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Technical Who has the capability of narrowing connecting rods?

Discussion in 'The Hokey Ass Message Board' started by Roothawg, Oct 20, 2017.

  1. C69A
    Joined: Jun 6, 2008
    Posts: 83

    C69A
    Member

    I built a 390 cad a few years back, first set of Pistons the customer got were Badger,told him to get his money back as some were 12 thou bigger than others second set were from Kanters two broke at the top ring land at 1500 miles then got some made by Venolia, has done 30,000 miles now with no issues.
     
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  2. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

  3. southcross2631
    Joined: Jan 20, 2013
    Posts: 4,271

    southcross2631
    Member

    that is the way I would go. Polish the beams and check them for cracks and straightness and change the bolts.
     
  4. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    I have a couple of NDT guys that work for me. I'll have them run the eddy current and mag particle on them.
     
    SquintBoy likes this.
  5. I read an article back in the late 50's about building a Caddy . The article said to side clearance rods to .030" , this will prevent rod bind at hi rpm . I run the .030" in my Caddy and constantly run 6500 rpm when going through the gears .
     
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  6. tjet
    Joined: Mar 16, 2009
    Posts: 1,272

    tjet
    Member
    1. Early Hemi Tech

    Joel, when you narrow the rods, what do you do with the bearing tab indentation? (not sure what it's called)

    Do you cut them in after?

    I need to narrow a set of rod big ends from 0.935" to 0.765"

    Thanks
     
  7. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    I still haven't pulled the trigger yet. The 455 rods are about .125" longer or you can run BBC rods. Both have to be narrowed or the crank has to be opened up and radiused. Not sure if there is a "best" option.
     
  8. ottoman
    Joined: May 4, 2008
    Posts: 293

    ottoman
    Member
    from Wisconsin

    I have narrowed many sets of rods and moved bearing tangs to make oddball combo's work. Narrowing the rods can be done by any good machinist
     
    Desoto291Hemi likes this.
  9. jimmy six
    Joined: Mar 21, 2006
    Posts: 8,372

    jimmy six
    Member

    That's an 1/8" and quite a bit. No one here has said they're doing an 1/8". I've clearances all the rods I've used in my GMC's on a mag base surface grinder but never near that much, definitely not needing to narrow the bearings.
    I'd like to hear more from guys who have taken .060" from each side.
     
  10. jaracer
    Joined: Oct 4, 2008
    Posts: 766

    jaracer
    Member

    When I put together Chevy engines for my sprint car, I always opened up the side clearance. Did it one time with a surface grinder that had a magnetic table. Ran those rods in engines for years without failure. The very last one I put together I didn't have access to a surface grinder so I used a stone in a drill press. Took off a little at a time and kept checking with a mike to ensure they were square.
     
  11. Hemi Joel
    Joined: May 4, 2007
    Posts: 908

    Hemi Joel
    Member
    from Minnesota

    I have never taken off enuff to get into the bearing tang indentations. If I ever had to, I would use a T-slot cutter to put the indentation where it needed to be. The rod and the cap would have to be done separately, so some careful measuring would be needed, and maybe a fixture to hold them in the mill straight.
     
  12. nrgwizard
    Joined: Aug 18, 2006
    Posts: 1,275

    nrgwizard
    Member
    from Minn. uSA

    Hey, Jimmy6;
    "I use 8" rods in my 302 GMC6's. Long rods work, at least for us."
    Where did you find 8" rods & from what, if not totally custom?
    Rpm you're running in the gmc 6?
    TIA. Marcus...
     
  13. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,823

    seb fontana
    Member
    from ct

    Keeping the rod side clearance tight [around .012] helps piston stability, rod small end stability. Think on it, slightest pin angle or bore angle [there is always some] and the rod small end will slide back and forth on the pin and transferring some force to the piston ..If you want to let oil out then just grind some 1" wide reliefs about .020 deep on both sides of the rod big end parallel to the beam. The reliefs will not bother each other on the crank pin. I would not remove .110, .050 is about most I would be comfortable with.
     
  14. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    The stock 390 rods are 6.500" long, the Pontiac rods are 6.625 and the BBC are 7.100.
    The widest point on the cap is .995 for both the Pontiac and the BBC.
    The original caddy rod had an .8775 cap width. So, approx. .117" plus your side play.

    If you are into rod ratios, the Pontiac works out to be 1.83, The BBC is 1.83 and stock was 1.68.

    The recommended stock side journal clearance for the Caddy is .0008-.0014".
     
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  15. jimmy six
    Joined: Mar 21, 2006
    Posts: 8,372

    jimmy six
    Member

    Custom. Stock head port size limits the RPM in my kind of use. Lower gears can see a lot more probably over 6500.
     
  16. Hemi Joel
    Joined: May 4, 2007
    Posts: 908

    Hemi Joel
    Member
    from Minnesota

    A gen 3 hemi has a rod width of .787. maybe you could turn the rod journals down to fit the big end of the Hemi rod, while widening them slightly at the same timeand find an aftermarket rod that is up sufficient length.
     
  17. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    What's the journal size?
     
  18. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    Lots of things to consider here. After Machining the rods, what bearing will you be able to use in it, and will it need to be narrowed? Is there enough material on the crankshaft to widen the journal ? If so, you also might look into enlarging the fillet as much as possible. Corvair engines adapted for airplanes had some crank breakage problems due to small poorly ground fillets from the factory. I don't know if it was a problem in any other GM engines, but a larger fillet properly done means a lot. If you can't get all the width from widening the journal, maybe a combination of widening the journal and narrowing a rod might be feasible. Also you have to be sure any rod you buy will clear your camshaft and your block at the pan rails.
    You could have your stock rods shot peened and resized if you don't plan any real abuse from your driving.
     
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  19. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    I would probably just run the stock Cadillac bearing, since the rods would match the factory specs. Oh and I abuse all my cars, so shot peening would be a false sense of security.....
     
  20. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    How about a little more info on the Caddy. A Chevy 400 has the same stroke as the Cad (3.875) and a 9.025 deck height with 2.100 diameter rod journals. Edit: Big OOPs there.
    The Chevy stroke is 3 3/4 (3.750)
    NOT 3 7/8 (3.875) Big time Brain Fart:confused:

    What are the Cad's Deck Height, Rod Journal Dia, Piston Pin Dia. Also do you have the early or late 390 ? Have you looked into other than GM rods ?
     
    Last edited: Feb 10, 2020
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  21. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    You have to be sure that the inside diameter of the rod you select and the Cad rod are the same. Not all bearings are the same thickness so its possible a different brand of rod would be a different diameter inside.
     
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  22. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    Well, since you asked nicely.......

    Early 390, last year of the early style 1962
    Yes, I am looking at the Pontiac and BBC rods.
    I am buying custom pistons, so don't feel that we have to stick with these numbers. Some we have to get as close as possible, others, not so much.
    I can size the rods appropriately also.

    Deck height 10.650"
    Rod diameter 2.2488"
    Rod Dia w/o bearing 2.3470"-2.3745"
    Mains 2.625"
    Stroke 3.875
    Rod Length is 6.500"
    Factory pin height is 2.0938"
    Pin diameter .9994"-.9997"
     
  23. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    If I haven't had another brain fart.........Big "IF" there. The deciding factor seems like how much "compression height" you need to have room for your rings . If you take something in the range of 1.430 I think you have about .660 you can add to your rod length. There is room for some fudge factor on ring placement. I'm wondering if since 4.00 in bore is such a common size, if it might be possible to find a standard aftermarket piston that would work. Also, the diameter of the piston pin will affect this.

    Compression Height Diagram.jpg


    Roothawg Compression Height Diagram.jpg
     
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  24. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    It looks like you did your homework pretty well. I looked on the Scat site and the best option I could see was the Pontiac rods with the 2.249 rod bearing. I think I would at least check and see if there is room to widen your journals and put good radaii on the fillets before I tried narrowing the rods. If it has room, you could then just use stock Pontiac bearing that you know fit the rod correctly. There might be a little room to narrow the rod slightly without getting all the way to the side of the bearing, but anything you can do to keep the Pontiac bearing in the Pontiac rod seems a better choice than using the Cad bearing.Both rod bores are the same per my old Federal Mogul catalog but the part numbers are different. May be due to width or maybe tang placement.......or both. I guess you could just insert one of your old bearings in a new rod before you decide. Got any buddies with an old Pontiac laying around ?
    Buy one of these and see how your plan works before using high dollar stuff.
    https://www.ebay.com/itm/PONTIAC-V-...643647?hash=item2cdac1043f:g:ybMAAOSwznVaz2ch

    To answer your original question: If you have the rods milled, any decent machinist can machine them down easily. Be sure you stipulate that you want material taken from both sides equally. You may assume he knows that, but since I'm a former machinist I will tell you to make sure he understands. The best thing is to finish them on a surface grinder. A conventional mill is somewhat precise, but a surface grinder will make them all the same because they put them all on a table at the same time. Turn the magnet on to hold them. ZThen as the table reciprocates back and forth under the grinding wheel they all come out exactly the same......and smooth. If you only mill them, then leave a .001/.0015 on each face and finish them by putting emery paper on glass or a flat surface and lapping the last .001 off them.

    Since you will probably be having your crankshaft ground anyway, I would look at the idea of widening the journal as a first consideration (along with nitriding it). If thats feasible then the rod narrowing becomes moot.

    Sounds like a nice project.....keep us up on it !;)
     
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  25. Fordors
    Joined: Sep 22, 2016
    Posts: 3,361

    Fordors
    Member

    I have no idea, but I’ll ask- do crank grinders not like to use the sides of their wheels? I understand that they might dress the thrust surfaces but grinding .056 per side off the journal might be a deal breaker.
     
  26. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    That's way cooler than what I put together.
    It's a catch 22. I don't want to order the rods before knowing if they can grind the crank or surface the rods, but I need the rods for sizing.
     
    Last edited: Feb 10, 2020
  27. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,709

    GearheadsQCE
    Member

    Cutting the rods is no big deal. As others have said, they can be done on a lathe, mill or surface grinder. (.117" is a lot of passes on a surface grinder.) I use a mill and the surface finish is good enough that no sanding or grinding is required.
    The big deal is that the bearings have to work or be modified to work.

    I mess with a lot of stuff like this and there is no substitute for mock-up pieces. Recently put together a loooong rod Chevy 292 six. I sectioned a stock piston to determine if the theoretical compression height was correct in real life. What I found was that the C.H. was pretty close, but that the bolts on the new rod hit the cam. I think that I can replace the bolts with ARPs for a Ford and get the required clearance. But, it was a bunch cheaper to do a mock-up with old parts than buy a set of custom pistons and find out that it wasn't going to work. Now to find somebody with an old Ford rod bolt or better yet the ARP.

    Maybe a new thread asking for a Pontiac guy to measure and take pics of a rod like you intend to use would help. I'd read it!
     
  28. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    They won't use the side of the wheel. It will be a plunge cut just like they do for the rest of the journal. They probably will take several cuts rather than just one and then finish it when they actually grind the journal. Probably use an old wheel to rough all journals close and then dress the finish wheel and go back over all the journals. Getting a good large fillet will be important too.
     
    Last edited: Feb 11, 2020
  29. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,825

    ekimneirbo
    Member
    from Brooks Ky

    I suggested buying the used rod off Ebay for about $30, but you won't actually know till you get the actual rod you are going to use. I'd go to the Scat site and look at their "All Rods" listing. You can scroll down and see every rod and its dimensions. I saw some H beam rods that look better suited for clearing a camshaft. Don't know how they will do as far as clearing your pan rail. Bout halfway down look for 66625 Pontiac Maybe you can buy one rod.........

    https://www.scatcrankshafts.com/product-search/
     
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  30. Roothawg
    Joined: Mar 14, 2001
    Posts: 20,961

    Roothawg
    Member

    One rod wouldn't be too bad, I don't think...
     

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