internal-external bypass system

[Bill Ford]

I have a 46' P-15. I understand these came with external bypass system. It looks like I have a aftermarket internal thermostat housing. My head does not have the hump. I talked to Roberts and Bernbalm this morning concerning a water pump and neither one knew what I was talking about when I asked about a external bypass water pump. NAPA did not have any thing saying it was internal or external. I think I need a external bypass water pump and hope to locate parts to convert mine to external. Am I own the right track? When purchaseing a thermostat do you specify internal or external?

[Young Ed]

The new water pumps you'll find are setup for both. They rely on just the gasket to block the hole if you don't have internal bypass or you can switch the backing plate or weld up the hole. Beyond that you'll need one of the cast thermostat housings with the proper thermostat(also available at napa) and the little bypass casting that bolts to the top of the water pump.

[greg g]

small sheetmetal themostat housing = internal bypass, large cast iron housing with hose connection to the water pump fitting = external bypass.

[Bill Ford]

Greg, how can I know without removeing the water pump whether it is internal or external? I assumed that since it was a 46 and I had no hump on the head that it was external bypass. What should I look for when the pump is removed? I have the cast iron external thermostat houseing. The other piece has a tube that goes to the heater. Where does the heater hose go on a external set up.

[greg g]

If you have a large cast iron thrrmostat housing you have an iternal bypass system. If there is a connection with a short piece of hose between the housing and a fitting on the top of the water pump you have an external bypass. The visual difference is much more obvious that the hump on the head.

[Bill Ford]

Greg, what I have on the car now is the small sheetmetal thermostat housing. I have a extra cast iron thermostat housing with the hose connection if I find out I have a external setup. With the external setup where do you connect a heater hose. My heater hose now is connected to a elbow on a plate on top of the water pump. Do have a picture of a external set up? What makes a internal or external set up? Is it the water pump or is it the way the cooling chambers are cast?

[Jim Yergin]

Here is a picture of the external by-pass set-up in my '41 Plymouth.

Jim Yergin

[LAKOTA169]

Don't know if this will help.

http://moparforum.com/Technicalarticles.html

[1941coupe]

I am going through a similar situation with my P11. I have the sheet metal housing on mine. I want to keep it original but want to know why there are two types of bypass systems? And what are the advantages of each? There seems to be no photos anywhere that I have seen showing the internal type. Is the hump head internal or external? I am attaching a photograph of my engine...is this the correct setup for the P11? The head on the engine now has no hump...is this correct?

Thanks

[Bill Ford]

The hump on the head near the water pump is for internal bypass. Is that a fitting on the radiator hose?

[greg g]

I believe the MOPAR fourm posting is full of hoowie as fars as there being no external bypass till 51. I have seen plenty of vehicles with either or from all years i the 30's 40's and 50's. the rest of the information looks based in fact.

[Bill Ford]

I can see now that the heater hose does not connect to the radiator hose. Did not enlarge enough to start with. This is the same set up I have on the P_15.

[james49ply]

My understanding of the internal/external waterpump, if the head has a "bump" on the right front edge the head is designed for internal pypass, but the block must also have the "bump" for it to work. Thermostat housing for internal bypass are like the picture from the 41. external bypass has a large cast housing with the connection between the pump and the housing. The thermostats are different and connot be interchanged. using the small stamped t housing with the small stat is probably the cause of the cracked head, the engine actually overheats before the stat opens. with the external system you have to have the large thermostat and seal that fits in the large cast t'stat housing. I tried the small t'stat housing and t'stat, and removed it after one day and got the correct t'stat and housing. NAPA waterpump that I got from them (new) had a bypass opening for the internal, but the is no corresponding hole in the block, therefore it was mute. I don't belive they started the internal bypass till the 50's.

[TodFitch]

I believe the MOPAR fourm posting is full of hoowie as fars as there being no external bypass till 51. I have seen plenty of vehicles with either or from all years i the 30's 40's and 50's. the rest of the information looks based in fact.

External bypass was introduced in 1934. I can get you a scan of a photocopy of the factory service manual if you like. For much of the 1930s after that, and maybe into the '40s, the DeLuxe models had it while the standard/business/Roadking models did not.

Interestingly, near as I can tell from the parts book, they used the same block, head and water pump. Which implies that some sort of internal bypass was possible if not fitted with the external bypass. On my '33, which differs in several details from the later engines, there is no bump on the head and the bypass is entirely in the block. I think that is the way the 201 engines worked too if not fitted with an external bypass system.

I am guessing that the external bypass did a better job because the water in the head, not just the block, was circulating before the thermostat opened to allow flow to the radiator. Maybe the bump on the '50s head with internal bypass was to allow flow in the head too which got around the need for the fancy thermostat and external plumbing. Would make sense as a manufacturing cost savings change.

[1941coupe]

Thanks Tod...very good insight as to this confounded mystery. I was led to believe that my P11 had NO bypass system altogether. It's quite possible that a cross section drawing of the earlier cars would show how the water was bypassed if at all like on my 41.

And I think James is correct about the bypass having gone internal in the early 50's as some photos do show.

[Bill Ford]

I am assuming I have a external bypass system and will be buying a water pump soon. I have the cast iron thermostat housing for a external set up but will need the housing for the water pump that will connect to thermostat houseing and heater hose. Do any of you guys have one or know who does? I have been looking for a brass water tube that I could replace while the water pump is out. What type of thermostat will I need for the external setup. Will the thermostat be listed as internal or external? I appreciate all the help I have received. I have talked to NAPA, Roberts and Bernbalm and none of them list their pumps as internal or external. Roberts has a new one for $95 and a rebuilt with grease fitting for $105. Wonder how the rebuilt ones hold up?