The DOHC 250 Velo Engine: The Lubrication System – Part 1

The DOHC cambox was designed with 2 oil feeds at the extremities which direct oil directly onto the cams and a drain pipe which would have scavenged the oil by gravity back into either the oil tank or the crankcases, all circled in the picture below.

There are essentially 2 ways in which oil can be provided under pressure to the cambox. The easier way would be to tap into the lower bevel drive chamber (for example, via the tapped hole for the drain plug) and add a pipe with a bifurcated union to provide oil feeds to both sides. However, there is a problem with that, namely that by combining 2 essentially different lubrication systems – pressurized lower bevel/upper bevel chambers (the existing design on the Mk 1 engines) together with a directed and targeted approach to the cambox it would not be straightforward to control the different amounts of oil going to each part.

On the Mk V KTT engine that is in my workshop the lubrication system was completely revised to provide oil under pressure directly from the pump to a quill in the end of the crankshaft for the big-end (see picture below)

and a jet that is directed between the inlet and exhaust cams

plus another feed that provides oil to the upper bevel box and which then drains back to the lower bevel box by gravity. This approach avoids pressurisation of the bevel gear chamber.

The castings for the timing-side crankcase and inner timing case on the Mk V KTT engine were substantially revised compared to previous cammy Velo engines to support internal oil passages and there is an oil take-off in the timing side crankcase, indicated with an arrow below, to provide oil to the upper parts of the engine.

There is also a drain from the lower bevel chamber back to the crankcase. This picture is of the engine “as received”.

My preference is to replicate this approach on the DOHC 250 engine so that key parts of the system are targeted directly. This avoids pressurisation of the internal cavities of the engine and the oil flow rate to different parts of the engine can be controlled by choosing appropriate diameters for the various holes feeding them.

The disadvantage of choosing to modify an existing “K” engine to strategic lubrication is that it is not a simple 5-minute job. As I indicated in a previous blog, a number of modifications are required:

 1)  The small passage on the end-cap of the pump that connects the feed side of the pump to the lower chamber (indicated with an arrow on the picture below)

 needs blocking off. (I have made this modification previously for the Velo oil pumps on both the AJcette and the AJS V-Twin – see here) by building up with braze and machining.

2)  The drive-side crankcase needs modifying with a passage to take the oil directly from the pump outlet to a union to connect to the “outside world” - as in the picture of KTT 581 above

3)  A quill to provide oil into the end of the crankshaft for the big-end and a modification to the timing gear cover, K45/2, for an oil feed needs to be added

4)  The crankshaft must be modified at the end of the timing-side mainshaft to accept the inserted quill

5)  Oil feeds to the cambox (2) and to the upper bevel will be needed. A drain back to the oil tank will also need to be added.

6) A drain needs to be provided to scavenge the oil from the lower bevel chamber back into the crankcase

7) Some means of regulating the flow/pressure will be required.

The first step in making these changes was to modify the K-45/2 cover with a small (5mm) hole aligned with the end of the crankshaft. I have found on previous projects that the easiest and most accurate way to do this is to insert a centre into the end of a piece of solid bar, 22mm diameter in this case,

to pass through the bearings in the assembled crankcases and with the timing gear cover screwed into place, drill a 5mm hole in the cover.

The next step was to make a boss/oil channel to feed the hole (which will support the quill) out of a piece of aluminium. I didn’t have any material of quite the right size but I did have a few lengths of 40mm diameter 7075 (thank you Christer) and used one of these to carve out an appropriately shaped piece

and with a boss on the inside plus a 6mm LH threaded piece of steel

the collection of bits could be assembled

and then laser welded in place.

The piece of 6mm diameter steel is only to aid setup to unsure that the holes are aligned (and a cable tie was also put around the whole assembly to hold it together prior to welding); it will be replaced with the quill. A 6mm LH thread is used because if there is any interference between the crankshaft and quill then the quill would tend to tighten rather than unscrew. There is a 1/8” BSP thread on the end to feed oil to the assembly.

The quill was made with a 1.3mm diameter hole and fits perfectly into the hole in the crankshaft.

The second part of this project was to cut out another chunk of metal in a similar way and attach that to the crankcase

where the boss would be needed for the oil feed to the upper part of the engine and to then carefully drill

a hole through in exactly the right place to meet the exit of the oil pump

and to tap 1/8” BSP for a union.

It’s always a good moment to see the drill emerge in the right place - THIS was another instance, because a “fix” would not be straightforward.

There was insufficient material in the crankcase casting to allow an uninterrupted hole

and so, a 6mm OD/4mm ID aluminium tube was added to transport the oil from the pump to the boss. It’s held in place with JB Weld.

This completed the work on the aluminium components.

The external pipework to supply the oil to the upper parts of the engine – the bevel drive and the cambox and also means of controlling the flow rate - will come later.

The return paths for the oil must also be considered. The cambox oil will go into the oil tank under gravity but the oil that is provided to the upper bevel and which will then flow down to the lower bevel needs to be scavenged from the lower bevel chamber.

On KTT 581 there is a passage that is cast into the timing-side crankcase which connects the lower bevel chamber to the bottom of the crankcase. Here, rather than drill more holes in the structure, I have simply modified the K-163 Drain Plug with a 3/16” OD copper pipe silver soldered inside that will take oil from inside the chamber (at the same height as the scavenge hole on KTT 581) and which then connects with a modified B-38 crankcase drain plug to return the oil to the crankcase.

The last part, for now, was to modify the pump. I indicated earlier the opening that needs to be blocked and this was done by adding braze to the gap

and then carefully machining back to a cylinder.

to prevent the oil discharging into the lower bevel chamber.

 

…..And Finally….. While I was machining one of the pieces of aluminium on the milling machine there was muffled “pop” that appeared to come from the motor. Everything seemed fine and so I kept working until a few minutes later the machine lost power and I was forced to stop. The electric motor was so hot that I couldn’t touch it and, clearly, something was seriously wrong.

The next day, when the motor had cooled down, I found that the electrical junction box on the side of the motor looked like this inside.

It seems that the capacitor had exploded. I am not an “electrics” person but Google tells me that a capacitor can explode for any number of reasons – I have no idea which applies here.

Anyway, I ordered a new motor; this turned up the next day and was easily fitted.

As you can see, it’s made by Polestar – a Chinese manufacturer of electric vehicles. I fitted the last motor (the one that blew up) about 12 years ago so we’ll see how long this one lasts.

KTT 581: An Update

In an earlier post – here – I mentioned that the owner of the original chassis had contacted me. He also lives on the South Coast in the UK and subsequently visited me and was able to provide quite a bit of further information about the history of the bike.

Firstly, and for me extremely useful, he has a picture of Doug Pirie astride the bike after coming 4^th in the Junior TT.

The bike now sports the number 4 rather than his racing number of 19 to indicate that he came 4^th. I don’t know how to interpret his expression - he does not look happy! It may be that having raced very hard and come 4^th he was disappointed. Alternatively, having just raced over 7 laps of the Mountain Circuit (yes, it was 7 laps in 1935!) – 264 miles at an average speed of over 77 mph on roads that are not the lovely smooth tarmacadam surface that we have today that he was simply exhausted.

Anyway, the picture is extremely useful to me – details such as the form of the mudguards, mudguard stays, whether wheel rims were chrome plated or painted etc will help me in recreating the bike.

The other information that is of interest is the history of the bike immediately after Doug Pirie’s death in the lightweight race. It seems that his brother, John Pirie, then took charge of the bike and had it road registered. A copy of the old-style buff log book

shows that he became the registered owner in June 1936 (the first owner is “Stevens”, the London Velocette dealer to whom the bike was originally delivered).

It is believed that John Pirie did not ride the bike but simply kept it as a memorial to his brother. The minimal wear inside the engine would support this,

The bike was then sold to Geoff Monty, a very successful racer of 250cc bikes in the 1950s and who subsequently teamed up with Allen Dudley-Ward to develop a business producing racing motorcycles (see here). Although there is no date in the log book it is believed to be around 1950 when Geoff Monty acquired the bike and it was at this time that the engine and other parts were removed, to be replaced by a 250.

Thankyou Mick for providing all of the above information.

Earlier this year I put an advert in the “Wanted” section of Fishtail seeking an “oily rag” Mk 2 KSS or KTS that I could use as a basis for the rebuild – the chassis of these bikes are the same as the Mk V KTT – the plan being to use a donor bike and replace the engine, exhaust pipe and petrol tank from the original KTT 581. After quite a few months I have heard nothing.

However, it seems that every time I go on holiday something turns up. I was in Marrakech when I bought the engine and my wife and I were in Cornwall a couple of weeks ago for a short break when a 1936 KSS turned up on eBay. After a brief negotiation I have now acquired this.

By coincidence, it was also first registered in June 1936 and it will now form the donor bike to build the replica of the original TT bike.

There will be quite a few bits coming up for sale from this bike, notably the engine, petrol tank and exhaust system but also other extraneous parts such as the entire electrics, primary chaincases (screw type), mudguards, toolbox etc. The engine runs just fine.

I know that some people will be horrified that I am effectively breaking an original bike, however, I consider it more important to see and hear the original KTT engine running again as it did 90 years ago at the TT. And if the parts from this bike help to get someone back on the road, then so much the better. So, if you are interested in any of the parts that I won’t be needing then please contact me via email.