Tuesday 23 March 2021

Exhaust Rings

This might not sound like a very weighty topic but the exhaust rings are pretty important for attaching the exhaust pipe to the exhaust port. And, here, there was a problem!

Exhaust rings for various Velocette models can be purchased from GroveClassic Motorcycles. The picture below shows one of these on a Mk IV KTT cylinder head in my workshop.


These are well made and work absolutely fine ….on a single cylinder Velocette engine installed in a Velocette frame. However, there was simply not sufficient clearance between the exhaust port and the frame tube on the front cylinder of the V-Twin to fit one of these.

Luckily, I also have a very early Velocette Model K flat-tank that is awaiting restoration and these earlier bikes use a much smaller finned exhaust ring, Velocette part number K66.


I tried this on the V-twin front cylinder exhaust and it fitted without fouling the frame tube …just! However, I am not aware of anyone that is making and selling these and, as I only have one good one that I would need for the Model K, I needed to make a couple more for the front and rear cylinders.

One might ask the question: “why bother with castings and rather have new ones CNC machined?” There are 2 reason for not going down this route: firstly, I do not have CNC machining capabilities and, in any case, the overhead in setting up for a maximum of 4 copies (I will use 2 on other projects) would be a relatively expensive exercise; secondly, the original was made in cast steel and the appearance of a fully machined finned exhaust ring would, quite simply, look out of place on a 90 year old motorcycle.

The starting point was to prepare the original for making a mould for casting resin copies to be used as foundry patterns. This involved filling in the threaded portion with an aluminium ring and the remaining part smoothed over with body filler. The aluminium ring is tapered so that the resin part that will become the pattern can be withdrawn easily from the sand mould at the foundry.

A couple of pieces of stainless steel locking wire were also fixed to the top surface with epoxy resin; these would be used to suspend the original part in the resin moulding box.

None of these additions do any damage to the original and can be easily removed later when the mould has been made.

The next step is make a small plywood box and to suspend the original part in the box

Which is then marked out and 2BA screws inserted a few threads into the wood around the periphery

The original can then be suspended in the box, surrounded by screws, about 1cm above the base

The small pieces of metal are to balance the ring so that it lies horizontally in the mould.

RTV silicone rubber is then mixed and poured up to the top of the part

and allowed to set.

After removing the supporting wire, 2 glossy paper cones are attached to the top of the exhaust ring to act as a feed sprue and riser for casting the top part of the mould. The silicone is also coated with Vaseline to avoid the upper silicone part adhering to the lower part.

Silicone is then poured onto the mould up to the base of the screw heads and, after setting, can be disassembled to remove the original part.

The mould can now be reassembled and used to cast one or more resin copies of the original

These resin copies can now be fettled (removing the sprue and riser and filling in any small holes with filler) and then sent to the foundry.

I made 2 good resin copies and sent these to New Pro Foundries. I have been using New Pro for many years and they have made many casting for me in AB2 Ni Al Bronze and heat-treated LM25 aluminium; rockers for Nortons, the AJcette timing cases and all the crankcase and timing case castings for this V-Twin. Having visited New Pro many times in the past and seen the quality of their work on extremely complex castings I wouldn’t go anywhere else.

In due course, a batch of new shiny castings arrived in the post

ready for finishing.

Machining these is fairly straightforward. The number of fins is divisible by 3, which means they can be easily held in a 3-jaw chuck, and after facing off, screwcutting the 16 tpi thread for the exhaust stub and putting them in the tumbler for a few hours they turned out pretty well.

The picture below shows 2 of the new completed exhaust rings on top of the original.

And, in comparison to the later K66/2 variant, the difference in size is obvious and with a lovely cast surface finish.


And, most importantly, it fits without fouling the frame tube

Shrinkage: These bronze alloys have a quoted volumetric shrinkage of 4% which gives a linear shrinkage of 1.3%. I measured the shrinkage of the whole process, ie original > resin > casting as 2% as best as I could determine for an irregularly shaped part; in practice, this isn't a problem for a component such as this.

You may ask: "why go to the trouble of making a resin pattern - why not just use the original part as the pattern?". Well, you could ...if it had been suitably modified with the aluminium ring and filler as I described. However, I have a golden rule of never sending a one-and-only original part to a foundry as a pattern ever since a good friend of mine sent an aluminium timing case to a local craft foundry  ...who mistook his original for a piece of scrap aluminium and tossed it into the melt ...never to be seen again.

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