Strange discovery on Mc 7 restoration

[steveohara]

I was assembling a Mc 7 today that I'm building up from parts gathered here and there and found something very surprising. After installing the crank it had a lot of drag so I was looking very close at everything and noticed that the PTO side crank journal seemed to be offset to one side in the seal. It was far enough off center to nearly touch the metal part of the seal on one side.
Took the crank back out and removed the bearing and the bronze sleeve and started measuring everything.
To my surprise the bronze sleeve is way out of concentric.... one side is .030" thicker than the opposite side!!! That means the hole for the bearing is off center by .015"!
So I'm thinking this must be an anomoly and I go to the parts drawer to look for any bronze sleeves I have saved out of junk blocks and I find two of them. One is off center by .008 and the other by .003"
The only way this makes any sense is if the bearing bores are machined after the bronze sleeves are pressed into the block??? Any old timers here know any of the manufacturing history that might explain why they made them that way?
Anyone else notice the same problem? This obviously means each bronze sleeve is only useful in the original block it came in and if you remove it you better put it back in the same orientation as it was before removing it.
Steve O'Hara

[Terry Bentley]

I have the same problem with a Mc 7 parts donor. I stopped at that point after noticing the bushing was running out about 0.020". I was changing over a Clinton A-400 to needle bearing on mag side, but needed a thrust face against the crank. The Mac looked perfect with the PTO needle bearing, until I checked the runout. I also thought thru reasons why this to be. But the problem then comes about for the seal. It doesnt last long if that much off center line.

[Rob Voska]

Even 91's are that way. I noticed it by eye & got to measuring some bushings I got in a pile of stuff & they were all out different amounts so after that I kept a watchful eye on them & never take them out.

[steveohara]

I've never had a reason to remove one before and always assumed they were concentric parts made to be interchangeable. I would not have removed this one either if not for the visible misalignment of the seal to the crank.
I line bored the block to the crank centerline with that bushing in there so the train has left the station on that and it will have to go back in the same position is came out.
I will have to bore the seal pocket oversize and sleeve it and then re-bore it concentric with the ID of the bushing
Steve

[Rob Voska]

OUCH !

[Scott Kneisel]

Yup,
I ran across that last year so now I never press the bearing out from the outside of the block.

So I had a couple engines with the bronze pressed out and had to develop a method to get it back in place.

I made a tool using a good tight side cover from a super engine, made a "solid bearing" the same size as the 6203 and pressed a stubby saw crank into both. After installing the side cover and crank I used a ball gage to measure the gaps at 4 quadrents around the PTO bore between the 3/4" crank boss and the bore and recorded the thickness and position.
Then I took the bronze bushing and measured the thickness at many points around the OD. To my delight, I found only one combination of the same 4 measurements I made on the bore. Bingo, I pressed in the bronze in the correct position and I had correct alignment. I did this on both engines with success so I keep the tool handy just in case I run into more.

Give it a try it really worked.

[Rob Voska]

Steve after thinking about it for a few minutes even though you line bored it to the CL of the crank the flywheel side bearing will still be in a bind. You don't have a true crank CL to go from anymore. Also when you start thinking your only off @ .010 on the bearing when you extend that dimention to TDC the piston will be off a lot more than that. Besides that even the bushing will be in a bind.

I think you need to make a new undersize bushing up & bore it after it is installed to the CL of the side cover side of the block to get the crank to align, then rebore the cylinder it so everything is straight & true. Or just add that block to the oops pile.

[steveohara]

Rob,

After taking some measurments it's clear the factory must machine the side cover details first and then the PTO bore in the bushing is cut to line up with it and they don't worry about the misaligment of the seal to the crank. I have tooling similar to what Scott described and the bushing has to be cut the way I found it in order to have the bearing hole line up with the dummy crank and side plate. In other words, the machine work for the side cover is really the source of the misalignment. Fortunately I marked the block before I removed the bushing so putting it back in the same position was no problem.
The bore job was done square to the crank so any correction of either side of the crank mounting surface would change the relationship of the crank center line to the bore and the bore would have to be redone square to the crank.
Here is what I find odd... the machine work on the PTO side of the block is all concentric and the crank centerline dimensions measured from the bottom of the block are identical with the bushing removed. On this example the thin side of the bushing is at between the 1 and 2 oclock position if you are looking at the pto side with the carb at 12 oclock so there is very little effect on the crank alignment to the bore in the plane that matters.
The correction on this block is needed because the machined surfaces on each side of the block are not parallel to each other!
These old models are new to me and I have found a lot of odd stuff that never showed up in the 91B1s and 101A and later models that I did all of my serious racing with. Maybe I was lucky and never got a screwed up motor in the later models.
Steve

[Rob Voska]

This is starting to remind me of a TKM BT82 engine a guy was having trouble with. Someone had put bearing pockets into it & squished it tight together. I try to forget but as I recall it was about 6 hrs of indicating things & 20 minutes of machining to get things back in line. Oh did I forget to mention I cussed under my breath the whole time....lol

[Terry Bentley]

I checked over some early kart, saw, and 90 series motors when I realized this problem. The early ones as you described were definitely off more than later models. BUT what I did notice was the seal bore. It is smaller for the interference fit of seal and not concentric with the bore in block for bushing. It is concentric to the bushing ID tho. At least on the ones I checked. Om the 91's I checked the amount is barely noticable. There is 0.005" difference between seal bore at 1.119" and bushing bore 1.124". I am guessing the block is indexed off side cover face and bushing is machined to finish size along with the seal bore. This is just based on what I have found on the half dozen different blocks I checked out so far. Last year I was rounding up parts to assemble a Mc 8, and looking at what it would take to convert Clinton A-400 over to bearings on crank, instead of bushings. I realized the bushing not being concentric and parked everything until I looked further into why it was that way. On blocks that bushings are severely off center, it may be worth machining the block out large enough for a full cleanup. Then make a bushing long enough to fit all the way thru bore and for seal also. Then the bushing would not have to be indexed if ever removed.