McCulloch Chain Saws

[kbeck]

Piston height would effect the port timing , right?

I would think piston height would effect exhaust port timing

 

[PogoInTheWoods]

Yes.

 

[PogoInTheWoods]

Skirt characteristics would affect intake.

 

[kbeck]

Well got the Super Pro 60 cleaned up took the bar off it and put it on the 800. Didn't check compression yet but doing the drop
test seems to have plenty of that. Was going to start it today but gas line needs replacing

 

[vinnywv]

Doesnt look like the same 60 looking at the ebay ad. Very nice job.

 

[kbeck]

I know when a saw gets used without cleaning afterwards then sits for years the crude just seems to spread and get hard
that's what happened to this one, cleaned it pretty good should really pull the flywheel and clutch to really get it clean.
Happy with the results so far just need a gas line so I can get it running.

 

[vinnywv]

Thats a true statement. Most of the old macs have that crud all thru them. They were used and used and put away. Sit for years then one of us guys buys it to fix it up and spend hrs just cleaning. They rarely dissapoint though. Great engines.

 

[Maintenance supervisor]

Skirt characteristics would affect intake.

Could you put the old piston back in it (crazy I know) and since you have the cylinder apart put a degree wheel on it to see if the timing is bad? Too kind of get a feel if that is part of the problem?

 

[vinnywv]

Speaking of engines...ive been tinkering on a project build. After all the 70cc chat and information i decided to go for it. Ive started my next antivibe 70cc saw. I used a nos sp oil tank. And attached a 7-10 cylinder to it. So far its going great. Up to the fuel tank, it has a leaky seam. I was able to get the tank apart so i can reseal it. One of the 4 screws broke off and its currently at work so a buddy can remove the screw or drill it out on the machining equipment. But other than that its going very smooth. Im on midnights so its slow going till my next days off. Heres the progress so far though. And im calling it a "SP 71". Lol

 

[heimannm]

I get rather confused as well. Anyway, here are four pistons side by side, I didn't have a 69030 but did have the 0.020" oversize 69031.



As you can see, the pin height is different between the 0.4375 diameter wrist pin and 0.375" diameter wrist pin pistons.





I did the best I could to determine the rod length between the two styles of pistons as well as the distance from the bottom of the piston skirt to the center of the wrist pin and by calculation the distance from the center of the wrist pin to the top of the piston. It seems the stack up for the smaller 0.375" wrist pin piston/rod combination is 0.010" taller than the larger 0.4375" wrist pin piston/rod combination...

Wrist pin 65389 dimensions 0.375" x 0.993" long
Piston 85240 1.735" tall, 0.9805 bottom to CL of wrist pin, 0.7545 CL of wrist pin to top of piston
Connecting rod 65401 2.5065" CL connecting rod to CL wrist pin

Wrist pin 69202 dimensions 0.4375" x 1.550" long
Piston 85239 1.735" tall, 0.8015" bottom to CL of wrist pin, 0.9335 CL of wrist pin to top of piston
Connecting rod 85352 2.317" CL connecting rod to CL wrist pin

I hope you all are confused as I am about the subject.

Mark

(P.S. I am preparing this from scribbled notes that were clear to me today when I wrote it down, not sure I am following my logic now)

(P.P.S. Nice slippers Vinny)
​

 

[vinnywv]

Ha ha, thanks. Ive had those for years. They are old new balance shoes with the tongue cut out. My basement slippers. Lol

 

[kbeck]

I noticed the slippers too I'll be following your sP71 build. I do know that when I tear my 7-10 down going
to think twice before I do anything

 

[PogoInTheWoods]

I hope you all are confused as I am about the subject.

Excellent stuff there, Mark. I for one appreciate it very much. The only anomaly is the .010" difference between the two at the top..., which essentially only affects the squish and nothing else if each rod provides the same piston stroke otherwise. As shown in the 6-10 IPL earlier, both scenarios can be applied to the same crank and cylinder in that particular model. There shouldn't be any reason they wouldn't interchange in other instances as well.

Could you put the old piston back in it...

I've thought about it, but intend to wait until I get the one that the IPL says is supposed to be in it..., which looks like it's just gonna be a new version of the old one anyway and not a full skirt version like I've been theorizing it was supposed to have. What nags me now is whether it has the correct cylinder...

I truly appreciate everyone's input on this..., not to mention your patience with my continuous posting about it.

And a great looking project there, Vinny!

 

[vinnywv]

So far its going great. 7-10 cylinder and piston. Sp81 oil tank. Sp 81 oil pump and manual oiler. Sp front and rear av. Sp60 rear shroud (modified for pop up decomp. Valve), the lower brace is from a sp70 60 or 80 im not sure exactly, its yellow. The oil tank front cover came from a 10-10. The ignition was from a 10-10. The exhaust is from a 2-10. The leaky fuel tank is from a sp81. The carb is gonna be a zama c2. Starter is a 10 series. Clutch cover im not sure yet, it will be either a short brake style or short no brake style. Ill see when i get that far. Next week one day.

Forgot to mention i also drilled and installed an impulse nipple. Pretty easy to do. Jb weld is holding it.

 

[Maintenance supervisor]

Excellent stuff there, Mark. I for one appreciate it very much. The only anomaly is the .010" difference between the two at the top..., which essentially only affects the squish and nothing else if each rod provides the same piston stroke otherwise. As shown in the 6-10 IPL earlier, both scenarios can be applied to the same crank and cylinder in that particular model. There shouldn't be any reason they wouldn't interchange in other instances as well.



I've thought about it, but intend to wait until I get the one that the IPL says is supposed to be in it..., which looks like it's just gonna be a new version of the old one anyway and not a full skirt version like I've been theorizing it was supposed to have. What nags me now is whether it has the correct cylinder...

I truly appreciate everyone's input on this..., not to mention your patience with my continuous posting about it.

And a great looking project there, Vinny!

Hell for me it's not patience but anticipation! It'll be cool when you get her running.
I won't get into rod stroke theory too much but rod length has alot to do with how an engine runs, the closer you get to a 2:1 stroke (rod to crank throw )the more torque you have available (in a valve engine) , sure the .010 squish difference might be slightly off, but if that cylinder is designed for a longer rod that would change the whole torque and timing of a 2 stroke with piston port.
The only part that confuses me is that there is no open skirt piston with the smaller wrist pin height.
Even if your not stroking an engine changing rod length can really improve or ruin an engine, and I imagine that it would be alot more pronounced in a single cylinder engine.
I'm not totally up to snuff on port timing so I'm only commenting on things I have knowledge of, but I appreciate the effort your doing in keeping us in the loop of this build.

 

[Jethro 2t sniffer]

So is it just a simple case of swap in the rod that goes with that piston?

Did someone just put the whole crank/rod and piston in from another 7-10 parts saw?

 

[Jethro 2t sniffer]

Anyone into BP1s?

https://www.arboristsite.com/community/threads/mc-culloch-bp-1-saws-and-parts.340631/

 

[Goinwheelin]

 

[PogoInTheWoods]

So is it just a simple case of swap in the rod that goes with that piston?

Did someone just put the whole crank/rod and piston in from another 7-10 parts saw?

These are certainly interesting questions. According to the 6-10 documentation I posted earlier, the connecting rods are interchangeable on the crankshaft listed, and presumably on any crank that would use either one. A later 7-10 crank assembly with either rod and piston configuration should interchange.

Swapping connecting rods in order to use the full skirt piston would evidently reduce squish by 0.010 according to Mark's calculations and would need to be taken into consideration, but probably only marginally. I've never measured squish on a Mac. The different rod lengths should be offset by the different pin diameters (and locations) to effectively provide the same piston stroke.

Another possibility is that the cylinder was simply changed and that's been the problem. That's really the main unknown factor now. This theory would only be applicable if a full skirt piston is actually required for the three finger transfer cylinders as has been my theory..., misguided as it seems to have been. It has finally become quite clear to me that the piston originally called for in a 10 prefix 600052 7-10A is indeed a windowed design after all. The correct piston for the IPL of this saw is on the way. No way to really know if the cylinder is correct (or original).

The only other thing left is whether a windowed piston should work just fine on all 3 varieties of transfer types now known to be used on the 70cc saws. Indications are that it should, but I've not seen definitive evidence that is the case. It certainly would not apply to the earlier 70cc saws with small pins. I'm unaware of any windowed 2" pistons with small pin bosses..., only full skirt versions..., which are typically what is found in the three finger transfer cylinders.

 

[Maintenance supervisor]

Rescued another one from the flea market! Looks like a lightly used 10-10 , was missing the brake flag and a screw in the bottom of the foot pad/ handle. Has a decent bar and chain and a spur sprocket, which I usually don't see. So for 50$ its a running and cutting saw.
Now if anyone has a oiler button? I have stuff to trade and $$$s, P.O. told me he had it riding around in the back of his pickup truck.