308 rear main seal alternative

devindevin Posts: 7Member
Greetings, I'm receiving a 308 long block soon and will be swapping all the parts from my cracked motor. While I have access, I thought I would swap out the existing rear main seal with a Mopar?? one. Can anyone clarify which one for me and if this is a good idea. Thank you.


  • devindevin Posts: 7Member
    I should clarify that this is for my '52 Twin H Hornet 308
  • GlowplugGlowplug Posts: 1,907Expert Adviser
    edited March 23
    Put the Original style seal into your new block.  You are starting with a bare block and that seal will give best service. Dale cooper has these seals with the gasket sets he sells or separate if seal only required.
  • Ken U-TxKen U-Tx Posts: 3,772Senior Contributor
    I did this  on several Hudsosn. Pulled the engine out of the car. Be sure to get the new Best Gasket Graphtite Seal #5206. http://www.bestgasket.com/admin/uploadedimages/5206.jpg You can get this 5206 rope seal from either Randy Maas or Dale Cooper. DO NOT use the Chrysler 318 rubber two piece seal. I had two failures with that due to the fact the grooves in the Hudson block and cap were not machined to close enough a tolerance for it to seal against the crank seal journal. The two piece seal has worked on some 232-262-308 blocks, but the chances it might not work are at least 50% in my opinion. I don't like having to do something twice and losing time and money.
          When you are ready to install the Best Gasket 5206, be sure to clean all the oil & residue out of the groove in the cap and block. Obviously this step cannot be done on the block groove with engine and crank in the car, which is why that type of attempt often fails. Now with the grooves cleaned and dry, you take the half of the rope seal, and smear a thin film of Permatex black silicone on the sides and outer perimeter of the seal, keeping the silicone off of the inner sealing face. Next carefully push the seal down into the groove, and use a large round socket of about 2-1/2" to press it firmly into the groove. Wipe any excess silicone that pushed out. repeat with the other half.  Now it would be best to let this sit overnight so the silicone can set, before installing the crank. Next you take the razor provided in the kit, and use the cardboard template to trim the excess rope seal sticking up past the block and cap at the ends. The tongue depressor thing is placed against the side of the rope seal  to allow you to cut it cleanly. It should stick up about at least 0.030" on each side on both the cap and block. The next day , just before reinstalling the crank. you can smear a bit of silicone on the ends of the rope on either the block or cap side. Smear some assembly lube on the sealing faces of the rope seal. Don't forget to put assembly lube on the main and rod bearings before putting the crank in!
          Now you can put crank back in and reinstall the front and rear caps, and the two center caps. You best have a helper when reinstalling the crank! Now you will need to pack the vertical grooves on the rear cap with the cotton mop yarn that comes in the kit. I mix some black silicone into the yarn, and push it down into the groove with a long round punch about 3/16" diameter. Need to pack it good! Again trim it near flush with block oil-pan flange, a little protrusion (0.030") is Ok as the pan gasket with squeeze it, Do the same with the front cap vertical grooves, and then you pack the shorter horizontal grooves on the front of the front main cap. I used blue silicone on the first engine , 308, and black on the second a 262. Doesn't matter what color as long is it is a good quality oil resistant silicone. If any big globs appear, wipe them off.

  • devindevin Posts: 7Member
    Glow Plug and Ken U-Tx than you for your generous wisdom! I will abandon the Mopar trick and follow your advice. I just ordered some goodies from Mr. Mass. I'll get back in touch with him.
    Once again thank you!!! Stay safe

  • KdancyKdancy Posts: 2,416Senior Contributor
    Prier discussion-

    Rob FayetteRob Fayette Posts: 101Expert Adviser
    I sent an email to Best Gasket asking why they recommend to not apply sealant to the ends of the rope seal.  This is the reply I got back.

    RE: 5206 Rear Main Rope Seal kit question

    Roy Lucero<roy@bestgasket.com>
    3/21/2019 11:44 AM
    We do not recommend sealant on the ends or our GraphTite rope because the sealant will harden and actually damage the rope causing a potential leak passage.

     Other people might recommend sealant on the ends of the old style asbestos rope., not sure though.  

    Roy Lucero

    Best Gasket


    Ph (888) 333-2378 - Ph (562) 699-6631 - Fax (562) 699-5034

  • KdancyKdancy Posts: 2,416Senior Contributor
    Added from another post-

    Ol racer Posts: 2,495Senior Contributor

    October 2017 edited October 2017

    Tip on Installing a Rope Rear Seal 'In Chassis'..My Hudson seal drip was annoying so figured simply roll in a Neoprene Seal, however, afterward dripped worse. Like I mentioned above, crank seal journal may have minute scratches, or the cap groove is slightly larger.
    After researching learned a Rope Seal is far more 'forgiving' however modern Rope Seals in gasket sets (except Best Seals) are made from inferior material & unreliable, thanks to E.P.A. Reg's.  Fortunately, I had an old Gasket Set with 'old material' Rope seal, but had to figure how to roll it up over the crank...Here is what I did...

    Simply take a 12Pt curved wrench (also called starter, hoop, or half-moon wrench, but needs to be a 12Pt) then patiently loosen the three 1'4" bolts right above the crank, but do not unscrew them too far and pry back the plate slightly. I didn’t have a 12Pt so I made a wrench. You need put a slight bend on handle to clear a rear Plate Bolt at 3:00 and 9:00. (Pic below)

     Next soak the Rope Seal overnight in light Oil. (Yes, need to soak old material) then form fit the Seal in the Main Cap using a large socket or Pipe. I formed it to be .029 to start in groove then remainder was .031 - .034 depth. Next, buy a cheap Sneaky Pete Tool (Lisle #27000), use the Corkscrew provided to pull the old seal out while rotating crank, then discard the 3 tiny Lisle 'grab' attachments designed to pull the new Seal up (which won’t work), and instead, insert the Lisle piano wire above the crank then hook the end of the wire into the Rope Seal as far as possible & approx. 1/4" back from Seal end. Next, start the Rope Seal into the upper groove then rotate the crank while pulling on the piano wire handle. (suggest practice with pulling the old seal around). Lastly, trim the top Seal Ends flush then place a dab of Black Permatex RTV on each end and re tighten three top bolts..

    For the Cap, Fit the Seal in groove tightly and leave .015 protrude out of Cap providing a 'crush' when Cap is torqued 75 - 80lbs.

     I’ve done it twice now while lying on my back with no drip.... The key thing is don’t back the 1/4" screws all way out and be patient fitting the wrench on the bolts. Once get the rythm its pretty easy...This install was in Stepdown Hudson with Hydromatic.

    As a Foot Note: The Best Gasket Co Graphtite Rope Seal measured .030 before fitted in Main Cap groove so probably would pull up easier over the crank. It requires pining in the Cap so elected installing N.O.S Rope.


    Ol racer Posts: 2,495Senior Contributor
    Attached picture showing hooking Seal to slide up over the Crank after seal compressed using Main Cap.  Note The Sneaky Pete Tool is actually reversed from theit instruction since the 'Fingers' provided slip right off.

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