SQ4 - under the tank

 I've taken the opportunity to remove the petrol tank because it's time I did the valve clearances and had a look at a persistent oil leak that has been going on for years. I thought it was fixed; I changed my mind and decided it was a head gasket problem - but then careful observation of the rocker box oil line revealed some seepage. It's obviously been quite bad in the past at some point, before I fixed it last time, because there's oil on the steering damper plates under the steering head. There must have been quite a jet to get it out there. 

Some years ago I did repair a leak here but it appears I didn't do it very well:

When I took this apart it was clear that they was some oxidization within the joint which had prevented the solder wicking down inside the fitting. I cleaned that up with wet and dry and tinned the pipe with some solder, using plenty of flux to sweat the joint into place. It resists that twisting now. 

The next job is to do the tappets and pulling off the rocker covers reveals some lovely clean valve gear. 

These are all supposed to be set to 0.001" with the engine cold. I used the famous 'Rule of Nine' to set them up. 

Valve clearances all proved to be very close to where they should be - I adjusted a couple of them but most of them were ok. I moved on to the spark plugs - cleaning them, checking their gaps but again they were all ok.

This one is from cylinder number 4, but it's fairly typical of all the others. You might think it's a little dark on the centre electrode and indeed we might consider adjusting the mixture a little. We could put another washer under the float valve to lean out the mixture across the whole rev range.

I took the opportunity to change the thumb nuts holding the HT cables to the plugs for some larger ones that I made some years ago.

With the rocker cover off I took the opportunity to snug down the head bolts and nuts. This is quite straightforward except for the pair that are buried in the fins - these two at the back are particularly difficult and you have to take the carburettor off to get at them. The one on the timing side of the bike is almost impossible because the casting has not been cleaned up sufficiently for you to be able to get at the nut.

Next time that I take the head off, there will have to be some grinding in there.

Reassembly was quite straightforward, apart from getting the oil tank back in. I've now 'adjusted' the area above the engine plate and below the frame rail with my favourite (small) hammer. It goes straight in now, but you have to remember to take the oil supply/return manifold off the engine first. You can't get the oil tank bottom bracket past the oil lines with the lines in place.

Rule of Nine

Many moons ago, I had a burning interest in classic cars, many with British Motor Corporation 'A' Series engines. One of the useful tips I learned working on these was the Rule of Nine, a way to easily make sure your engine was in the correct position when adjusting valve clearances.

The thing is, when you adjust the clearance in a particular valve train you need to make sure the cam follower or tappet is on the base circle of the cam, or the clearance will be excessive. You do this by checking the clearance on both valves in a cylinder when that cylinder is at TDC. The thing is, wild valve timing or quietening ramps in the cam profile means you have to be accurate when positioning the engine.

But there is an easier way. Have a look at this camshaft:

That's a spare camshaft for my Square Four - a lot less worn than the one in the engine. On the right hand side of the picture, you see the thread retaining the cam chain sprocket, and that first cam is the exhaust cam for number one cylinder - notice it is pointing straight up and the valve will be fully open.

Here's a Stormy Petrel's eye view of the rockers, so you can see what I am talking about:

Move to the left hand end of the camshaft and we are looking at the last or eighth cam, the cam for the exhaust valve on number three. It's pointing straight down, which means the valve is fully closed and the tappet is on the base circle.

Now look at this picture. The cam that is pointing up is the inlet cam for number two cylinder - it's the fourth cam on the shaft.

The one pointing down is the fifth cam on the shaft, the inlet cam for number four cylinder, so

4 + 5 = 9

Remember the first example where we looked at the first cam and the eighth cam? Well:

1 + 8 = 9

Guess what? It works on the other valves too! So, if the valve on the second cam is fully open (which is very easy to discern) then you know that the valve on cam seven (the exhaust valve for number four) is on the base circle and can be adjusted.

Easy. The Rule of Nine.

By the way, I found that after 1000 miles, most of the clearances needed adjusting - maybe because I didn't do it very well last time.

Finish off with replacement of the rocker box gaskets with Threebond. The fibre gaskets specified by Ariel are not suitable for the sorts of temperatures the cylinder head sees.

State of our Roads

There are some outrageous pot holes around here. There's one I always forget about on a hill nearby, which has been filled in but is now sunken again making a smooth but significant hole.

So sunken in fact that Amelia's forks haven't recovered yet:

I think we have a fork bush clearance problem!