The Stovebolt Site Forums The Parking Lot Big Bolts Temp sender/sensor for 503 motor

The original temp sender on my 55 GMC with 503 motor is not functioning and replacements are 'unobtainium'. The thread size is 7/8 NPT. I can easily obtain a universal sender with 1/2 NPT and could use a face bushing/reducer. However, it seems that 7/8 to 1/2 NPT reducers are also generally unobtainium. The original GMC part # is 1513385, a web search for which leads to 0 results. There are a lot of NOS senders on Ebay of the same type, and one or more of them might fit, but being of different manufacturers, and application/measurements not known, that is also somewhat of a dead-end. I don't need to know the exact operating temp of the motor - just need a relative indication. Your thoughts/ideas?

You could have a machinist make a 7/8 NPT adapter, or enlarge the hole to 1" NPT with a pipe tap. Tapping the hole out would be the simplest solution, as 1" to 1/2" NPT reducer bushings are a standard hardware store item. Of course, if you modify something GM quit making 70 years ago, all your kids will probably be born naked!
Jerry

7/8 npt? That is so oddball google doesn't have any mention of it! You have tried other pipe sizes like 3/4 NPT?

Drilling it up to 1 inch NPT is probably the safest bet. The other thought is to remove the sensor part of your old sensor and drill the plug into your adapter bushing.

jaycoop, According to a search through GM Partswiki for GM# 1513385, that part number was superseded by GM# 6402752 as listed in the "Part Number Replacements and Supersessions 80TS-RP September 1979" publication. The replacement part is listed with a latest application of Chevy 366 and 427 Big Block engines in 1980-82 School Buses per the "School Bus Chassis Parts Book 82TM-SB-F July 1983".

A further check on the web found an NOS GM# 6402752 that looks like it might fill your need??? At a very reasonable price too. Since it's a direct replacement for the earlier part number, the dimensions should be right for your application. You can always contact the seller and get them to measure the part and confirm before ordering. Hope this might help you get what you need and not have to modify your sender port. Let us know how this turns out.

Thanks everybody for the ideas, leads and a bit of humor! I will followup on the superseded part and the particular listing on Ebay....and I'll let you know the outcome.

There's no such thing as 7/8 NPT. You may be confusing it with the OD of the hole. A 1/2" NPT thread has a major (outside) diameter of 0.84" which is pretty close to 7/8" (0.875"). A hole for a 1" NPT tap would be 1-5/32". Pipe threads are designated by the nominal INSIDE diameter of the pipe (1/2" NPT would be roughly 1/2" inside diameter.)

Sorry, it's the plumber coming out in me.

Whoops! I'm glad you explained how it's measured - and it's quite likely that 1/2" NPT will fit! Thanks again everybody - this forum never disappoints.

Quickest way to double check is to get a 1/2" pipe plug or nipple and see if it threads into the hole. Then you'll have your answer.

You beat me to it KL, we are brothers, retired Refrigeration Pipefitter/HVAC/pipe trades

Indeed it ended up being 1/2" NPT, so I locally purchased a universal aftermarket unit made by VDO, but sure do appreciate your leads on NOS options. Before installing it, I removed the 'T' pipe fitting which has the sender at the top, a plug at the bottom, and screws into the side of the thermostat housing. It was packed full of hardened sediment, so I replaced it also. Then I discovered that the gauge did not work properly. I tested it by grounding the lead which connects to the sender and the gauge went to the cold end - but my understanding is that it should have gone to the hot end. Then I remembered that this truck was a 12 volt positive ground system originally, and likely the gauge was designed for that system. I had an NOS Stewart Warner gauge from the 60s, so I installed it and BINGO, it works fine!

jaycoop, Great to hear you got it figured out! BTW - we don't get to see lots of pictures of a 503. Care to take and share some???

Thermostats are not available for these engines, so if yours is intact, take good care of it. Later GMC( not Chevy) stats can be used with different plumbing.

Ed

Ed, how about machining an adapter ring for a later stat? As long as it won't restrict the coolant flow excessively, any relatively large diameter stat should work.
Jerry

Thanks for the heads up on the thermostat. I'll try to attach some pix of the 503.

In the pix I just posted, one shows the dual chamber oil bath air cleaner which is normally mounted on the firewall, but removed to work on the carb. The left engine bay pic shows the formerly used selenium rectifier and large regulator for the charging system. The truck now has an internally regulated alternator. The 503's were made from 49-59, used in 630, 660 and 670 series trucks. The mid 50s had the huge Holley 2bbl as seen in pic; the late 50s had a Holley 4bbl (a modern double pumper bolts right up); the early 50s had a Zenith. The later engines had little coolant squirters in the head, aimed at the exhaust valve area. The block is a 2-part, with the jug section bolted onto the lower section.

Jerry, that would probably work OK, GMC small block(302 etc.) stats are larger in OD than Chevy stats. I just machined a surface in one of the horizontal water necks half the thickness of the stat OD and let the rubber gasket seal it, works just fine.

Ed