Thursday, 3 December 2020

The Cylinder Heads

There were 3 main tasks to prepare the cylinder heads:

1)    The cylinder head for the rear cylinder needed an exhaust port that exited on the left rather than the right side.

 2)    Some broken fins needed repairing

 3)    New valves/guides/springs and valve-seat inserts needed fitting and machining.

 Luckily, there was a straightforward solution to #1.

Velocette succumbed to the fashion for twin-port motorcycles and produced the KTP in 1930/31; the bike was not a success and they soon discontinued production. However, for my purposes, I am extremely grateful that they had this brief excursion into making cylinder heads with 2 exhaust ports as this has provided me with a straightforward route into making a cylinder head with a single port on the left side.  By chance, I had acquired a KTP “project” some years ago and as I didn’t plan to rebuild the engine with its twin-port head I had a spare KTP head for this project. The solution, quite simply, was to chop off the right port, insert a blanking piece into the hole and machine some “fins” on the outside.

The first step was to use a disk cutter to crudely chop off the port and a few protruding fins to make sure that this solution would allow a carburettor to be fitted on the front cylinder head.

It seemed that I would be able to fit a carburettor and the next step was to make a steel blanking piece and insert it into the hole. The blanking piece was, at this stage, secured in the hole with JB Weld. This was never intended as a permanent fix, as will be seen, but solely to secure it sufficiently so that the combined cylinder head and blanking piece could be machined without the latter moving during machining.


After rounding off the edges, the insert (which is mild steel) was laser-welded to the cast iron cylinder head by EMP Tooling.

At the same time, some fin repairs were made. The process here is to shape pieces of cardboard that fit correctly, cut out the “fin pieces” in mild steel and then laser weld these together.  I have, in the past, welded pieces of fin to cylinder heads but I do not enjoy welding cast iron and even with pre and post heating in an oven I have found it difficult to avoid cracking somewhere along the weld. The laser-welding of the fins is much stronger and more elegant that I could ever achieve.

The method of sealing the cylinder heads to the barrels is a “ground fit”. The time-honoured method for preparing this joint is to put coarse or medium grinding paste on the main flat face and fine grinding paste on the top of the spigot on the barrel and to then rotate clockwise and anticlockwise, by hand, the head on the barrel. In this way, the intention is to get the face on the top of the spigot to seal yet the head will still “pull down” onto the barrel when fully tightened. This does seem to work fairly well; other vintage bikes use the same approach and I have good sealed joints on Nortons and AJSs for which I have used this.

New valve guides, valves and valve seat inserts were put into both heads and then rebuilt with new springs, spring retainers and collets.

A few interesting points came to light whilst working on these heads, namely that, apart from the obvious differences of one versus two exhaust ports, there are some other differences between KTP and single port heads/barrels, namely:

The distance from the top of the valve to the machined face for the spring is less on the KTP head, so much so that I had special valve springs made to reduce the otherwise excessive spring pressure.

The spigot protrusion on the top of the cylinder has different heights: 0.25” on the KTP and 0.33” on the single port head with correspondingly different recess depths in the heads.

The inner diameters of the heads are different – the KTP is ~3.5mm greater diameter – see picture below.  This has consequences for the compression ratio ….more of this later….

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