Inlet manifold vacuum pipe take off fitting.

[Geoff Kingston]

We have just put a brake servo on a 401 and had an interesting time trying to find a fitting to replace the blanking plug at the front of the head. We had expected it to have a different thread but eventually I found some thread gauges and it turned out to be 7/16 BSW.
Thinking I might have to get Brian to make a suitable fitting I pinched one off the 403 Cylinder Head which is awaiting repair, not sure what was attached to it as the car has no servo and it was not ideal but did the job.
I want to put a servo on my other 401 so are these fittings available commercially and where from, have tried BCL but they have not found any yet. If anyone has any good second hand fittings available including the blanking plug part no FS.105/3D I would be interested
Geoff.

[Clive Foster]

Are you sure its 7/16 BSW and not 7/16 UNC. Both are 14 TPI but thread angle is different, 55° for BSW and 60° for UNC. Can be hard to tell the difference on old components. Frequently the wrong one will screw in but not be reliable, especially on something like a cylinder head subject to heating and cooling cycles.

UNF is the norm for 7/16 sensors, plugs et al for UK / USA vehicles so no surprise that BSW / UNC is hard to find.

Blanking plugs and ordinary push on pipe connectors are trivial to make. I rarely bother to even google for odd ball ones as making takes about half an hour from scratch including set up time if I have suitable hex about the place. Say an hour if I only have rounds. Once set up probably 10 minutes or less floor to floor. I have toolroom style machines. Old style capstan, whether proper or an attachment, or new fangled CNC will be way faster.

I'm a little surprised that BCL don't have a tame "old boy in a shed" on call to knock out a few when needed.

Clive

[Kevin H]

Originally Posted by Clive Foster

I'm a little surprised that BCL don't have a tame "old boy in a shed" on call to knock out a few when needed.

I'm sure there would be a market for such a service...

[Geoff Kingston]

Clive
I will check if I have a UNC thread gauge or get my pal Brian to check it for me if I don't, all I can say at this stage is that the tread gauge we tried in it gave a perfect fit at 14G/7/16 BSW. We are assuming therefore that this is telling us what the thread size is though I notice Brian cross references gauge readings to his ruler and a table of measurements so I might be wrong.
Another engineer I spoke to when looking for fittings told me it would most likely be BSP but nothing we had would fit.
I doubt very much at that tine that Bristol Cars would have used UNC threads anywhere on the car and there is no apparent wear in either fitting or the head.
Taking up on Kevin's point If I have to ask Brian to make some fittings I will see if he can make a few extra's. I have a friend who has made a jig to reproduce the wood frame for the back windows of the 401/403's but so far has not had any orders so it is a bit of a thankless task sometimes trying to help fellow owners.
Geoff.

[Geoff Dowdle]

Clive ,
remember a 400/401 was developed in the late 40's in England.
All the threads used by Bristol were BSW , BSF , BA and a few special threads , but are all British threads/forms .

UNC &UNF are American or Canadian threads , used on the Chrysler engines & auto in the Bristol V8 cars . I'm not sure about the threads of bolts and screws used in the suspension and body of the V8 car.

Geoff

[Clive Foster]

Geoff

Agreed. Whitworth is what you'd expect given the date and that its a British developed motor. But its always wise to check. Post war British vehicles are a total minefield when it comes to the BSW / BSF or UNC / UNF thing. I've learned the hard way to check everything when folks ask for a one off replacement on something rare.

If no one else steps up and folk do want a small batch of blanking plugs and take-offs made I have the necessary tools. If they are simple hex headed plugs and straight push on pipe take offs I guess something like £4 each plus material in a batch of 10 or 12 would work.

Clive

[Geoff Kingston]

Clive,
As you and Geoff D have said their were quite a mixture of threads on the car and I was of course forgetting the brake pipes which have some UNC fittings.
I am sure I have somewhere the fitting for a servo pipe I took off the engine when I put it into another 401 years ago because I pinched the blanking plug off another engine, if I could find that it would serve as a pattern. A least I have one spare blanking plug to copy. I will see what Bristol Cars turn up next week and may well take you up on your kind offer to make a small batch of plugs and take offs.
Geoff.