Thursday, 15 October 2020

W/NG - Assembling and fitting the head

I finished the last post with the cylinder head paint drying, and I went and had a day on the railway which I recorded in this blog post so that now, the paint has been curing for a couple of days.


It's dry but not very hard. I scrape the paint off the gasket faces with a Stanley knife blade, and set about cleaning the bolt holes with a 5/16" plug tap. Apart from this damaged one, they are all in pretty good shape.


Next stop, put the valves in. We know that these fit, because in the last episode we ground them in so this time it is a matter of fitting the valves, Halite washers, spring seats, springs, spring tops and collets. Collets are a bit fiddly to get in, so the best thing to do is put a lump of grease on the stem and then introduce your spring compressor. When you have the spring compressed, fit the collets with a small pair of pliers, tweezers or a chopstick with a bit of grease on the end:


Push them into place with the stick and slowly, gradually, gently release the spring compressor. Tradition tells us to tap the valve stem with a hammer to make sure the collets are seated and the valve/spring assembly is secure.

Next, check the head seal to the barrel. these engines have no head gasket, but the barrel is machined with a spigot which is ground into the head with grinding paste; the area around the spigot is machine a few thousandths deeper so the pressure from the head bolts is born by the spigot alone, sealing the joint. When you are happy that you have a clean joint all the way around the spigot, you can think about putting the head on - but before you do, you need to sort out the pushrod tubes and there seals.

This is the bottom seal - the old, compressed black one and a new one from AOMCC Singles Spares:


Here's the top seal. You see there is a shorter spigot here, which fits around the spigot pointing downwards in the head. I've assembled these dry, with no sealant:


With the pushrod tubes standing in the crankcase, you can drop the head in place, watching the pushrod tubes settle onto the spigots at the top. Next you can drop the 3/8" CEI head bolts in - which I have previously cleaned on the wire wheel. Tighten these down firmly - there is no recommended torque figure. when that's done, the pushrods can go in. When I dismantled the bike, I stored the inlet and exhaust valve gear separately, so I could ensure they all went back in the same place.


Next I took the opportunity to refit the exhaust, with Threebond 1215, and the carburetter - again with Threebond and a new gasket.


Don't forget to put the little caps back on the valve stems:


Next, you can seal the new gaskets (I'm using Draganfly's new aluminium gaskets) and fit the rocker boxes. Be careful with the bolts - three are the same length (about 2 5/8") but one, the one above the pushrod, is longer - 2 7/8". the one shown in this picture is wrong - it's a short one. The danger of this is that you will pull the thread if you overtighten it.


This looks easy doesn't it! What you can't see in this picture is that I had refitted the exhaust rocker box without aligning the rocker in the top of the pushrod. Later, when fitting the decompressor I found it wouldn't work, and I had no compression. What was happening was that the misplaced rocker was holding the valve open, and preventing the eccentric pin on the decompressor reaching the pad on the rocker. Secondly, one of the long rocker box bolts is missing from the exhaust rocker box, so that's two I have to find.

I've now managed to fit both rocker boxes without missing anything out! I've added the rocker oil line, and this time I have fitted a cable tie as shown in the parts book for this bike - it's very unusual to see these fitted.

There is still a rocker box bolt missing (two actually), which I will have to make. I've used the opportunity to introduce some oil into the rocker box through this hole - this is not normally possible, but recall that this hole has a chunk broken out of the head and so provides a passage from the bolt hole into the head and down into the pushrod tube. The objective is to get some oil onto the camshaft.

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