Mcculloch sp125/101b kart saw build thread

[jacob j.]

This picture shows why the small end of the SP-125 rod is much larger - it contains the needle rollers which are capped by a thrust washer on either side:



This picture shows a dismounted SP-125 piston next to the 1/2" wrist pin 101A piston:

 

[Bigmac]

I agree on the sp style pistons, nobody is making those, and some larger oversized pistons would be nice. And on the Kart pistons the thicker weisco ring would be an upgrade. Also don’t know if the sp style rod could get a boost port, that would make it more versatile

 

[Definitive Dave]

OK I am learning here, my sole experience with Macs is running from from LeeHa, Eccentric and a few 125s at the latest Randy GTG last fall.
I got an oily Super 797 and then bought all kinds of parts for it, like 5 coils, 8 pistons, 4 heads, 4 rods, I had no friggin idea what I was doin and started taking it apart and it has set in a box for about three years now so I pawned it off on @Redfin
Where do the CP125 and 797/Super 797 fall in this kettle of fish? would any other models use the same piston as an SP?
Also I figured I should attach the two high resolution Mac manual scans I did awhile back in case they are helpful to anyone.
You Guys Rock!!!!!
Dave

 

[Bigmac]

Like jj was saying there are a few different rods, and that’s were It gets a little tricky. But the common rods are the 1/2 kart rod, same as the cp rod, and the sp rod. The problem with the sp rod/piston in the kart is it doesn’t have a boost port window, but it will fit and work

 

[jacob j.]

Would any other models use the same piston as an SP?

Like Nathan said, with the SP rod, a person can use the matching SP piston in any of the 123cc Macs - 797, CP-125, SP-125, or Mc-101/101 A-D motors.

SP rods are abundant so that's a choice that will be a quick seller, plus there's still tens of thousands of these saws in regular use, or in the hands of collectors who
stockpile parts for the future. I would gladly take more SP pistons for future builds, especially like Nathan pointed out - if we got away from the .024" thin rings
and stepped up to the 1mm Wiseco rings or 1.2mm Caber rings. We were talking about the limits of the .024" thin rings the other day, and they're only good
for so much power. The Wiseco rings could handle a lot more power.

I can supply several sizes of SP pistons along with pins and clips, even used rods for the manufacturer to mike-out when setting up tooling.

 

[Bigmac]

I don’t know how feasible it would be to shorten the wrist pin in the SP style, if a person could incorporate the boost port notch and have weisco piston rings With the stepped ring for a recessed pin boss, the cart guys would be extremely happy with that piston rod combo, I my opinion. And as jj said it would work in the entire product line.

 

[Bigmac]

And as a fall wish list option a taller piston would really help the saw blocks, Would lower the port timing a little bit allowing someone to do more port work in the future And tighten up the squish, which is fairly large on several of the saws

 

[Definitive Dave]

I am gonna read through this all again on Tuesday morning when my brain is fresh, but so far I don't see any real stumbling blocks. You guys are a wealth of knowledge.

 

[Lee H]

Good topic, I have been away from computer for a couple days.

Since Terry ives has had Burris pistons made for years now for the common 101B, AA, MC motors
i think as Nathon and jacob do the SP125 piston would be a great candidate for a repop. As for the
CP125 it used a different piston part #69301 thick ring and an optional 69241 thin ring piston. The
CP125 used the 1/2 wrist pin press in style like the 797's, The super 797 also used the 101 1/2 wrist
pin style with press in wrist pin but the Super 797 used a piston with thick rings same piston as the
CP125 # 69301. Not sure where Mcculloch got the Super from since the Super also lost the boost
ports somewhere in the production line.
From left to right, 101B piston, Super 797 and CP125 piston, CP125 Optional thin ring and can be
used in Super 797, Stock SP125 piston, Early 797 with 9/16 pin.

Also keep in mind that you may want to consider cast rings as they can be used in cast or Nikasil
bores. A chrome ring can not be used in a chrome bore which a stock SP125C has. The early SP125's
have cast bores and can be bored to oversize pistons. There have been but not by me folks that have
had SP125C motors bored and replated in Nikasil.

 

[jacob j.]

More kartzilla action:

 

[jacob j.]

 

[Frank bierce]

Looks pretty fantastic!!

 

[jacob j.]

Getting there....

 

[jacob j.]

Making some progress:

 

[jacob j.]

This is one of dad's old saws, "kartzilla". This one got the .068" over 101b motor, freshened up by Terry Ives. The original ignition that my dad set up with an Atom chip in 1979 still works...

 

[jacob j.]

This is a build for a member here - "Junkyard Dawg" -

 

[heimannm]

How did you get the tank apart without destroying it? Many have tried, few have succeeded.


Mark

 

[ManiacalMark]

0.068” over? What displacement does that make it?

 

[Bigmac]

How did you get the tank apart without destroying it? Many have tried, few have succeeded.


Mark

Mark, I think it’s just a 797 style chassis

 

[jacob j.]

Mark, I think it’s just a 797 style chassis

Yeah, that's affirm (if Mark was referring to the second saw). It's a saw a member here picked up that's a combination of several different models.

I think the tank and rear handle are off of a 440.

0.068” over? What displacement does that make it?

I think it calculates out to around 130cc.