Tapping 5/8 UNF to 3/8 BSP

Hi,

I have an inlet manifold which has a 5/8” UNF threaded hole.

I need to fit a pipe for a Manifold Absolute Pressure sensor so I was advised to re tap the thread to 3/8 BSP and get a 3/8 - 1/4 BSP adaptor with a take off pipe. I was advised that 5/8 threads tap out nicely to 3/8 BSP?

Sorry but I always struggle to understand thread types.

I bought a 3/8 BSP Tap and a 3/8 to 1/4 BSP adaptor.

is this correct?

Hi, I would make an adapter to get from 5/8 unf to 1/4 bsp ,this will screw into manifold .

If you retap manifold ,you could damage it and get swarf in engine.

Hope this helps

Maurice

Edited By Maurice Taylor on 27/04/2023 21:23:08

Sounds OK provided you mean the taper version of BSP, not the straight. The tap drill for 3/8 BSP (tapered) is 14.75mm which is very close to 5/8UNF, which calls for a 14.73mm hole.

Not so good if you have a straight BSP, because the tap drill is bigger - 15.25mm. That could be a bit loose, if got my head on the right way round! Anyone tried it?

I find BSP confusing because the nominal sizes don't relate to the actual holes. It's because they're specified by inside diameter to match the outside diameter of a pipe. Oh, and BSPT is the taper version, but the T stands for "thread".

Dave

Edited By SillyOldDuffer on 27/04/2023 21:36:23

Looks like I bought a set of 3/8 BSP straight Taps.

Tap

But the Adaptor looks like it’s 3/8 BSP Tapered

Adaptor

Hi , Easiest way to make adapter is to get a 5/8 unf bolt and drill and tap 1/4 bsp .Job done.

Maurice

Not sure there will be enough metal in a 5/8BSF adaptort with a 1/4bsp hole as minor dia of the BSF is 13.5mm and major of the BSP 13.2mm

Usual way is not to drill 1/4" BSP tapping but just go straight into the BSF thread with your 1/4" BSP tap

Edited By JasonB on 28/04/2023 06:56:02

Jason made a mistake! He meant "just go straight into the UNF thread with your 1/4" BSP tap". But it needs to be a 1/4" BSP taper tap, not a 1/4" BSP straight.

Sorry if I confused the issue by mentioning tap drills - I didn't mean there was any need for drilling, only that based on the tapping diameters, a hole already tapped 5/8" UNF would take a 1/4" BSP tap and let it cut a new BSP thread in the hole, replacing UNF with BSP as William wants.

I've used a similar bodge to convert M10 to 3/8" - by chance the two systems are close enough.

My sympathies are entirely with William. Yesterday I ordered the copper pipe, elbows, tee-joint, vacuum gauge, adaptor, ball-valve, and compression fitting needed for my clock project. It's a minefield - couldn't find anyone selling all the parts in the same system. Had to mix and match and will only be certain it fits together when the parts arrive.

Dave

Please comment on the statement in the page below:

https://www.valvesonline.com.au/references/threads/

"BSPT - Female thread is parallel and the male thread is tapered (also know as R/Rp) (the female thread can also be tapered it is then Rc, these are fairly rare to find). Within BSPT it is also common to call the female thread BSPP (parallel) and the male BSPT (tapered) even though they are both technically a BSPT thread form (the female would be parallel and the male would be tapered)."

It is easy to cut a BSPT male thread with a die (thousands of Ridgid pipe threaders do it every day). Cutting a tapered female thread (of any standard) with a tap is challenging. It is akin to reaming a Morse taper - half a turn too much and the taper disappears into the spindle. Please try to find someone selling Rc taps.

Hi, I agree with DC31K. In most cases the female will be parallel and whatever is fitted into it will be tapered, this ensures a good seal as both will have an interference fit with each other, but it is important to use a sealant on the threads as well, to help stop galling, and even some molecules are small enough to get through a metal to metal threaded joint. If the part you are fitting is a parallel thread, then unless it has a shoulder where a sealing washers can be used, it would be better to have the female one tapered.

Regards Nick.

We used tapered female threads for gas pipe bosses where I worked. We had to have a suitable thread gauge for them before they were welded onto the pipe as too small or too large would produce a scrap part as the matching male part would either not go in far enough or too far, both were not good outcomes for gas pipes. We used a suitable sealant on assembly.

Martin C

5/8 UNF has a core diameter of 0.5649", 1/4 BSP has an OD of 0.518". A difference of only 0.051" or 0.0255" a side

So I wouldn't ne inclined to put in a 5/8UNF plug and tap it 1/4 BSP. The BSP tapping should be separated from the 5/8 UNF by at least 0.100", in my view.

This will make the fitting longer, but mire durable.

Howard

Without getting out my tables, Contact a hydraulics firm and ask if a JIC male size uses 5/8" UNF thread, if yes then you will be able to use a standard adaptor.It could be a JIC fitting that was used originally. In hydraulics there is a VAST array of adaptors and materials. Noel.

I was half asleep this morning. What I should have said is usual way is to tap the 5/8" UNF hole with the 3/8" BSP tap and not bother drilling tapping size for the 3/8" BSP tap.

As the OD of 3/8BSP is larger than the OD of 5/8" UNF you will get a good enough thread depth for a sensor on an inlet manifold which is no doubt why it was suggested to use the 3/8 to 1/4 adaptor.

So here are some images of what I have.

The manifold is refurbished and not installed yet, the plug hole is I believe 5/8"UNF.

The adaptor I have (according to the eBay link I included above) has an outer 3/8" BSP tapered thread.

The Taps I bought are 3/8 BSP parallel.

From what I have read, I should be able to run the 3/8" Tap through the 5/8 UNF thread OK and the Tapered 3/8" external thread on the silver 3/8" - 1/4 BSP adaptor should thread in fine.

Second problem is my Tap Wrench is not large enough to hold the 3/8" Tap, any thoughts on what I can hold it with?

Well I though I had better check that I was awake so while in the workshop earlier I put a 14.5mm hole in abit of brass and tapped to 5/8UNF

Actually found I could not start teh 3/8" BSP tap so put a 15mm drill down the hole and then tapped 3/8" BSP, going by the effort needed to turn the tap I would be happy to hang off that thread with my anvil in my pocket

Can just about see a trace of the first thread towards the bottom which is to be expected with only 1tpi difference in pitch between the two

If you can set the manifold up on a drill press or mill then use the chuck to guide the tap and you should get away with a spanner on the flats.

That seems a lot of work to me for such a simple, unloaded part. Why not just drill a hole through a short 5/8UNF bolt and solder(epoxy!) an appropriate barb in place? Make a complete part from one piece if you have time to waste. I've made brake servo adapters from delrin expoxied together, although they went on the cool servo not a hot A-series inlet manifold.

Come on chaps I really am struggling with the amount of discussion this simple problem has generated.

Two obvious solutions IMHO.

First simply retap the 5/5 UNF thread with the BSP thread you require.

Second take a piece of 9/16 or 3/4 bar (metric equivalents are apparently available. Turn one end down and die cut or screw cut a 5/8UNF thread. Drill through to produce the bore size you need to suit your pipe work. Then Drill and tap the second end with the appropriate BSP thread form

You might consider using a piece of hex bar to allow the use of a spanner.

EDIT: I forgot to add. The answer to the question posed in the initial posting is YES.

Edited By MadMike on 29/04/2023 09:53:14

Thank you for taking the time to try this out.
I didn’t have a 15mm drill, but did manage to nip the tops off the thread with a 14mm drill. I then managed to re tap the 5/8” UNF thread with the 3/8” BSP Tap using an adjustable spanner and a quick release wood clamp!! Anyway Job Done ✅

To answer the question 'How do I hold a tap if it's too big for my tap-holder? -

Although the tap drive-end is square, you will find that a double-hexagon spanner should fit it - but you might not find one exactly the right size. Try Metric, Unified, and Whitworth sizes. If you use a socket spanner, your tool kit should include various handles to turn the spanner - a ratchet can be useful, and for starting the thread, a tommy-bar version is best, as you can press equally on both sides.

No-one has realised, yet, how useful it would be to have taps with hexagon ends.

Cheers, Tim

Edited By Tim Stevens on 01/05/2023 17:23:00

A tap that bis too big for the available Tap Wrench can be driven by an open ended spanner.

Ideally, it should be supopoirted / centeredin the lathe by a centyre in the tailstock.

Metrich spanners / sockets will drive Whitworthm , A/F or Metric squares / hexagons, since they drive on the flat, not the corners.

Since one size will cope with all three types the fit seems rather slack, but a Metrich will safely apply a lot more torque to a stuck / corroded fastener than the "correct" spanner with far less risk to the hexagon.

Howard