No temp sensor hole in heads

[aggiegrad]

I have a low millage 68 gmc that everything appeared very original. 307, TH400 and even has all original shocks. I am trying to convert it to a gauge cluster as the aftermarket 3 gauge dash mount were all broken. I have prepped a new gauge cluster and purchased all oil lines and sending units to do the swap. However, I cant find the plug to screw the temp sender into. The green wire connector is laying across the intake manifold and a mechanical temp gauge line was added to the heater hose outlet in a pipe tee. I assumed I had a light sender or a plugged hole in the heads like every other picture I have seen but cant find it. I have felt the rear of the passenger side head between the rear two plugs and there is no threaded plug. I looked in the drivers side front two plugs and no hole there. I took pictures with the cell phone to confirm there is no hole in the head. Am I going crazy or is it possible that these heads were replaced sometime with ones without sending unit holes. My original intake doesn't have a spot either. If the heads were switched out they kept all of the other original parts still on it. I recently bought this truck from the 87 year old that owned this truck the whole time.


thanks for information.

[davepl]

I don't know what year heads lacked the hole, but I've tapped the big (13/16?) hole in the intake to add an extra sender, so that's always possible. The tap and drill bit won't be cheap (around $50 at least) but it's doable.

[Bigdav160]

I can't say I have ever seen a SBC head without one, OE or aftermarket.

With ramhorn manifolds it'll be under the manifold.

If there is a plug in it like this one you'll be better off locating the sensor to the intake manifold. It take a lot of heat for these plugs to come loose.

[aggiegrad]

Here is a picture I took of both the drivers and passenger side head locations. Can someone confirm this I where the sensors were on a 68 307. The wire harness is really long and could reach all the way to the fan shroud. They never put them in the top of the water pump did they. I have a plug there and it is long enough to reach that location?

[ray_mcavoy]

Originally Posted by Bigdav160

I can't say I have ever seen a SBC head without one, OE or aftermarket.

Most of the 1967 & earlier factory SBC heads did not have temp sensor holes. During those years, the temp sensor was located in the intake manifold (usually just off to the driver side of the thermostat housing).

Originally Posted by aggiegrad

Here is a picture I took of both the drivers and passenger side head locations. Can someone confirm this I where the sensors were on a 68 307.

Yes, that location (on the driver side) is where the temp sending units are typically located on 1968 & newer small blocks. And that is where it should be located on your truck so it does look like you could be dealing with an older set of heads that were installed on your engine some time in the past.

Originally Posted by aggiegrad

They never put them in the top of the water pump did they. I have a plug there and it is long enough to reach that location?

No, they never put the temp sending unit in the top of the water pump. That is on the "cold side" of the cooling system (after the coolant has passed through the radiator) so it would give an abnormally low reading there. That plugged hole in the top of the water pump was used for a bypass hose in some applications.

[garyd1961]

"Most of the 1967 & earlier factory SBC heads did not have temp sensor holes. During those years, the temp sensor was located in the intake manifold (usually just off to the driver side of the thermostat housing)."

Contrary to what a lot of people say on here, I have my sensor in the intake manifold and it works just fine.

To the OP, I wouldn't be too sure your motor and trans are original. I could be wrong but I never heard of a 307 with a th400 from the factory. I'm no expert but I've never seen that combo before.

[aggiegrad]

The truck was definitely a 307 with TH400. I have all the original documentation and protecto plate. I do question the gearing however as it seems to top out at 60-65 mph but the plate says it has 3.54 which I assumed was the rear end. It is a dana 44 which I don't know if that is what it was from factory. It will always be a discovery. I am trying to make it a daily driver for my daughter. I think it is but just worry about all the unknowns with it. Any knobs or screws that had been replaced were placed in the glove box.

Thanks for the info so far. I thought I was going crazy and just not seeing it. I know this truck was an early 68 made in 1967 based on different items I found and sold in Feb 68. The shocks even have early 67 dates. Could it be they had changed the heads mid year? The wiring loom does seem to go to the front of the engine.

I will probably just put the sensor in the pipe tee that was added out to the heater hose line even though it want be in the flow of water all the time. The temp shouldn't be too much off from the main water temp.

[KQQL IT]

Could get a newer t stat housing with a hole for a vacuum switch for your temp sender.

https://m.summitracing.com/parts/fss-84842

[ratty69]

Originally Posted by KQQL IT

Could get a newer t stat housing with a hole for a vacuum switch for your temp sender.

You could, but then you'd have zero temperature indication until the thermostat opened. Not a big deal maybe, unless/until your thermostat fails closed ( does happen).

[KQQL IT]

Originally Posted by ratty69

You could, but then you'd have zero temperature indication until the thermostat opened. Not a big deal maybe, unless/until your thermostat fails closed ( does happen).

I'd think 4- 1/8" holes in the stat would give some indication and still better than no sender?

[garyd1961]

"The truck was definitely a 307 with TH400."
Would this truck be a 3/4 ton?

[davepl]

My Dad's 68 GMC half ton, bought new, came with a 307 and TH400. It was "optioned up" for towing in most every way except the engine.

That was a GMC910 built in Canada, but I doubt it'd be any different in the US.

[Money_pit]

One of these will do the trick. thinking I put my sender below thermostat. Seems ok