I have intentionally not disturbed the cambox up to this point in the engine build. Why? Well, because it appears to work just fine and there has been so much else to do but, as now the engine build is nearly complete, it was time to look inside.
This is what it looked like before stripping.
However, before dismantling the cambox I figured it would be a good idea to check the “as-received” valve timing. The relative valve timing, ie the relationship between the Inlet and Exhaust is determined by the position of the gears on the camshafts at the ends relative to each other and I have assumed that the setup reflects how it was run on the engine.
The absolute valve timing (ie the actual valve timing) is governed by the positioning of the bevel gears. Now, the large top bevel gear that is on the end of the shaft driving the other gears – part number K-18, has 44 teeth. One complete rotation of this shaft equates to 2 revolutions of the crankshaft and so the crank-angle change that would result from changing the engagement position of this gear by one tooth would be (3600 x 2) / 44 which equates to ~160 crank angle. This would be a very significant change and this helps in determining the absolute timing by avoiding unfeasible extremes.
After a couple of changes to the relative positions of the bevel gears I came up with the following valve timing:
IVO 600 BTDC
IVC 700 ABDC
EVO 700 BBDC
EVC 300 ATDC
The 2 more reliable figures are EVO and IVC as the other readings can be affected by the loading of the other cam during valve overlap.
These can be compared with valve timings on other engines
and also indicates that changing the bevel gear by one tooth would put the valve timing outside the range used on any of the engines listed above.
After removing the outer steel plate supporting the intermediate gears all of the gears can be seen more clearly.
I have marked the tooth engagement of all gears (and also the bevel gear engagement) so that they go back in exactly the same place. The position of the intermediate gears is not actually critical as they have been set up with hunting teeth. The number of gears from the camshaft on one side to the camshaft on the other side is:
30 – 31 – 32 – 30 – 32 – 31 – 30
After removing the gears (which are all bushed with an oilway) the main structural support plate is revealed.
I initially wondered where there were 2x 2BA threaded holes in the plate that seemed to have no obvious purpose but it soon became clear; they were there to be able to insert 2 screws to lift out the steel plate from the aluminium casting.
After removing 4x ¼” BSW Allen screws and heating the entire structure, the plate plus everything attached to it was separated from the outer casing.
Both cams had minimal wear
but I replaced the balls (10x 7/32” diameter) in 2 of the Hoffmann bearings
which were showing a bit of play. I measured the base circle diameter at 1.008” and the lift + base circle came to 1.315” which gives a valve lift of ~ 5/16”.
All of the parts were in excellent condition, however, there was one conundrum, namely: what was the lubrication strategy? There are clearly 3 different ways that oil can flow in or out of the cambox. The oil drain is obvious – the pipe at one end of the cover, shown in the picture below.
An oil feed to the cams is also obvious, indicated by the arrows in the picture below.
There are quills of ~2mm diameter at the ends of pipes that would supply oil directly to the cams. However, it is questionable how much oil would be delivered using this method; the oil would have been provided from the upper bevel chamber via a slot in a K-12 phosphor-bronze bush in the same way as a standard “K” cambox oil feed and although there is an oil seal in the centre and immediately behind a bearing (to prevent the oil simply discharging into the cambox at the point of entry). I am sceptical that this approach would provide the amount of oil required to lubricate the cams effectively and, indeed, the entire gear train. This also indicates that the cambox was originally designed to work with an internally pressurized system rather than a targeted distribution approach.
I suspect that whoever designed the cambox must either have had the same concerns or, alternatively, the cambox started life on a pressurized “K” type system but transitioned to a strategic lubrication system (as on KTT 581) because that would explain why there are oil feeds that are positioned at the ends of the cambox. Alternatively, these could have been additional oil drains to lower the level in the cambox when the engine was stationary to prevent it leaking away via the pushers and covering the cylinder head in oil. The inside and outside (it is a 1/8” BSP thread) of one of these is shown in the picture below.
With the lubrication system that I have decided to use, the central feed system is redundant – the upper bevel chamber will not be pressurised and, instead, I will introduce a quill into both ends which will be fed under pressure directly from the oil pump.
Similarly, the upper bevel will also be provided with a direct oil feed ….more next time.
No comments:
Post a Comment