Correct radiator pressure & cap...

[WrenchTurner]

I have a 1956 MM 445 gasoline and I'm looking for the CORRECT cooling system pressure rating and radiator cap. The 'reputable' parts places I've found on the internet do not agree...is it 4# or 7#? My MM parts catalog says the correct part number is 10P689, which is, of course, unavailable. I can't find a good cross reference chart that includes this application. Neither MM parts catalog nor shop manual tells what the pressure rating is, at least not that I've found.

 

[molinegb]

Crosses to 10A10056. Common to all MM's in 60's. 670 Jet Star etc......

 

[Alan K]

I have a 1956 MM 445 gasoline and I'm looking for the CORRECT cooling system pressure rating and radiator cap. The 'reputable' parts places I've found on the internet do not agree...is it 4# or 7#? My MM parts catalog says the correct part number is 10P689, which is, of course, unavailable. I can't find a good cross reference chart that includes this application. Neither MM parts catalog nor shop manual tells what the pressure rating is, at least not that I've found.

I believe most of the MMs were 7#

 

[WrenchTurner]

I appreciate the replies but I'm really looking for something definitive. I checked the new number and it takes me back to the websites that disagree on the pressure rating. Has anyone seen any MM literature that addresses this? Maybe a cross-reference chart from a major manufacturer? Something else along these lines? Thanks.

 

[molinegb]

Todays parts people do not know how to cross anything and don't care. If not in computer not available. MM did not make cap. 1s a 7# like Allan said. AC RC-1

 

[WrenchTurner]

Todays parts people do not know how to cross anything and don't care. If not in computer not available. MM did not make cap. 1s a 7# like Allan said. AC RC-1

Well that's not really fair. How can they be expected to look up a part number if they have no access or means to do so?

I believe that it is likely a 7# system too, "like Allan said", but even Alan wrote that "I believe most of the MMs were 7#", leaving room for the possibility of other sizes.

I also don't believe that MM made radiator caps, but the part number they used may cross somewhere. I'm guessing it was made by Stant, but that's just a guess.

I'm just looking to corroborate info through a reliable source. Maybe that source isn't here, but when I find info to a query I've posted, I usually post it for the next guy that happens along looking for the same.

So where does your recommendation for the "AC RC-1" come from? Do you have a catalog?

Sorry for my delayed reply. I don't access the internet daily.

 

[molinegb]

AC Delco RC-1

 

[MMDEL]

It is not that critical!!!! 4Ibs or 7lbs. For years nothing was pressurized. Then someone discovered if the system was under pressure it could run at a higher temperature and not BOIL Saved a lot of coolant from overflow and the engine could run hotter to get better fuel economy. If you have a weak system with a seepage problem release the pressure cap to the first notch and it will -probably last for years. Most systems use 7lb and I am sure all MM s did.

 

[WrenchTurner]

I understand cooling systems and the science of pressure. I've worked on automotive systems for decades. I've already agreed that it is likely a 7# system. I don't know why you seem to take a basic question so personally. If it bothered me as much as it seems to bother you, I'd just leave it alone. I figure that as long as I'm looking for a cap, and I'm in no big hurry, why not get the right one?
I went through a box of old caps today and found a couple of old AC RC-1's. Turns out that they're for a 3/4" inch deep filler neck. The one I'm working on is 1" deep. I've ordered a catalog that should help. Hopefully I'll have the answer, if anyone is interested, in a few days.
Have a fun weekend!

 

[Alan K]

I understand cooling systems and the science of pressure. I've worked on automotive systems for decades. I've already agreed that it is likely a 7# system. I don't know why you seem to take a basic question so personally. If it bothered me as much as it seems to bother you, I'd just leave it alone. I figure that as long as I'm looking for a cap, and I'm in no big hurry, why not get the right one?
I went through a box of old caps today and found a couple of old AC RC-1's. Turns out that they're for a 3/4" inch deep filler neck. The one I'm working on is 1" deep. I've ordered a catalog that should help. Hopefully I'll have the answer, if anyone is interested, in a few days.
Have a fun weekend!

I looked in the parts books. As was stated, a 10A10056 crosses to a 10P689 then it is a 7# cap. The 5 Star and M5s use the 10A10056. The 5 Star operators manual lists it as a 7# cap, as does the Jet Star 3 and U302. Steiner tractor parts lists a 4# and a 7# I am betting the 4# is for the Avery models, whereas the 7# is for all the other MM models listed, which includes the 445.

 

[MMDEL]

I understand cooling systems and the science of pressure. I've worked on automotive systems for decades. I've already agreed that it is likely a 7# system. I don't know why you seem to take a basic question so personally. If it bothered me as much as it seems to bother you, I'd just leave it alone. I figure that as long as I'm looking for a cap, and I'm in no big hurry, why not get the right one?
I went through a box of old caps today and found a couple of old AC RC-1's. Turns out that they're for a 3/4" inch deep filler neck. The one I'm working on is 1" deep. I've ordered a catalog that should help. Hopefully I'll have the answer, if anyone is interested, in a few days.
Have a fun weekend!

I was not taking it personalty!!! Just thought you was making a big issue out of something so insignificant.. I
have been in the grease since I was 7yr old, Was in charge of service Company in Korea, And have have a Repair Shop since 1954.
MMDEL

 

[WrenchTurner]

Hey, some of us are more into details than others. I don't fault you for your views, but I have my own. I don't know why trying to get the right part is "insignificant" to you, but I understand if my problem isn't a big enough deal for you to waste your time on. But I love detective work sometimes.

After installing the cap I found for a 1" deep filler neck, I started wire brushing on the old cap for a little bit and I found (barely) that it also was an AC RC-3. Maybe the original? I don't know which brand MM used originally.

I appreciate the input and info from those who took the time. For some reason this turned into almost entirely a pressure issue. That was not my intent. There is much more to a radiator cap than just pressure.

Attached is some radiator cap info that might help someone else. For whatever reason, neither this catalog nor the other ones I've seen reference the 445. All I can say is that the RC-3 works on mine.