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FT: Frame (Twin) Oil Tank Inspection Port


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VOC Member
Oil tank inspection port

Recently the forum has seen a flurry of activity concerning the cleaning out of UFMs via an inspection port let into the top of the oil tank. As many of you like to make parts I thought that I would put the design of one of these that I came up with several years ago. Originally I wanted two for my own use and local Kent and Sussex Section member Eric Priestly also wanted a couple so, as he has the necessary skills and equipment, he offered to make a few for our own use. These were used on two of my bikes and two of his. Later, another Kent and Sussex Member, Paul Champion, asked if he could have the design to make one for one if his own bikes. The original design meant making the ‘ring’ which has to be fitted into the top of the oil tank robust enough so that when it was welded in it did not weaken the tank in any way. It was 2.5” diameter and 0.5” thick. I designed the internal thread to be the same as that of the exhaust ports and rocker inspection caps. The reasoning behind this was that the welding was likely to cause some distortion of the internal thread and people were likely to have a 1.875 x 20 tpi tap for cleaning up the various threads in the cylinder heads. This proved to be necessary as we found that both brazing or silver soldering still caused enough distortion to require the threads to be cleaned up. It is possible that a modern TIG welder might circumvent this problem. With this experience Paul decided to follow a slightly different route and instead made a large flange to the insert and glued and screwed this to the top of the tank.

Below I will show three drawing which should allow anyone with the skills and equipment to make their own and they can choose which type to produce. Note that the centre of the later design is exactly the same as the earlier design; it is only the flange, which contains four countersunk holes, which is different. It should also be noted that the slot in the top of the screwed plug was designed to take the curved end of either a K1 or K3 spanner for tightening or undoing. Not all these spanners are the same thickness so ensure that you check the thickness of the spanners before finishing the slot.

Here is the drawing for the plug which is the same whichever design is chosen.


Here is the design for the original insert to be fastened into the top of the oil tank. It is 2.5” diameter and 0.5” deep
Note that the specification of the four countersunk holes has not been made as one can make these to fit whatever screws are chosen, e.g. self tapping, machine screws in to tapped holes etc. Several types of adhesive could be used. A thickened epoxy would suit if the metal can be guaranteed grease free or one of the modern structural metal to metal mastics, black RTV would be one I would try.
The 'o' ring, which is intended to fit into the groove below the top flange of the plug was a 3mm cross section by 45 mm I.D.

Next page shows four images of Paul Champion’s mod. All pictures due to Paul.


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Well Known and Active Website User
VOC Member
Thank you for this posting, great idea!
What can we do to motivate you to produce these bolt on clean out ports please?

I do not have a machine shop that can do this at this covid time.


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VOC Member
Hi Craig, there is no problem getting a batch of these made. After all I have had over 200 of the JE steering stems made, My motivation for publishing the diagrams was to allow people to make their own and if some of those who are competent enough wish to make a few extra then I am more than happy to send larger diagrams by email if that would help. The commercial alternatives are either they are made in one offs and cost a lot or I get them made by CNC machining and those chaps like to make a minimum of ten at a time. There is also the problem of cutting the hole in the top of the UFM. I did this by using a hole saw in a pillar drill so it is not a particularly difficult job but if you wanted someone else to do it then you have to be prepared to ship your UFM. If there is someone here on the forum who is a competent machinist and prepared to make a few as a favour to fellow Vincent owners then I am more than happy to let them have any more information they might need to allow them to make a batch. All I would ask is that this is done at a reasonable cost as my aim here is to help fellow Vincent owners, not to make a profit.
If there are ten people who would be potentially interested in these then please contact me on enw07@btinternet.com and I will get a price for a batch.


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VOC Member
Yes, not trying to compete with Conways. Just trying to provide a way for people to make their own if they wish. I do not have the details of the Conways system but I imagine that they must be quite similar. Just looked at the photos on the Coways site and it looks as though they even have the same idea for the slot in the top of the bung. Their plug is bronze or brass and hollowed out, probably to save weight. Mine is just aluminium alloy and has worked well enough for many years.

Black Flash

Well Known and Active Website User
VOC Member
Had some made by my cnc guy a few months ago in ss. Already fitted 2 to my bikes.
I silver solder them. The plug iirc is 52mm.thread pitch is 1.5mm.
The four indentations in the cap make tightening easy.
That picture was taken before removing the flux.


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