Filter assembly, from the manual

As you will no, correct lubrication is essential to the long life of your Matchless…or any other bike of course. The 1966 G12 came fitted with the high capacity oil pumps and a return to the tried and tested oil filter setup. High oil flow and effective filtering are just two of the reasons why most later G12 engines have little wear. The first oil filter is in the outlet feed in the oil tank, and consists of a fine wire mesh. Following that there is a cylindrical felt oil filter fitted towards the front of the crankcase; it’s the latter filter we are concerned with here.

Whilst working through my bike, prior to running it, I drained the oil from the sump and tank, then performed the messy procedure of removing the felt crankcase filter; it’s messy because there’s a ball valve to keep oil in place that’s heading for the top of the engine, when it’s stopped. It reduces the time it takes oil to reach that part of the engine; it’s a non-return valve.

Old and new filters side by side

Once the filter assembly was removed, the cavity was carefully cleaned of sludge and the filter felt was washed in petrol. I discovered that the end cap was missing from the filter assembly which means the felt filter would have been sitting at an angle in the chamber, and possibly leaking around the edges. The felt itself looked like 1966 vintage and was thickly matted; it was quite hard to blow air through the filter and I’d imagine at least some of the oil would have found its way around the filter and through the gaps. When I cut the filter open, it looked very home made, but I was later told that it resembled original items.

I obtained a replacement felt filter from Steve Surbey at AMC Classic Spares and found an end cap, part number 01-8120, on eBay which then found its way to my house.

The end cap was a close fit in the filter cavity and wouldn’t pass over the join in the crankcase halves. When I checked, there was a very tiny misalignment of the cavity halves; I’m guessing this is why it was missing.

Main filter assembly

Clearly resolving that would be a nightmare job but as the misalignment was so slight, the outside diameter of the end cap was carefully shaved a little on a lathe; this time it passed across the join and into place.

Examination of the new filter itself revealed wire ‘stumps’ sticking out on either end, which would obviously cause a poor seal at either end. This would allow the oil to bypass the filter.

I pressed some wire cutters on the ends of the filter and cut off the protrusions; the seal at each end would now be good. Blowing air through the new filter was very easy, so it was soaked in oil and the assembly reinserted, making sure the end cap was correctly placed over the end of the felt filter.

If you are fortunate to own a roller starter, I’d recommend you use it with both G12 plugs out, to turn the engine over until oil flows into the tank from the return pipe. Failing that, carefully kick the bike over with the plugs out until there’s a return of oil.

Oil filter end cap 01-8120

When you start the bike for the first time after replacing the filter, let it tick over for a few minutes to make sure the oil has circulated. I can’t find any end caps for sale right now, so grab one when you see one.