7hp Main bearings

[Dhbangham]

Having measured that I have 8 thou end play on my crank shaft, when measured without any gasket or sealant, Rob Abineri and my self investigated how this might be adjusted. On close inspection, we found a shim washer between the flange of the white metal main bearing and the rear conical bearing support (end plate). We therefore put the end plate in the freezer and when frozen, warmed the aluminum support and the main bearing pushed out. The bearing is indexed with a cutout, so will go back in the same orientation. I will now make a shim washer 4 thou plus the thickness of the gasket paper and install that washer behind the flange.
I am also pretty sure that the main bearing is exactly concentric inside and out, and one can rely on the original line boring of the aluminum crank case to align the crank shaft. This means that it should be straight forward to make new main bearing and install them without a line boring machine. I will report back if I am successful.
Update on 6th Dec. measured my gasket material at 10 thou, I want to reduce the play by 4 thou, so 10 + 8 -4 =14 thou. I rolled some brass sheet to 14 thou, cut a washer from it, froze the end cone, removed main bearing, re-assembled with new washer (and original) and 10 thou gasket for the cone and and I have 2 thou endplay. So something went slightly wrong. There are two possibilities, a) the sheet brass I made the washer out of was thicker than I thought, the other b) is that is the bearing did not seat perfectly back into the cone. It was touch and go as if I had aligned the lug well enough, so next time i am going to score a line down the outside of the bearing, to help alignment. By the end of the day we had re-assembled with Lucas assembly lube and grease on the 10 thou flixoid gasket for the cone, and the whole engine locked up! I lost confidence in my workmanship and dismantled it again this morning and re-measred "dry" and it again floats 2 thou. The important lesson is to follow measure dry and maybe not use assembly lube on big bearing, but save it for tappets etc.
10th Dec, I can confirm that the journals are offset towards the center of the crank and the con rod have a kink in them to compensate. The con rod will work in either orientation, but the small end won't be near the middle of the piston.

[Dhbangham]

I have just made a new Main bearing for my spare 7hp engine. Casting the metal, used the same principle as casting the big end bearing, Machining the bore and outside, required many "spring" cuts. Spring cuts are when you machine the component using exactly the same dial setting as the previous cut. Unless you have an perfectly set up "tool makers lathe" each machine will have small amounts of flex and wear, which allows the cutting tool to deviate from the specified dimension. By re-machining on the same settings, each pass will put less stress on the tool and machine and it will cut closer to the set dimension. Cutting the main bearings needed 2 spring passes on my machine to get it exactly to the desired diameters. As you can see from the photos, I bored the bearing first, then mounted the crankshaft with bearing, on the lathe and machined it on the crankshaft. I stopped the bearing rotating by using a drive dog, which I cable tied to the crank web to keep it firm.

[Dhbangham]

At last the broken main bearing is made and fitted. Next I tried fitting the conical aluminum bearing support for the back end main bearing. I had noticed that removing the cone was a monumental struggle when taking it apart, so I was not too surprised that it was difficult to re-assemble. I backed out the bolts that are screwed into the crank case and hold the cone support in place and on inspection, it was clear that either the crank case was distorted and causing the housing to out of round, or the conical aluminum main bearing holder was out of round. Since the easiest part to check and machine was the conical support, I mounted the crank and support on the lathe between collet chuck and center and found the housing was distorted. So I machined both the mating face and the location surface until it fitted the crank case.
I could then re-assemble the crank with the original big end bearing in the conical support, but it was not smooth, or comfortable. It would "stick" when left for a few minutes, then if you got it going, it was reasonably smooth. So I googled the topic and found to my surprise, that old white metal bearings can, suffer from micro cracking of the surface, which in turn allows oil to be squeezed into the crack and tighten the bearing. So I made another main bearing. It took three attempt to cast the second bearing, as I was in too much of a hurry and made mistakes.
The new bearing worked perfectly.