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Need to re-tap 3x Exhaust Flange Stud holes? What tap's?

Hi, I have a 1981 VC Commodore with Starfire 4cyl Motor.

I'm re-doing the manifold gasket but the 3x exhaust Flange studs are heat rusted solid.
One has snapped off and I've drilled it out close to the thread sides.

The other two are complete but I plan to remove them also.

One of the local stores suggested they were 3/8" studs? Is this correct?

Also I now need to understand what the correct "Tap" is to buy.

I see many Tap's that state 3/8" but they have other specs after wards that I don't understand.
(16/18/24 etc TPI; UNF; NPT; BSW; BSF; BSP; BSF; Left Hand; Right Hand; etc etc) What do I pick please?
I am wondering if this perhaps relates to the thread type or pitch etc?

Can anyone tell me the correct Tap's to buy please?

vk4akp at GMail
.-.-.

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3/8" UNC RH thread

Dr Terry

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Hi DR Terry.

Are you sure on the Tap type?

I've just managed to do one hole.

I used a thread gauge and matched up the spacing against a starter Tap we already had here.

Thread Gauge - 16G 3/8 Whitworth. (What's the "16G" ??)

Tap used was ...

--
3/8" WHIT
16 - P&N
Australia
--

Drilled up to size 19/64 (5 short of 3/8) before running the Tap.

I always thought Whitworth was an old English standard from around the War era?

The first hole goes straight through which allowed me to use the starter Tap.
However the other two are "Blind" so I will still have to buy the 3x set to complete. (Start, Middle, & Finish).

The studs supplied have a non threaded section as well so I am thinking I might have to run a Die across yet to further the thread for the manifold hole.

The studs are supposed to be Brass. I did question that at the time as I assumed the originals were hardened steal. However they assured me that Brass was correct??

I was surprised how clean the final outcome was. As specially since it's close to impossible to drill the hole perfectly center. I really didn't want to go up that many drill sizes but all seems ok.
The peaks of the threaded hole re-cut do seem a little blunt however this could just be from age and heat etc. I did not see any shine indicating that I had drilled the peaks off etc.

All in all it's a tight strong fit though so I guess all is good.

Pictures and Video here.

http://shazam.zapto....

.-.-.

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In 3/8" Whitworth and UNC are functionally interchangeable. However the correct thread is 100% UNC. Also if anyone in Holden land was to know it would be Dr Terry......

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Cool, thanks.

Sorry for asking but I have to mail order the Tap's so I just wanted to be sure.

Been without the car now a couple of months and I have a lot to do yet before it's back on the road.
And Christmas is just around the corner!! :(

So what's the 16G / 12,14,16,18,24 TPI ?

I know it's not Tracks Per Inch cause that's a Disk Drive! ;)

Perhaps it's Thread Per Inch? In essence simply the length of the Tap?

As long as it's not a thread pitch thing?

.-.-.

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Bunnings or Masters will have a 3/8" UNC thread tap in stock. Repco also have a 1/4-5/16-3/8: UNC tap kit in their $10 bargains bin. They are in every Repco store GV Brand, orange blister pack.

http://www.threadche...

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It is definitely UNC, Holden never used Whitworth for anything other than some outsourced items such as some battery terminals & exhaust clamps. Everything made by Holden back then was UNC, UNF, NPT etc. Later stuff went metric & BSP.

In 3/8" both UNC & Whitworth share the same pitch (16 TPI) but the included angle of cut is 55º for Whit & 60º for UNC so they're not exactly the same but in many cases will do the job.

BTW the studs & nuts were originally steel, but many aftermarket suppliers use steel studs with brass nuts. Brass studs would be useless.

Dr Terry

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Myeh

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forget repco - get on down to your closest nut and bolt shop, they will have "real" taps and dies for sale ie sutton, p&n, etc - quality brands not chinese mild steel that will snap first time you use it. and the bonus that the guys behind the counter will not argue with you when they dont know about something, but will even actually know what you are talking about!!

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Hi Ben

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All you need is a bottoming tap. If that not available just buy an intermediate and grind the end off.

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OK, So the Tap's came a couple of days ago.

I checked them against the nut's off the new 3/8" studs I bought (Champion "Bulk Pack" Manifold Studs. 3/8 x 2" (Holden) P/NO.CGMS1 QTY 10.

(Local rip off store sold me their last 3x separately as $2 per Nut + $2 per Stud + GST %10!)
$13.20 total for 3! :(
But they were stupid enough to include the now finished packaging of the 10x pack).
So will look into getting more somewhere at a realistic price.

Anyhow, strangely enough I was not able to run them through the Nut's.
Oh Fark! I went. :( Typical.

However tonight I thought I'd try it in the existing Whitworth Tap'd hole I had already done.
And the End (Finishing) Tap fed in quiet well!
Got to the bottom and it was able to cut out the remaining thread that the starter Tap wouldn't reach before.

And once again the Stud screwed in cleanly.

So ASAP I will try drilling out the remaining two holes and see how I go there.

Looks like they sent me two intermediate Tap's though and no starter.
They both have the exact same chamfer at the start.
.-.-.

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OK. So I had some time today to have another go at the remaining two studs.

I was very pleased to find that I could remove them with the stud remover.
I had been giving them a small squirt of CRC every few days over the past weeks and it has done the trick.
They came out quiet easily after the first little crack as they let go.
No drilling needed this time! :)

I ran the Tap through just for good measure. It cleaned out the threads nicely and the new studs screwed in nicely.

I've run a Die down the new studs as the threads do not meet enough from either end but because of the way they are machined you don't get the full peak on the newly cut thread.
So I will most likely pack the nuts up with washers etc but otherwise all looks good.

The face of the manifold (Flange section)I have run a flat file across lightly trying to keep a good flat edge (I have no machine shop) :(
It does look to have a few burn in spots. I may have to take it down further yet.

I'll put more photo's up soon. Currently my laptop is at a friends place (No car to transport stuff!) :(

.-.-.

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OK Guys, Here's the latest pictures.
Sorry for the delay, I have not been near my PC for some time.

http://shazam.zapto....

I think I need to skim the gasket faces a lot more.
There seems to be some burn in around the flange gasket rings.

I'm hoping I can do this with a file if I go across it flat.

I know this is not a great way to do things but I have no machine shop at home. ;)

.-.-.

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