Main Bearings

The main bearings need to be in-line and parallel in 2 axis to allow the crank to run smoothly. This is best achieved by boring the bearings in situ. Before this can be done the basic shape of the 2 bearing halves has to be machined leaving just the final honing to be done in-situ at the end.

First the 2 bearing tops were placed in the 4 jaw (one at a time) and the lower face was turned to give the final thickness of the bearing mountings.

Skimming the lower face to thickness

Next the 2 halves were stuck together using double-sided tape (solder can be used here if you prefer) and set to run centrally in the 4 jaw chuck as shown below. In this setting the round end of the bearing was turned to size and the shoulder of the bearing clamp was also faced square. Next the assembly was placed in the 3-jaw chuck clamped by the round profile just turned. The second shoulder was now turned to size and the other side of the bearing clamp faced square as before.

These parts were now put to one side and the bearing bases were addressed. These parts were clamped and turned in the 4-jaw chuck to make perfect 6 sided shapes. I made a mistake here and turned the height to the height of the bearing housings in the sole plate. However I should have left extra material to form a shoulder on both sides at the bottom. I decided that this wouldn't matter and continued with the parts.

Next the top and bottom halves were made into pairs and again stuck together using double-sided tape. Back in the 4-jaw chuck they were turned as a pair and then centre drilled and drilled undersize to 3/8" diameter.

At this point the thickness of the bearing was set. I made the shoulders on the outside the same dimension but had to carefully measure the inside shoulders and make them to suit the off-set of my engine standard.

Next I took each pair in turn and clamped them on the vertical slide as shown. Here I drilled the holes for the oil cups and then co-ordinate drilled the 2 clamping bolt holes.

Drilling and tapping the bearing cap

The vertical slide was then clamped in the height setting and the lower bearing was turned to face the chuck. The channel was then milled in the sides of the bearing using a milling cutter of 15mm diameter. Equal amounts were taken from each side. If you have made you bearing correctly then you will also need to cut the channel in the bottom of the lower bearing component.

Cutting the seat in the lower bearing

Finally the bearing was assembled in the sole plate and the 2 holes drilled and tapped to take the 2BA stud.

Bearings assembled in the sole plate ready for boring.

To get bore the bearings to the final size and to ensure they would be totally in line a small 'boring between centres' bar was used. To do this the sole plate with the bearings screwed in place was packed to the correct height and adjusted square. However the deflection on the small boring bar made it impossible to get an accurate size and so in fact the finishing was done with an adjustable reamer.

Once the axle was a tight fit in the bearings, the con rod and axle were put in place and bedded in by hand using Brasso. Once a tight running fit was made the con-rod was removed and the lathe used to bed in the axle to a smooth free-running fit.