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[Solved] Rear Main Time of Labor Estimate


Topic starter

Well, the other day I loaned my 2017 Mustang to my girlfriend and my parents had my DD 1999 Ranger to move something, so I had to drive my 1979 Pontiac Catalina with the Pontiac 301 and THM-200 automatic. I was going 55 on the highway and noticed some chattering valve noise. I got off on the very next exit, about a mile down the road, and checked the dipstick. Oil level was below add, but not off the stick. I threw the quart and a half I had in the engine, started it up, no chattering, so I went the rest of the way to work. I had my girlfriend bring 5 quarts of oil for the drive home, no problems. I had to use the thing again the next day, my parents kept my DD longer than they were supposed to. I heard chattering valves again, so I got off within a half mile and added more oil. Started the engine, it was quiet again, so I drove home, and the car has sat since Thursday night. 


In the daylight yesterday, I looked under the car and the transmission oil pan looked like a mirror. I fully suspect the rear main seal has practically gone out; it went through at least 6 quarts in 200 miles. The motor still sounds fine and runs fine, so I asked my girlfriend's parents to find out if their mechanics can do an old rope seal rear main. Other than a compression test, possibly a leak down test, and inspecting the filter media for metal, is there anything else I need to do to verify the engine's health? 


Here's a screenshot of the procedure to replace the rear main. Roughly how much time are we looking at, if a shop were to do this? I like the car, so I am willing to spend the cash to fix it. I should've done it sooner than later.



Does All Data have the information for a car this old, or should I offer the Pontiac service manual I have on CD?








4 Answers

What we used to do to replace those old-style rope seals is use a "Sneaky Pete" to do the job by dropping the pan and removing the rear main bearing cap. You'll probably need to find a greybeard to to the job. I don't think most of today's mechanics have ever dealt with those old rope seals.


You may be right about the new breed mechanics not knowing how to deal with rope seals. I'd try to do the job myself, but the engine needs to be lifted up a bit to get the pan to clear the bottom of the oil pump uptake. I'm glad I didn't brush that noise off and kept oil in the car at all times, or the situation would be quite different right now.

I'm surprised that Pontiac was still using rope seals in 1979. My '65 Rambler has a rope rear main seal in its 327 V8 (referred to as a "hemp seal" in the factory shop manual) and I thought that was pretty much obsolete even by mid-1960s standards. The database confirms it though as does your shop manual.,1979,bonneville,4.9l+301cid+v8,1246474,engine,crankshaft+seal,5604


Yep, I ordered that seal Friday morning. If the shop can do it, they let us give them parts and warranty the work, not the parts.

The Pontiac V8 dates from 1955, they were updated as time went on to improve fuel economy. The 301 in 1979 was referred to as a short deck engine. They're built with lightweight components to save weight. The block casting, connecting rods, etc. Are all lightly built.

I'm guessing that GM anticipated phasing out the Pontiac V8 and only updated it "just enough" to get better fuel economy until newer generation engines came out. I've heard these 301s are old man V8s, they're not made for speed. The 301 Turbo in the Trans Am was, but that's very different from mine, so the dinosaur technology would make sense. 

The 1st-generation Rambler V8 in my '65 dates from about the same time period, 1956. It was designed on a crash program basis when the deal that AMC had made with Packard for their V8 engines went sour. From concept to production took less than 18 months, a record at the time. So it's a heavy, overbuilt engine with forged crank and rods. A version with optional electronic fuel injection was developed in 1957 but problems with the system caused it to be withdrawn before it was offered for sale to the public.

AMC finally debuted a more modern, lightweight V8 in 1966 that was actually a clean-sheet design - and no more rope seal. (My '67 has that engine.)

It doesn't seem like it would be a big deal to re-engineer a rear main from a rope seal to a rubber one. It seems relatively straight-forward, but I guess it could alter the entirety of the engine, depending on how it's done. I wonder how old airplane engines in the P-51s did their rear main seals. Rubber seems like the more intuitive seal material, but I'm also looking through the lens of over 100 years' worth of engineering. Haha.

I found out from my girlfriend's dad today that their regular shop contracts with an old-time mechanic, he used to work on 1965 Dodge Darts and the like, so he probably knows how to do that rope seal. He sounds like he's Scotty's age. I can't picture Scotty raising an engine to get an oil pan off at his age, but I could picture him packing a rope seal if someone put one in front of him.

The shop called back today. It'll take 3.5-4 hours to do the rear main, and they said to bring a pan gasket as well, so it sounds like they know you get at the seal through the bottom of the engine. I'm surprised it's not more, they charge $75 an hour.

I'll need to find an oil pump gasket. RockAuto didn't have any.

$75 per hour sounds like a deal to me.

Posted by: @justin-shepherd

RockAuto didn't have any.

try these


Topic starter

I got the car back today, free of leaks. They held onto it for three days to make sure the new seal didn't leak. It cost $625, parts and labor. 

Sounds like a fair price, IMHO.

Topic starter

The Catalina fiasco continues. I went down to the shop today to cover the car, the windshield leaks on the passenger side and we're supposed to be getting some nasty weather in the next few days. I asked what's going on, they're still messing with the rear main seal. My car is sitting outside, with half of the exhaust cut out. Apparently they were unable to replace the rope seal with the one I bought from RockAuto. Mechanic said it's the wrong seal, then said he tried anyways, and it wouldn't fit without pulling the crank. He's definitely not an older guy who would know how to work on that era of a car. He's probably in his early-mid 30s, roughly my age. The car is definitely older than he is. 


I talked to the guy doing the work. Apparently they ordered a rubber replacement seal two weeks ago, I didn't know they made them for Pontiac 301s, or I would've ordered that instead of the rope seal. They were waiting to hear back from the business on whether or not they need to RTV the seal, the directions were confusing. They didn't communicate with us or provide any updates about any of this. He did say there wasn't any unpleasant surprises in the oil pan. 


I showed the guy this video to try to answer his question about RTVing the seal.



He did not order that brand, but they are very similar, it's a 1-piece seal. He's going to take a look at that video tonight and they'll try it if he thinks it'll work. Seems to me that unless I have a 3.25" rear main journal, the only thing that might need RTVing is where he cuts the seal to fit it around the crankshaft. 

This post was modified 10 months ago by Justin Shepherd

Thanks for sharing the update. Always curious to hear the challenges in these unique situations.