Threads on DTI’s

[MetalhackerParticipant @metalhacker]

After reading the article by Gary Wooding in MEW 309 on making an ‘Elephant’s Foot’ for a DTI, I decided to make one for my 3 dti’s. But what thread to use? Gary’s was 6BA but he suggests the majority would be Metric. Far from it! I have a British built long travel DTI……. 3/32 x 48 whit. Small Mitutoyo…… 3-48 UNC. Larger Mitutoyo……4-40 UNC. What diversity of threads; have others found this to be the case ? It would be interesting to know the extent of this diversity!

What have others found?

Andries

[MetalhackerParticipant @metalhacker]

[Brian GParticipant @briang]

Hi Andries

Starrett 25 series have a 3/8" stem and use what they describe as "English 4-48 thread", (I'm unsure what is English about UNF) and other AGD indicators to be the same, whilst DIN indicators have an 8mm stem and M2.5 thread

Brian G

Edited for smileys

Edited By Brian G on 08/11/2021 18:49:38

[DMBParticipant @dmb]

My thoughts on this problem as I have had the same. Measure dia. of thread and pitch, if you possess a thread gauge and turn up a trial short length. Failing that. guess by apparent age, units of measurement, make, as to likely thread used, e.g., Starrett and Mitutoyo, UNF/UNC. Metric units by say German manufacturers would probably be M2.5 or M3 and same for the British makes, but if instrument is old and uses Imperial div's, then probably 6BA.

DMB

[Howard LewisParticipant @howardlewis46836]

Possibly every one will be different from that of another manufacturer.

It will depend on the preferences of the designer , so one might choose a 40 tpi Whit form, another a very small BSF, while another chooses a Metric Fine, and some one else a ANF or ANC.

Some may even be a special, unique to that indicator.

I have an industrial Compaq bore gauge that uses a 5 x 0.5 mm thread for the extenders and anvils.

Best not to assume anything but to measure very carefully in metric and Imperail units, and then look up to see what is most likely.

Howard

[Peter Cook 6Participant @petercook6]

I have some cheap (Chinese?) digital ones. They have M2.5 threaded tips.

[Brian GParticipant @briang]

Posted by DMB on 08/11/2021 20:14:47:

…, e.g., Starrett and Mitutoyo, UNF/UNC. …

DMB

Mitutoyo's catalogue includes both AGD (3/8 stem, 4-48 UNF) and ISO/DIN (8mm stem, M2.5) indicators. Oddly, although Starrett list indicators with an 8mm stem these are to neither standard, having 4-48 UNF threads even if the ball end is a metric diameter.

Brian G

[Oily RagParticipant @oilyrag]

For engine builders – A good tip for dial indicators for an extended foot is a bicycle spoke. Old spokes will be right for use on a Mercer or a 'new' metric spoke for a Mitutoya.

We used these for setting up a degree plate on engines where we did a 'swing about' TDC off the piston crown down the plug hole (swing about is determining the 0.005" drop either side of the TDC point and splitting the difference ). Also good for setting cam MOP (maximum opening point) by the same method. The spoke can be bent to fit the available space without losing its strength.

Martin

[John ReeseParticipant @johnreese12848]

Starrett back plunger indicators had a male thread on the plunger. The kit included an adapter so standard 4-48 indicator tips could be used. The back plunger kit came with 2 female thread tips that fit the plunger.

[John ReeseParticipant @johnreese12848]

Large flat contact tips are readily available and quite inexpensive. It is hardly worth making one. Check with MSC.

Contact tips are available up to 2" dia.

[Howard LewisParticipant @howardlewis46836]

We used to find tdc by bring a piston close to tdc and dropping a valve onto the piston crown and marking an equal depth (Measured with a plunger DTI ) each side of tdc. tdc was the point half way between the two measured points.

As a safety precaution, a piece of sealing wire was wrapped around the collet groove to prevent any risk of inadvertently dropping the valve into the cylinder.

Howard

[Pete RimmerParticipant @peterimmer30576]

Posted by Oily Rag on 09/11/2021 13:05:38:

For engine builders – A good tip for dial indicators for an extended foot is a bicycle spoke. Old spokes will be right for use on a Mercer or a 'new' metric spoke for a Mitutoya.

We used these for setting up a degree plate on engines where we did a 'swing about' TDC off the piston crown down the plug hole (swing about is determining the 0.005" drop either side of the TDC point and splitting the difference ). Also good for setting cam MOP (maximum opening point) by the same method. The spoke can be bent to fit the available space without losing its strength.

Martin

I keep a couple of spokes for that very reason. I have also found that you can screw the red straw from the spray cans of penetration oil straight in to many dial gauge threads if you need a makeshift extension.

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