WELCOME to my blog about restoring vintage overhead camshaft AJS and Velocette motorcycles

 

Apologies to anyone that has come to this page expecting to see exclusively vintage AJS motorcycles .....scroll down the page a bit and you will find plenty of them.

However, I recently sent all of my cammy AJSs to Bonhams and they were sold at the Autumn Stafford Sale. 

The URL of "vintageajs" for this blog is now somewhat misleading but it's too late to change it and there is still plenty to read about the AJAY projects. If you happen to be the new owner of one of the bikes that I recently sold then there is plenty here to read about its build. I still have 2 early Big Ports waiting in the wings ....but they will have to wait until I've completed the Velos. 

In 2023 I started the restoration of 2 early Velocette KTTs plus another Mk 1 OHC cammy special - a few details about each of these bikes can be found here and here. 

A lot of work has been done on these bikes over the past 2 years and the INDEX PAGE provides links in chronological order of the project so far.

Earlier this year I acquired a DOHC 250 Velo Engine and now have a Resurrection Plan. There is an INDEX PAGE for this engine rebuild and work is progressing well. The crankshaft has been rebuilt using MOV flywheels and "K" mainshafts and I have completed work on revising the lubrication system to mirror that of the Mk V KTT engine that is in my workshop.

The most recent work on this engine rebuild has been to make eccentric studs to be able to bolt down the cylinder + head + cambox onto the "K" crankcases.

 

This is a solution that Veloce applied to their race engines to enable cylinders with a different bolting pattern to be fitted to a standard set of crankases. Making these is far from straightforward - details of this project can be found HERE.

During the last 5 years I have posted quite a lot of information and to aid navigation the "Labels" section on the right side of this page lists the various projects.

The labels marked "INDEX" give a link to a page that provides a complete list and links to all of the separate sub-projects related to that main project.

Alternatively, scroll down this page and see what's here...

When I started this blog I owned a 500cc AJS R10

that I've been riding for many years and wanted an early 350cc bike. I bought one at a Bonhams auction; this is what I brought home....

....a bit of work was needed to bring it back to life 

Full details of the restoration can be found here.

During the restoration of the K7 I figured that I could put an early overhead camshaft Velocette cylinder, cylinder head and cambox onto the crankcases of an AJS 350cc engine from 1931, convert it to chain-driven OHC and make an engine that looks like a K7 but has a Velocette top-end. I had a 1928 350cc AJS sidevalve that I had bought on eBay and used that to create the AJcette ....giving credit to both manufacturers.

It looks pretty similar to the K7 and to demonstrate that there really are 2 bikes, here they are both together.

Details of the AJcette project can be found here.

I have quite a lot of early Mk1 OHC Velocette parts and after completing the AJcette I decided to use some of these to make a replica of a one-off bike that AJS built in 1929/1930 for an attempt on the world speed record. The original is a huge V-Twin beast that started out with a naturally-aspirated engine but, having failed to gain the record, was supercharged ...and again failed. The bike ended up in Tasmania for many years and, after being repatriated to the UK and restored, it is now in the National Motorcycle Museum.

This is what the original looked like:

and this is my recreation.

 

 

Like the AJcette, the V-Twin uses Mk 1 OHC Velocette cylinder components. The full story of how this bike was built can be found in the links here.

There is also a 14 minute edited Youtube summary of how these bikes came about here and a longer unedited version here. 

This bike is now in the Sammy Miller Museum.

 

In January 2022 I started the restoration of a 1933 AJS Trophy Model

and this was completed in March 2023.

 

The Index Page for this project can be found here.

I also reported briefly on a couple of my other projects ....vintage OHV Nortons

 and putting a Marshall supercharger onto my 1934 MG PA

 

I hope you find something of interest.

The DOHC 250 Velo Engine: Eccentric Studs

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 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.

The DOHC 250 Velo Engine: Completing the Bottom End

I mentioned at the end of a previous blog on rebuilding the crankshaft that one of the consequences of using “K” mainshafts and MOV flywheels is that the assembled inner distance between the flywheels is about 1/8” less than the standard “K” crankshaft which means that the mainshafts are 1/16” further inboard.

This ended up having a few knock-on effects; in no particular order:

-       The big-end nuts foul the bearing outers

-       The crankshaft shock-absorber meets the crankcase casting before the taper grips on the drive-side mainshaft

-       On the timing side, the woodruff key on the crankshaft barely reaches the slot to engage with the bevel gear

The first of these needed to be addressed before I could make any progress on the others because it was not possible to assemble the crankshaft into the crankcases. 

As I mentioned in previous blogs I have been using solid steel “bearings” that have the same ID/OD/t as the correct bearings

throughout the build rather than continually using the genuine items as the latter have a slight taper to ensure a good tight fit onto the mainshafts – it’s just easier this way. Modifications were first tested using the dummy bearings and then 0.040” was removed from one side of the bearing outers on both the new MC22 bearings that I had purchased

and making a similar modification to the inner side of the bearing housing in the crankcases (shown below with one of the dummy bearings)

and by using 0.015” of shims on both sides of the crankshaft allowed the crankshaft to be fitted without interference with the big-end nuts, gave a slight (couple of thou) oversize on the overall crankshaft length and positioned the connecting rod exactly in the centre.   

The second issue – non-engagement of the shock-absorber on the mainshaft taper - was quite straightforward to fix. The drive-side crankcase was put on the milling machine and material removed from the timed breather extension of the crankcase casting

until the crankshaft and shock-absorber tapers mated (plus a bit of clearance). There is also a consequence of this – the crankshaft sprocket will be further inboard and will be out of alignment with the clutch chainwheel but this can be easily fixed (at a later date – when the engine is in the frame) by making a thicker K-145 shock-absorber fibre washer.

The last “gotcha” was that the Woodruff key barely reached the bevel gear and had minimal engagement. There are 2 main factors that position and hold the bevel gear in place on the crankshaft: the key and the tightness of the LH threaded nut. As this engine has a lot of gears in the cambox (with their associated inertia etc) the drive train will need to be as strong as possible to survive.

A new key was made from an 18” length of ½” x 3/16” key steel. The picture below shows the new key together with a standard Woodruf key.

Fitted to the shaft (and with the dummy bearing in place) the new key provides the best possible level of engagement with the bevel gear.

The material is O1 tool steel and will be heat treated in due course to improve its properties (see eg here). And I have over 17” of this key steel left over!

The bevel gear is designed to be a close fit in the timing-side crankcase casting to maintain oil pressure in the lower bevel chamber. I found that the fit with this particular bevel gear and casting was a bit too tight – the bevel gear would not even fit into the hole. The crankcase was therefore back on the milling machine and the boring head used to open it up slightly.

As the changes to the lubrication system that I have made will not require a pressurized lower bevel chamber the larger diameter hole will not be an issue.

With a 0.020” gasket on the K-46/2 bevel gear housing and 2 K-111 shims (0.023" in total) behind the bevel gear

there is perfect engagement of the lower bevel gears.

Not shown in the above picture, I have made a ½” x 20 TPI LH thread nut (removed here for clarity) to replace the usual K-114 LH ball-valve nut and the crankshaft bevel has been pushed “home”.

In preparation for the next sub-project – making the eccentric studs, I have put helicoil inserts into the 4 3/8” BSF threaded holes that support the barrel. These must be put in very accurately to avoid the studs sprouting out at weird angles like the spines on a porcupine. Each crankcase half was bolted, in turn, to the large angle plate on the milling machine and the digital level box used to set the top face horizontal (with the milling machine table as the reference).

However, when I checked this by using the position of the vertical head when a 5/8” diameter ground bar first contacted the surface

I found that the level box method was not sufficiently accurate and I could get a better and consistent result by using the contact method.

For the X-Y positioning I machined one stud on the lathe with a small “pip” in the centre and used that in conjunction with a small centre in the collet chuck, under a magnifying glass, to locate the on-axis position.

Whilst this might seem crude it has proved to be accurate. At the time of writing I have completed most of the machining of the eccentric studs – and the barrel slides down over them perfectly.

For any particular helicoiling exercise I never seem to have the size I want and invariably end up buying another kit.

I have also helicoiled 2 of the 3/16” BSW threads that hold on the K-45/2 timing gear cover.

That just about concludes the work on the bottom-end for now. The oil pump drive gear alignment will be checked and modified if necessary at the final build (when the oil pump is inserted) and I will need to remove a bit of material from the back of the inner timing case to clear the piece of additional aluminium that I have added for the crankshaft oil feed quill.

The next step is to complete work on the eccentric studs so that the cylinder barrel can be put in place properly.