The distance between the centres of the threaded holes in the crankcase and those of the cylinder/cylinder head/cambox are different – the distance-between-centres on the DOHC engine is smaller.
As I mentioned in a previous blog, it seems that Velocette solved this by using eccentric studs to change the bolting pattern to allow different barrels to fit onto a standard set of crankcases. Clearly, this method could also be used to move the centres outwards rather than inwards. Ivan Rhodes sent me a sketch showing exactly this solution that was applied to one of the works DOHC KTT engines.
Ivan also mentioned that he had acquired a large box of eccentric studs when the Velo Race Shop closed down 50+ years ago. I have never seen any of these “in the flesh” but a reader of my blog kindly sent me a picture of a bundle of these (thank you Paul).
Although I do not yet know the origins of the DOHC engine that I’m rebuilding, the same solution was applied, evidenced by the “pear-shaped” holes in the base flange of the barrel which are required to accommodate the change in section.
I have carefully measured the distance between the centres of both the crankcase studs and the 5/16” diameter holes for the cylinder head retaining bolts. Allowing for some manufacturing errors of a few thou, the dimensions are shown in the picture below.
The resulting design for the studs to give this offset and with a 5/16” BSCY female thread in the top 1” for the cylinder head bolts (the same thread as used on the “K” engines) is as follows
and would be made in EN24 steel.
I chose to make these from a 1m length of 20mm diameter bar which allows the 4” long ½” diameter section to be turned on-centre
An additional 1” length has been added to the unmachined section for which the reason will soon become apparent.
The question now arises as to how to machine the off-centre section accurately? There are a number of different ways in which this could be done and I have chosen the following approach to set up the 0.18” offset in the lathe.
The part-machined stud was put in the rotary table chuck in the milling machine (that had been centred) and then moved off-centre by 0.18” and a 0.75” deep ¼” diameter hole drilled into the end.
This was then put into the 4-jaw chuck in the lathe and a close fitting ¼” silver steel dowel in the tailstock chuck was used to adjust the position of the stud until the hole was on-centre.
This can be “felt” (and seen when it is rotated) accurately and small adjustments made until the dowel slips in easily. Unfortunately, it was only possible to hold the workpiece in the chuck using the 20mm section rather than the much longer ½” section to avoid the jaws interfering with each other. Other checks were made to ensure the workpiece was parallel to the lathe bed. This is a very substantial chuck and although there is less than ½” in length on which the jaws are in contact there were no problems in carrying out the subsequent machining.
The entire section was turned to give 3/8” diameter
before sawing off (in-situ) the end section with the hole and putting a 3/8” BSF thread on the remainder.
Three more were made
and the 4 then inserted into the 3/8” BSF helicoiled threads on the top of the crankcase.
And then the acid test ….would the barrel slide over the studs?
Success! After positioning each stud in the right place, the barrel slipped down without a problem. It would not go any further at this stage and required the “pear-shaped” profile on the yet-unmachined round transition section to be completed before it would seat at the base.
The pear-shaped section is essentially the merging of 2 different diameters – the ½” and the 3/8” sections. Most of this was done on the milling machine
and then finished by hand.
After a few hours of machining and fettling there were 4 studs with the required section,
all of which fitted inside their holes in the bottom of the cylinder.
After juggling the position of each – small changes to their angular positions,
the cylinder could be dropped down all the way.
This, at least, showed that the calculations and the machining were OK but I could see that trying to simultaneously get all the studs to be screwed in fully “home” and the cylinder to seat on the base flange would not be easy.
I had, in the meantime made a better estimate of the distance by which the cylinder barrel would need to be moved upward to avoid piston-to-head contact near TDC and decided that a 2mm thick steel spacer plus two 1mm paper gaskets either side should be sufficient.
The spacer was made from a square piece of stainless steel with the main hole for the barrel machined on the lathe
and the stud holes drilled on the mill.
A gasket was made and I then started to fit the barrel.
A couple of issue now arise. The first, and one with which I was aware of before I started, is that when each stud is screwed down onto the surface the orientation is unknown ahead of time – you can guarantee that it will be pointing in the wrong direction! This can be corrected by extending the threaded portion by an additional rotation with the knowledge that a 3/8” BSF thread has 20 TPI and therefore 3600 of rotation equates to 0.050”. For example, if another 900 of rotation would put it in the right place, then extending the threaded portion by 0.0125” should do the trick.
I could no longer hold the stud in the 4-jaw chuck on the lathe and so each stud was carefully set up in the 4-jaw chuck in the dividing head on the milling machine, offset and with rotation about the axis of the threaded section to remove material from the contact face to be able to orient the stud correctly in the crankcase.
All this went according to plan, however, one thing that I had not thought of was that when the lower contact face of the stud was screwed down onto the spacer the stud was forced slightly off-centre – away from the centre of the cylinder - by the bending moment from the contact side and this prevented the barrel from sliding down. The effect is amplified over the 4 1/2” length of the stud and although the movement was less than 1/16” at the top of the stud this was more than enough to cause a problem.
I therefore had to remove a few more “thou” from each to orient the studs correctly but with only sufficient contact to avoid this problem. After a few iterations of this I managed to get the 4 studs to seat in their correct orientations and with a cylinder barrel that could be fitted properly. During this sequence of fitting operations, the studs had been put in and taken out multiple times and I was glad that I had helicoiled the threads in the crankcase otherwise they would have been completely worn out.
At this stage, I also drilled and tapped the 5/16” BSCY threaded holes to attach the cylinder head.
There was still one more machining operation and that was to reduce the length of the pear-shaped transition section by 0.050” on all the studs to ensure there was clearance in the depth of the corresponding holes in the barrel. Compared to much of the previous work, this is a straightforward and I have used a carbide insert with a small tip radius to reduce the sharpness of the corner.
With another 1mm thick gasket, between the spacer and the barrel,
the barrel could be slipped down and the cylinder head put on and bolted down for the first time.
There is still a bit of tidying up to complete this setup – rounding the corners of the spacer etc but the hard work on making these studs has been done.
I would imagine that Veloce would have experienced the same fun-and-games that I have had in making and fitting these some 90 years ago. Whoever was given the task of making these at Veloce must have groaned when the foreman came along wanting more. Maybe they developed some jigs and fixtures to help? Anyway, I am pleased with the outcome and can now get on with the upper parts of the engine.
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