Register now to get rid of these ads!

HELP! rear main seal on 6 cyl Chevy

Discussion in 'The Hokey Ass Message Board' started by CayoRV, Jun 2, 2013.

  1. CayoRV
    Joined: Dec 19, 2008
    Posts: 356


    Can't seem to find the answer to rear main seal leak. Have a 1959 235 that has gotten the works. New cam, lifters, timing gears, pistons, rings, reconditioned rods, reconditioned (.020/.020) crank, all reconditioned 848 head, all new gaskets and seals. All machine work was preformed on the block by a reputable shop I have used for many previous builds. The only thing not done was to line bore the block, something I have never done on previous street motor builds. Running Fenton 2x1 with Rochesters and Fenton split headers, late model GM hei distributor and all backed up by a the stock cast iron powerglide in a 52 Chevy hard top.
    OK so thats all the information, now to the problem.
    Cannot seem to keep a rear main seal in this thing.
    We are on the the 3rd seal and its leaking again. Rubber not rope.
    In the car the pan can be removed and the seal changed but this is getting very frustrating. We have only been able to get between 300 and 700 miles on a seal before it leaks, and not just a drip, a major puddle and leaving a trail behind!
    Have checked all the clearances every time it has been apart and found nothing out of spec. The last go round the rear main bearing did show some unexpected wear toward the rear edge of the bearing and was replaced with a new bearing shell along with the seal. Once again checked with plastigauge and buttoned it up only to have it leaking again after only 300 miles!
    The engine starts right up, has great power, is smooth running and sounds great. Gets expected mileage and is in every other way a fantastic success.
    This car belongs to a very good friend and as such I cannot vouch for the way it is being driven (he tends to have a bit of a lead foot) but I would think that this motor could handle cruising the highway at 70 which would put it at approximately 3500 rpm without this type of failure. This thing carries 45 psi of oil pressure cold and 30-35 warm at idle. I can't believe that could be excessive and contributing to the problem.
    Anyways I just thought some might ask.
    I am looking for any thoughts on how to fix this thing before it gets pulled and replaced with yet another belly button sbc. They are a great engine but not as cool as a stovebolt.
    Sorry for the long post, I just don't know where else to turn for help at this point.

  2. H.G. Wells
    Joined: Mar 11, 2006
    Posts: 386

    H.G. Wells

    Been a long time since I have been into one but... Did you check the crank end play?

    Try one of the old antiquated rope seal and see if that does not work better than the new improved rubber.
  3. CayoRV
    Joined: Dec 19, 2008
    Posts: 356


    Yes, end play was withing spec each time also. I think the rope may be the last attempt before the sbc.
  4. chessterd5
    Joined: May 26, 2013
    Posts: 643

    from u.s.a.

    hello, Is your crankcase vented properly? excessive crankcase pressures can blow out seals. Just a thought ,thank you.

  5. incogneto
    Joined: Nov 25, 2012
    Posts: 26

    from Australia

    Might i suggest you ask this question on some Stovebolt specific boards like or
    This is not an uncommon problem, and someone there will have the answer for you.
    I have a gasket number from a Japanese engine somewhere if you get stuck. It requires the crank to be machined to suit it.

    If you cannot find the information from these bboards, pm me and I will find it for you.
  6. CayoRV
    Joined: Dec 19, 2008
    Posts: 356


    Checking with the Stovebolt guys. I think I may try the old rope seal before I give up. Just have to find the right one and get it installed properly. Seems to be some difference's in opinions as to the best brand and installation technique.
  7. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    from Phoenix AZ

    +1 on excessive crankcase pressure. Also the oil pressure is just fine. Ran a warmed up 261 years all fresh and had OP in the same area and no more than a bit of seapage from the rear main. It is normal for a rope seal to weep a bit,pre 70s stuff was not all that oil tight as a rule.
  8. CayoRV
    Joined: Dec 19, 2008
    Posts: 356


    Yes the road draft system is clean as is the breather cap on the valve cover so I don't think crank case pressure is to blame. At this point a weep would be great, just not the Exon Valdez!
  9. There is a permanent cure, but it involves some machine work. I machined the rear of my 261 for a one piece rear main seal from a Ford Powerstroke. The 235/ 261 crank flange o.d. is very close to the Power stroke diameter. It's a very slick update to these old blocks.
  10. waldo53
    Joined: Jan 26, 2010
    Posts: 863

    from ID

    I wonder if when they were doing your crank if they ground down the area where the seal rides? Might be your problem. I would try the rope seal, read all you can over on Stovebolt before you install it. I think the offset join-line is a must (don't have the 2-halves of the seal join right at the bearing cap line). Try to find a NOS seal if you can.

    Man, this has got to be pretty frustrating at this point, hope you get it fixed.
  11. CayoRV
    Joined: Dec 19, 2008
    Posts: 356


    If this were the original build on the engine I would look into the Powerstroke seal option but this thing has us beaten down pretty bad now. If we can't correct the problem with the engine still in the chassis by trying an old school rope seal I think it will be sbc time. Maybe after the 6 is our and has set for a year or so I would come back to it with a fresh outlook but for now he just wants to be able to drive the car without the problems.
  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 18,521


    I have had no fewer than 3 machine shops cut cranks where the seal rides. Not sure if this is the case with yours, but worth a measure. If it has been cut, I doubt that any seal will now fix it.
  13. afan
    Joined: Jan 1, 2006
    Posts: 283

    from michigan

  14. As a rule, there is no need to touch the seal surface when you grind a stovebolt crank. Usually just a polishing because you don't want to change the diameter of the sealing surface. Not all of the blocks are intended for lip seals, but I don't recall what the difference was. I know if you use the lip seals in an early block they will leak like a stuck hog. The rope seal should cure the problem. Read up on proper installation, first.
  15. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 943


    Did the machine shop line bore the block? That could be the problem, the seal is no longer concentric with the crank centerline. Very common on line bored blocks. A rope seal is very forgiving in this case, while a rubber seal becomes oval shaped when you torque the cap in place.
  16. fossilfish
    Joined: Dec 16, 2010
    Posts: 320

    from Texas

    The only rope type seal I have found that is any good is the rope seal used on Mercedes diesel six cylinders. If you find you need a rope seal this is the one to use.
    The new rope seals found in kits are paper wrapped with a fabric that last about 700 miles.
    I have found that too much rear main clearance will flood the seal area with oil too.
    Many engines have a drain back port to the inside of the seal area. The oil that comes out of the rear main drains into the pan. If that is clogged up or plugged the oil will pour out the seal too.
    Your Results May Vary

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.