Thread & hex data

 The following page provide descriptions of the threads in common use; the drill sizes, pitches and hex sizes associated with them, and a few interesting tit bits of information...

• Reference Page

It's handy to have it in the blog - makes it easy to find when you are in the workshop.

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.

SQ4 - rocker shaft seals

 These wet black rings are the fibre washers that seal the ends of the rocker shafts. They get very hot and are subject to oil seepage from the rocker box. 

I need some new ones.

They are not a standard fibre washer - they have a large outer diameter of about 3/4 inch but the hole in the centre is for 5/16 inch - unfortunately they are out of stock at Draganfly:

So here are the bits we need. At the top of the picture there is a 20 mm punch which I have turned from bit of handlebar on the mini lathe. Below that is the special washer and the nut that fastens the rocker shaft in place; top left is the punch used to make the central hole. The new washer is to the left - it's the grey one. 

Below is the mangled original.

SQ4 - sump plate

 The sump plate looked a bit mangled when I took it out after the oil was drained.

After degreasing I cleaned it up using some clock cleaner which is an ammoniated fluid used for stripping the corrosion from brass and copper. It works very well on the gauze.

When I got it clean, I manoeuvred the washer that stiffens the hole for the oil pickup pipe out of it's wire prison. I managed to get it back into place and squeeze it tight to retain the gauze.

With that done I could think about refitting the sump plate. I destroyed the gasket completely when I removed it so I needed to make a new one. 

With a sump plate it's easy to cut a piece of gasket paper to the right shape and to mark the holes.

Punching the holes is the easiest way to make them tidy and I used the corner of the sump plate to complete the ring of gasket around the holes.

I cut the centre out with a craft knife - it will be big enough to use for something else.

When I drained the sump, there was around a cup full of oil coming out and of course the gauze was damaged. The pickup pipe was the cause of both of these problems in that it was out of position - too high and off centre. It's just a copper pipe retained with a bracket inside the crankcase so it's quite easy to bend it into the right place.

With some Threebond 1215 the sump plate is back on and the plug fitted.

FH - building up the miles

 This week, we are mostly decorating in Cromer! The FH now has 205 miles on the clock, and is still smoking a tiny bit and leaking a bit of oil, possibly from the head gasket.

Here it is, collecting caulk from Screwfix and CO2 from Argos...

I've checked the points today and am pleased to record that they haven't closed up.

I also checked the QD hub nuts after an unusual sound began from the back - they had loosened off a lot.

There's no picture, but yesterday I rode a couple of miles along the main road at 1/4 throttle, killing the engine and coasting to a halt. The timing side plug was brown on the centre electrode, black on the outer. I won't be making any carburetion changes for the moment!