love the sound too!

 

[qurotro]

I really think that mc101 would make an exceptional saw motor, especially with an sdc20. The biggest downside to the saw motors is bad squish and lower compression. That is something that is much better in the kart motors, and can also be tuned, to what you want. The cp125 for example, has a horrible combustion chamber and massive squish, the squish on my cp is over .120, that is killing power! The sp blocks are much better

I'm actually thinking why my cp feel "loose" by hand and my stihl 076 feel like it's locked up when pulling the rope. The feel of compression from the rope is way softer with the cp.

 

[Bigmac]

I'm actually thinking why my cp feel "loose" by hand and my stihl 076 feel like it's locked up when pulling the rope. The feel of compression from the rope is way softer with the cp.

I seems like the cp had the lowest compression of all the 123cc Mac’s, the recoil is a little bigger diameter for the Mac’s, so that changes the feel compared to the 076, I have one too

 

[jacob j.]

I'm actually thinking why my cp feel "loose" by hand and my stihl 076 feel like it's locked up when pulling the rope. The feel of compression from the rope is way softer with the cp.

In addition to what Nathan mentioned, it may also be that the thin-ring piston isn't making as much static compression as it does dynamic compression when the motor is running. I have a couple SPs like that -
they don't feel impressive turning over by hand but they make massive compression when running.

 

[Bigmac]

In addition to what Nathan mentioned, it may also be that the thin-ring piston isn't making as much static compression as it does dynamic compression when the motor is running. I have a couple SPs like that -
they don't feel impressive turning over by hand but they make massive compression when running.

Good point! And a dry cylinder will also feel soft, even wd40 can give a big bump to the feel on a dry cylinder, my shelf 125 that is all new, dose not feel like it has much comp either, but it’s pretty dry inside, it has light oil, but mot wet with oil. And I know it should have well north of 200psi

 

[Scott Kelsey]

Hey guys. I have been trying to find information on how to identify the BDC carb on this 101? I have looked in some of the Kart forums and can not seem to find any concrete info how to identify the BDC series. I have found the carb has WR 1 stamped in it, but that is all I can find. Any input?

 

[Bigmac]

Definitely not a bdc expert, but I believe that is the earliest version, and isn’t marked bdc, and that being and early 101, it would make sense

 

[jacob j.]

Some goodies from the archives for you boys:

This is an early Wiseco standard 101 piston - The design of the box indicates that it probably dates from early-mid 70's. I've used several
other Wiseco pistons from this era and the literature states that they used a "low-expansion" alloy back then, so no relieving or special machining
was required, unlike some of the early bike pistons. Arrow on top indicates the exhaust side -

 

[jacob j.]

This is a later .040" over 101 piston, also Wiseco. I think looking at the design of the box, this one dates from the late 80's/early 90's. I have some bike pistons
from that era that have the same style box. This one is interesting because it uses two thick alloy thrust washers and a much longer wrist pin than stock. This one is also
cast and relieved for the boost port and has the feeder ports -




Unlike the older Wiseco pistons that use the .024" tool steel or chrome-faced rings, this one uses the standard Wiseco 1mm chrome-faced rings.

 

[jacob j.]

This is one of the most interesting pistons I've ever seen - it's an anodized kart piston for the Mc-8/9/45 motors. I don't know the brand on this one - I was told it's a Moss piston,
and that might be accurate as Moss made a number of custom kart pistons back in the day. I have a new old stock saw block I'm going to use this one in -

 

[Lee H]

I have a few of the old Wiseco pistons myself.