View Full Version : Starrett comb. square

Jim Hubbell
08-13-2010, 11:34 PM
I need some ideas on what type of setting compound to install a bubble vial in place of the broken vial. I would like it to be removable in case I need a second chance to get it close. If anyone has done this with any success ( or not ) I would be interested to hear of it.

The hole is cleaned out and I found a good vial in a crappy square.

08-13-2010, 11:57 PM
I think they use plaster of paris.

08-14-2010, 12:02 AM
Hot glue? :)

John Garner
08-14-2010, 01:45 AM
Jim --

US Gypsum's Hydrocal B-11 low-expansion plaster has been the de-facto standard for setting precision level vials for many years, but it's not available in small packages. Small instrument-repair shops usually use Plaster of Paris, but it sets fast enough that it can be hard to use. I prefer Patching Plaster, which is available in small packages from most good hardware and building-supply stores, because of its hour-or-so setting time.

But I don't use straight Patching Plaster, I mix roughly equal volumes of Patching Plaster powder and All-Purpose Flour. The mixture is stickier and sets into a softer solid than straight plaster, which I consider advantages.

White RTV (the stuff sold as bathtub calk works as well as the "industrial" RTVs from what I've seen) has been a fairly common level-vial bedding compound for the last twenty years or so. It seems to work very well if used in half-pea sized dabs which fully cure in a couple of days, but not so well if used in big globs.


08-14-2010, 07:23 AM
I read once (but have never tried it) that using cold tea instead of water delays plaster of paris from setting.


Davo J
08-14-2010, 07:36 AM
Here is a full right up for you on replacing one


08-14-2010, 09:44 AM
I don't know the necessary specs for the plaster or whatever used to cement the vials; however, if low expansion is your primary criterion consider using dental stone or dental crown and bridge die material. Both are considerably harder than plaster of paris; their expansion/contraction is tightly controlled and is specified.

You could, I'm sure beg a small amount of stone from the family dentist; the crown & bridge die material comes in pre-measured packets. The latter might be more difficult to obtain since most dentists no longer pour the crown & Bridge impressions. Dental supply houses would be a source. I can't imagine that you would need to be a dentist to buy it. Be certain to get the water:powder ratios exact since this factor is vital in controlling the final contraction/expansion of the set material. If you want to go "all out" then use distilled or de-ionized water and not tap water.

08-14-2010, 12:58 PM
Rather than buy a 50lb bag of plaster of paris, I would just send it back to Starrett, and have it done right.


08-14-2010, 01:04 PM
Silicone caulk,easily reversible with gasoline solvent.

08-14-2010, 04:15 PM
Rather than buy a 50lb bag of plaster of paris, I would just send it back to Starrett, and have it done right.


Good advice. It doesn't cost that much and it will be done right.

Jim Hubbell
08-15-2010, 03:30 AM
Thanks all for the replies. I didn't know about gasoline solvent reversing silicone caulk. This is strictly a DIY project so I won't bother Starrett.

Again thanks for the ideas.