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.

[Geoff Kingston]

Spent an hour with my engineer friend Brian yesterday afternoon and learnt something more about this plug or take off fitting,Brian confirmed we had the correct thread gauge reading 7/16 BSW but he then cross referenced this to a table of thread sizes, checked the measurements and said that the fitting was actually 5/8 BSF and to prove the point actually made one which fitted perfectly. Even he admits he gets caught out sometimes so no hope for the likes of me!
Geoff.

[Clive Foster]

Glad you got sorted in the end. As you discovered 7/16 BSW and 5/8 BSF are indeed exactly the same thread. Just different diameters.

Pipe threads are the classic ones for catching normal mortals out as they are specified by the size of the hole in the pipe not by the outside diameter like (nearly) everything else. Fortunately the other oddities are seriously obscure.

BA is the other one that can drive you nuts as they are a metric dimensioned thread specified in imperial units. Which reminds me. 0 BA will screw into 1/4 BSF and 2 BA into M5 and merely feel slack. Be careful not to do this as the joint will be extremely weak and probably not stay done up.

Here is a link to a really useful tabular Compilation of Thread Size Data originally created by Andy Pugh.

https://fromtheframeup.com/uploads/T...size_chart.pdf

As the list is sorted by bolt (or stud) diameter its an excellent resource for figuring out what size any particular fastener is. Conventional listings are by type then size so first you have to figure out (guess) which breed it is.

Nuts are possibly more of a problem for normal folk. Best way for normal folk is probably to screw the suspicious object onto a bolt. Alternatively measuring the size of the nut, an open ended spanner will do just fine, will rapidly narrow the list of possible suspects. Out in the normal world there aren't really that many combinations to worry about.

Its not a bad idea to invest in a test box containing one labeled sample of each size you expect to encounter. Something I have been threatening to do for at least 50 years! Best paint them pink so you aren't tempted to dip into the box to replace one that has gine walkabout.

Clive

PS If anyone is wondering that list is, from a wider engineering viewpoint, by no means comprehensive.