Read this first
I cannot justify doing what I'm taking on here. I have time, it's a puzzle and a challenge, I'm stubborn and I like exploring. The rational first option would be to buy new shims. Second option is take them to an engineering workshop & get the shop to surface-grind them to the right thickness.
The whole thing started when I checked the timing. I had to take the cover off to fit the dial gauge holder in the spark plug hole in order to determine that magical 0.24mm BTDC on No1. I thought, since I have the cover off may as well check valve clearance. The truth is, the bike was running okay except for a too-fast idle I was trying to find the reason for, and I had not intended to check clearance. When I did check I was a bit shocked at how tight the valves were.
The whole thing started when I checked the timing. I had to take the cover off to fit the dial gauge holder in the spark plug hole in order to determine that magical 0.24mm BTDC on No1. I thought, since I have the cover off may as well check valve clearance. The truth is, the bike was running okay except for a too-fast idle I was trying to find the reason for, and I had not intended to check clearance. When I did check I was a bit shocked at how tight the valves were.
Valve Clearance
At more than 80 000km one would expect to have to do valve clearance adjustment.
Inlet valves were all too tight, three of them were below the 0.15mm minimum. The exhaust valves were within tolerance. I had a shock when I determined the price for shims. So I talked to guys at the club and they said, lap; use a fine grit water paper on a sheet of glass. I did. And ended up with a shim that was thinner at the sides than in the centre. Fortunately I messed up only the one side; the other side was still flat. So, I have to do some surface grinding. I could take it to Johnston's but it was a long weekend. So I decided to experiment. The following topics are not in sequence yet. I may rearrange the steps at some stage. |
The Angle BracketThis is just a piece of angle iron mounted on the cross slide where the tool post normally sits. I'm not concerned about accuracy of the vertical right angle at this stage, the right angle between the face of the angle bracket and the top of the cross slide - the next topic will explain.
I am concerned about the horizontal right angle though, the right angle between the face of the angle bracket and the lathe ways. I used a dial gauge mounted in a steady to get this angle as close to 90 degrees as would make precious little difference over the width of the shim. |
The grinding wheel
I figured I could mount a straight cup grinding wheel or a tapered one in the lathe chuck. The shim would go on a right angle bracket on the cross slide.
I did not have a cup grinding wheel, but I wanted to start experimenting immediately. I did have a recessed centre 100mm diameter angle grinder disk. |
How to hold the work
The shim is about 2.5mm thick. I'm going to attach a suitable strip of steel to the angle bracket. Then take a few passes with the grinding wheel to get the face of the strip aligned with the working edge of the grinding wheel.
Epoxy the shim to the strip and reattach it to the angle bracket. This photo shows the strip and a experimental "shim" the shape & size of the real one - the real one don't have a hole, obviously. |
The Experiment
This photo shows the setup for the experiment. The wet cloth is to catch as much grinder dust as possible. The experimental shim is positioned so that a pass involves the full width of the raised front edge of the disk across the whole of the shim.
I have to pause often to allow the work to cool off. I don't want heat to build up to a point where the epoxy fails. Generally epoxy will fail at about 80 C. |
Proof of concept
Here's the experimental shim after grinding, still attached to the holder. To remove it I heated it slightly and it popped of easily enough.
The experimental shim after lapping with 220 grit followed by 360 and then buffed. It looks good. I checked it with a vernier at various points and I think it is okay, thickness variation is of the order of 0.02mm. I want to still check it with a micrometer, but I figure the concept is proven. |