Mc 45- at Risk?

[steveohara]

Scott,
I'm curious, is the information you offered in the previous post something you read in a McCulloch service manual or some other printed material? If so, can you post a copy... I would like to add it to my collection of Mac stuff.

With respect to the Mac 45, 75, 90, 100 models that have the partition removed between the third port passage and the main case there would be no pressure delta between the third port passage and the the rest of the case so I'm wondering what would make a/f mix travel in the manner you described in those engines?
Regards,
Steve O'Hara

[Bill Hermsted]

steve-
What is this "pressure delta" ? Does it have something to do with circulation? Scott might be on to something. I forgot about the hollow wrist pin, which was left slightly away from the closed bearing's end cover at assembly. We know that pressure exerts equal force in all directions, but could some sort of mixture flow occur through the pin since it is not a closed off passage like i thought? I've heard that the crank throws whip the mixture into a frenzy inside the crankcase. And maybe at the Mac's high piston speed the pressure or vacuum inside the crankcase doesn't get a chance to equalize in every nook and cranny.
bill

[steveohara]

Bill,
The term "pressure delta" refers to the difference between pressures in adjoining but segregated areas. The most basic example is inside the case and atmosphere outside the engine at different stages of the cycle.
The early Mc engines have a split intake system comprised of a very conservatively timed piston port arrangement coupled with a reed valve assembly. The design is pretty slick in that it allows a modest air flow to occur through the piston port component of the induction system even if the engine is not spun fast enough to create enough pressure delta to open the reeds. It makes for easy starting and better idle and very low speed carburetor metering than designs that are strictly piston port (such as a Yamaha KT100) or strictly reed valve such as all the Mac engines starting with the 45 and 75 and later models.
In Scott's desription it is possible to have a pressure delta that would try to induce flow through the wrist pin and bearing but it would only work in the motors prior to the 45/75 versions. Here is how it could work... as the piston begins to move up the bore from BDC the pressure will start to drop in the case until the delta is great enough to lift the reeds off their seat. During the phase where the delta is growing but is not yet great enough to unseat the reeds air would try to move through the side path on the intake manifold, down through the third port and across through the pin and bearing because the pressure in the case would be lower than atmosphere. In the 45/75 models the partition that seperated the third port (piston port) passage from the rest of the case was removed and with the partition gone there can be no pressure delta between the open end of the wrist pin and the case in general so there would be no flow through the pin and bearing in either direction. It strikes me as unlikely that the McCulloch engineers would effectively eliminate that theoretical means of lubricating the bearing if it was actually something they intentinally designed into the earlier motors. It's also intersting to note that Wiseco started building "9 port" pistons even before McCulloch released the 45/75 models and they were still using the closed end bearing. Of course a "9 port" type piston would effectively nullify the pressure delta just as the removal of the partition does so I'm wondering if they too were of the belief that flow through the pin was a means of lubricating and cooling the bearing? One other observation... some of the Mc chain saw motors have only one very small hole drilled through the cylinder into the third port passage and the hole is offset to the side so that it does not line up with the recess in the piston so it would not allow the kind of flow described above.
I'm no historian on the early motors so I am just thinking out loud here and I would be interested in seeing any service bulletins or other material from McCulloch that addresses the subject if anyone has some.
One thing I do know.... sometimes they wrote stuff that was wrong. Want and example... read the info they released on all the "improvements" in the Mc 70 engine... now that was a pile of you know what!
Steve O'Hara

[Bill Hermsted]

Steve-
Would a very,very slight crankcase leak hurt performance at the higher speeds? If not, why wasn't a tiny hole, like a .030", ever tried in the end of the closed bearing? This might help mixture flow through the wristpin as now there would be a pressure difference as that part of the piston lines up with the exhaust outlet. Maybe there would even be a slight cooling effect. I must sound crazier than ever, but please remember, i'm not a Mac man yet. Did you, Jeff, and the other karters ever think of writing a manual of performance and history? I will be at the top of your "buy" list!
bill

[steveohara]

Bill,

No, a small leak like that would not hurt power... every Mc model made starting with the 91/101 has a built in leak from the factory. A hole in the end of the bearing might allow flow to occur through the pin but is would not push or pull fresh charge through the bearing so I doubt it would do much more than cause a minor free port problem. It doesn't take much of a free port leak to really screw up the idle on a Mc engine.
I worked on a guys motor once that someone had heavily modified with wide eyebrows on the exhaust. The builder had also drilled some holes in the piston about half way down the skirt on the front and rear sides of the piston. Anyway, we went through the motor and everything looked great but the damn thing would race up to very high rpm even if I sealed the end of the carb with my hand. The only way to shut it off was to pull the fuel line. All the normal checks were made, it pressure tested perfect, timing perfect, carb right on the money.
After some time screwing with the thing I was looking closely at the exhaust and discovered that if you put the piston at about 1/8th inch below TDC the very tip of the eyebrows on the exhaust ports slightly overlapped the holes drilled in the piston. When I say slightly I mean that I could get a piece of .032 safety wire through the passage and thats it. If the piston was moved up or down a tenth of an inch the opening was covered. That tiny free port condition was enough to make the motor rev uncontrollably even with the carb chocked off completely.... a pretty interesting discovery! Simple solution.... change the piston to one with no holes and the motor idled just like a chainsaw. So, based on that experience, the hole in the end of the bearing idea would most definately allow air to move and I would expect it to move in when the piston was travelling up and move out when the piston is travelling down. Anything the prevents the air from being stagnant would probably remove heat from the region so it might help that bearing stay a bit cooler but it might screw up the idle as well.
Regards,
Steve O'Hara

[Scott Kneisel]

Steve,
I need to go thru my documents to find where I read that, I don't think it was a service bulletin but I will look. I also have all the early bulletins for the chainsaws, maybe it was in one of those. It did make perfect sense on the 3rd port engines and that is probably why the small hole was there for some of the old 4.9 chainsaws as well. Some of the really early saws; however, had an open bearing on both sides and had a plug pressed into the wrist pin on the exhaust side to prevent free porting. Lubrication on those models would not be any better.

You make a good point though on the engines where the rib was removed and are 100% reed induction. If all is working properly, pressure is pretty much the same at all locations in the crankcase. I suppose there could be some minor flow thru the pin but it wouldn't be forced. That makes that closed end bearing vulnerable on all those engines with the early 9 porting. This is most likely why Mac went to the 9 port pistons I guess. Much more reliable. I think my Mc45 is gonna get a 91 piston.

Thanks as always for your knowledgeable input,
Scott

[Bill Hermsted]

Steve-
I didn't know
that Wiseco was first with the nine port piston. As for the built in crankcase leak on the 91- 101 series, where is it so it can be sealed during a pressure test? Also- were the Quick Time pistons for Macs nine port, full skirt, or both? Thanks again for all the great info!
Take care, bill

[steveohara]

Bill,

The leak built into the later model motors is the duck bill circuit and since we pressure test the motors with the carb off that leak does not effect the pressure test. If we try to pressure test the motors with the carb on we get leakage around the throttle shaft so we eliminated the carb long before the duck bill carbs showed up. Fortunately, the "built in leak" is only one direction as the duck bill acts as a one way valve that only lets pressure escape but will not draw air in.
I can't remember ever seeing a quick time piston that was not also a 9 port design but I can't say one way or the other. Maybe one of our friends here has an old Wiseco catalogue and will look to see if they offered quick time versions that were not 9 port. I didn't do that much with modified Macs back in the early days as we ran the IKF stock Reed classes and the engines were kept stock... sort of
Good conversation guys!
Steve O'Hara

[Bill Hermsted]

Steve-
Is a piston to wall clearance of .0065 to .0075, as stated in this site for the Mc91, good for the Mc 45 with a nine port piston as well?

You mentioned some literature on Mc 70 upgrades that wasn't quite on the numbers. Well, Mac Service Bulletin K 139 says the crankshaft drive taper on all models is not intended to lock into the clutch. The hookup depends on the key and the nut torqued to Mac specs. I guess it's better to lap and make sure the clutch hub has puller holes. I had no options as my crank is the keyless L.H. threaded one. Even though a Power Products enthusiast, my belief is that Mac had the most advanced engineering of them all. This said, could there have been typos in these papers from time to time?
bill

[steveohara]

Bill,
That skirt clearance will serve you well. If you are only using an early exhaust, not an expansion chamber and only running gas the clearance can be a bit tighter, as little as .0055" Also, the minimum clearance you choose should be based on the brand of piston you are using. The OEM McCulloch pistons have very little or no taper and need more clearance than the Burris pistons. Terry Ives has had a run of pistons made that I believe are identical to the Burris versions. Most folks don't run the vintage engines all that hard or put a lot of miles on them so there is really no reason to push the limit on clearance. Too little can ruin your day but a couple thou too much only makes a difference when you have worn the piston out.
About the clutches... I always lap my clutch hubs to the crank I'm using to assure clean contact over at least 80% of the surface area. With the keyless version it is very important to make sure the fit is real good.
My comment about the literature on the Mc 70 is not about typos... it is just funny to go back and read all the promotional material describing all the "improvements" they incorporated into the motor that they quickly abandoned with the next model.
The 40/70 intake manifold is a perfect example... impressive looking but junk when it came to performance.
That being said... I agree that the McCullochs were the best of the various motors throughout the 60s although there are some folks that loved those WBs and I don't mean to offend any of the WB lovers
At least not here.... I save my offending for the vintage races where my stock 101s always blow all the 820s off the track!
Steve O'Hara