Another frustrating day....
Turbo Mini Forums

The problem here appears to be the fact that your block is tapped 3/8 UNF.

I've just checked two 1275 A+ blocks and they are both 1/8 BSPT.

And the oil pressure switches are all 1/8 BSPT.

Is yours a "Metro Turbo" block? Did the factory drill the turbo blocks differently?

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Dude not my block. All the tapped holes are factory standard all my fittings were assembled by Beachgrove engineering to my requirements. All my spares are removed facory fitted items so should fit yours.

I point you to below.

Own the day

The block is definately 3/8" UNF at 24 TPI parallel.

It is an early Aplus block (fully grooved bearing shells)

I agree the pressure switches I have measured are 1/8" BSPT.

The first fitting Avonbar sent was 3/8" UNF and fitted perfectly except it had no spacer incorporated so the hose got cocked over at an angle. It is clearly listed on Avonbar's websire as 3/8" UNF to 1/8" BSP converter and it has a 24 TPI (parallel) thread so requires the copper washer provided.

The subsequent bits are 1/8" BSP so lock up after a few threads.

I'll pull the oil switch from another random block later today and I'll bet a 3/8" UNF bolt screws in without locking up.....

Schrödinger's cat - so which one am I ???

PM me your details and ill throw them in a jiffy bag nothing like having the parts in your hand. Never seen two different types of bore for that location but I dont know everything.

Own the day

The thing is, what you are all looking at is fittings that you "expect" to only go in a short way..... and expect to lock up and seal. The expectation is that they lock up and seal because of tapered threads.......

So here's another Aplus block, factory MG Metro, its the one I'm doing my initial fuel injection testing on, with factory oil pressure switch.



And here's the oil switch out (note slight distortion on threads) and a 3/8" UNF bolt



Note slight distortion of leading threads in block



And here's the 3/8" UNF bolt screwed in by at least 20mm



Now with a 4 TPI thread difference, would I have got it in by nearly one "I" (inch) ???

And here's it removed



First few threads distorted as it tried to correct the damaged threads in the block and then just minor marking on the rest as it isn't a tap so couldn't perfectly correct the block threads.

There is NO WAY that would have gone in and out again by 20mm if the block was threaded BSP.

The problem if you try to measure the block with thread guages, is you can only put the guage on the first few thread and they are the damaged ones....

(edit - grammer)

Edited by Rod S on 13th Jul, 2009.

Schrödinger's cat - so which one am I ???

I would have drawn the opposite conclusion from this bolt:



The major diameter of the 1/8 BSPT thread is 0.408" maximum and 0.383", 4mm down the hole.

It will take a 3/8 UNF parallel bolt if it is forced in so that it damages the bolt as in the photo.

Trying to fit it by hand and it will only go in about 4 threads.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

It went in over 20mm, it was only "tight" (and not that tight) for the first four turns as it corrected the damaged leading threads in the block.

The pitch difference is 4 TPI. that would be nearly four threads differernce on the bolt in the photo.

The threads in the block are parallel and well over 20mm deep - if they were tapered in the block, the parallel bolt would simply have stopped after a few threads because, as you state, the diameter gets smaller down the hole on a tapered thread !!!!

If the threads were less than 20mm deep the bolt would not have gone in.

The damage to the bolt is exactly as I would have expected from a 3/8" UNF hole having had the wrong fitting screwed in the first few threads and the bolt being a slightly harder material so "correcting" the first few threads.

Schrödinger's cat - so which one am I ???

Right, here's a 3/8" UNF second tap next to the bolt (sorry, I don't have a plug tap).



And here it is screwed into the block, it took a small adjustable spanner for the first three turns to finish correcting the damaged threads in the block, then the rest of the way by hand on the tap shank.



And here's what it brought out.



OK, I'll have to spin the engine to pump some oil up the hole to remove the few remaining bits of crap (not metal) but here's the hole (as best as I can photo it..



Sorry about the shadow at the bottom but all threads remain, at least 20mm deep.
If I had just put a 24TPI tap down a 28TPI threaded hole there would be no threads on the 24 - 28 overlap.

And if it was a tapered BSP 28TPI hole (so the overlap would have disappeared down the taper) there would have been enourmous amounts of swarf on my tap when it threaded the small bit of the tapered hole.

EDIT - by the 24/28 overlap I meant that every 6th thread would be completely missing ( 24 / (28-24)) and the ones either side, mostly missing.

Edited by Rod S on 13th Jul, 2009.

Schrödinger's cat - so which one am I ???

I have to agree with paul S, The original avonbar banjo bolt was 3/8 unf, but because they are very close to the bsp they got away with it, after a while and stripping rebuilds etc they start to wear. I promise you that is deffinatly bsp. or 1/8th gas as its commonly referd to.

I ended up doing this properly, 1/8 bsp to 1/8 bsp out the block, into a 1/8 bsp t-adaptor. then 1/8th bsp banjos. so far its held up very well. and hasn't leaked, unlike my avonbar one.

stef

"Cars are a lifetime of pain"

i'd agree with rob if the tap went in that far just turning the shank with fingers and with such a minimal amount of swarf its clearly 3/8 UNF . perhaps, there was a changeover at some point from one thread to another?

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Well, on my two blocks, there is simply no way I could screw a 3/8" UNF tap down a 1/8" BSP threaded hole without removing every 6th thread and most of the threads either side.

Simple maths, 24 TPI vs 28 TPI, every 6th thread would disappear and loads of swarf would come out....

and this is what I got....



This project will be abondoned for a while until I can get a correct 3/8" UNF fitting for my unique blocks as both my blocks are obviously non-standard !!!!

Schrödinger's cat - so which one am I ???

but later on in life, i can understand that on the later mpi's, but 1/8 bspt has been on both my A+ lumps. Maybe someone has taped it previously? also the thread is probably not what it was when it came out of the factory.....and if all oil pressure switches are 1/8 bspt then surely we would have an option on threads from the suppliers?

stef

"Cars are a lifetime of pain"

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

I am convinced it is down to people assuming things tighten up because of "tapered threads".

I have screwed a 3/8" UNF tap into two A plus blocks with no metal removal apart from the first three threads that had previously been damaged by a BSP fitting being screwed in.

The tap has gone in 20mm in both cases (I had already done this to my "turbo" block before I sent it for offset boring because I knew the threads were UNF). This is why the correct threaded Avonbar fitting fits the threads perfectly - it is just too short......

The fact AvonBar have now changed to 1/8" BSP is, IMHO, just a sad reflection that the holes have been already badly distorted by wrong fittings, so it is better to supply a "screw thread locking" fitting rather than a parallel one that needs to go in properly to seal on a copper washer.

Schrödinger's cat - so which one am I ???

Agreed the threads in the block are parallel. I would not expect a tapered fitting to screw all the way home. If it does the fitting has been over tightened in the past or the tap has been run in to far, over done it on a pressure regulator before.

If your oil pressure switch goes in the fittings I showed will go in. If the tapered fitting winds all the way down then things are as stated above.

It’s the cost of a stamp and in your hand in a day.

Own the day

Well, I've just gone and used a thread gauge on the two accessible blocks that I have.

They are both definately tapped at 28TPI (1/8 BSP). There is no damage to the threads at all.

Now one of those I got in a 1990 Metro from a neighbour who had it from new and used to get it serviced by the Local Rover dealer. The oil pressure switch looks original. It's all 1/8 BSPT.

If Rover had used a parallel thread, then they would have spot faced the boss and used a parallel thread switch.

Rod, your blocks are obviously different.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

So maybe there was a change and that's why Avonbar moved from 3/8" UNF to 1/8" BSPT.

But mine are both parallel thread and both UNF - otherwise the 24 TPI tap would have wrecked the hole with 4 TPI difference.

My two blocks are 1985 (the MG Metro one) and the other one earlier.

So, for whatever reason, I need 3/8" UNF fittings.

I'll check my two old "A" blocks tomorrow.

EDIT - re the parallel thread and switch thread - no, many manufacturers compromise and put one parallel thread and mix it with a taper thread to get a simple (low pressure) seal.

Have you measured down your block threaded hole to confirm it is actually tapered as well as being BSP ???

Obviously if mine were, the parallel tap would have taken out loads of swarf.

And finally, does anyone have access to the original Austin/Leyland/Rover drawings to see what it was meant to be ????

Edited by Rod S on 13th Jul, 2009.

Schrödinger's cat - so which one am I ???

If I can find them ill compare both taps

Own the day

I've emailed Avonbar, who sent me the original UNF fitting (but too short) followed by their BSP fitting, to see if they can shed any light on thread size changes.

Mr Joshua, I'll PM you after I've had a response from Avonbar....

Rod.

Schrödinger's cat - so which one am I ???

Just to add a little more fuel for discussion, I went out and checked my two A+ blocks and, as Rod says,a 3/8 UNF bolt will screw all the way in by hand.I use one just to keep the crap out.

All you engineers !!!
you need a mechanic to do the job
:)

Schrödinger's cat - so which one am I ???

I just tried to put a 3/8 UNF screw in the hole.

Went in about 5mm by finger tips. Used a spanner for a few more turns and could feel a lot of resistance.

Took the bolt out to find the end threads of the screw starting to distort.

I think that we can safely say that over the years Rover used both threads.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I believe both thread types were used. MiniSpares sell this fitting:
http://www.minispares.com/Product.aspx?ty=...978&title=UNION FOR OIL PRESSURE PI...

Given the fibre washer I presume this is a parallel thread and is intended to seal on that washer.
I have one fitted one of these to one A+ block without a problem, however I have another A+ block that I couldn't comfortably get it to screw into upto the washer. I also couldn't get it to fit a A series comfortably.

Yes, I think all this debate points to there being two different threads.....

and why do I have the less common one..... on two different blocks

Schrödinger's cat - so which one am I ???