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Technical Anyone mix main bearing shells?

Discussion in 'The Hokey Ass Message Board' started by Budget36, Oct 12, 2020.

  1. Budget36
    Joined: Nov 29, 2014
    Posts: 4,265


    After checking mine, looks like I need to get .001 undersized. Put a search in Google and came across an article from SuperChevy ( I think it was) and reading through it they were talking about dialing in clearances-this was after align boring, etc. anyways, they said it was okay to use say a standard bearing size on one 1/2 and an over/under size on the other to achieve a .0005 you might need.

    I thought that odd.

    Anyone do this before, is it generally done? Not that I’ll be doing it, I’m just trying to satisfy my curiosity
    anothercarguy likes this.
  2. I don't see it happening but interested in hearing more on this.
    Elcohaulic likes this.
  3. jaw22w
    Joined: Mar 2, 2013
    Posts: 1,065

    from Indiana

    Just finished a 355 build. Mains 1-4 have a +.001 bearing shell and a standard shell. Clearance was .0031 with 2 std's and .0026 with split shells. They will tighten up clearance by .0005".
    In fact the 383 in my roadster has split shells in it. Pretty common thing to dial in clearances
    seb fontana likes this.
  4. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 1,221


    Yes. I have read those types of articles and even done similar things. When young, broke, and trying to build race engines I was fortunate to have a older friend who was the area Chevy warranty inspector. He would slide my way gently used bearings, and an occasional crank or rods. I would spend time measuring bearing shell thickness and mix and match in an attempt to blueprint bearing oil clearance. I not saying I recommend it, but never lost a bottom end.
    Last edited: Oct 13, 2020
    Elcohaulic and Truckdoctor Andy like this.

  5. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,513


    AMC and Ford offered .001 and .002 oversized bearings. I know Honda used selective fit rod and main bearings on some of their motors. The block and crank were stamp as to what bearing was on each journal. Most of that has gone by the wayside now with better machining being done and better bearings being made.

    Not sure if they are still made but at one time Delta Wall bearings were being produced for rod journals. The upper shell fit closer to the crank and lower shell had more clearance. This was to allow more oil to be distributed around the journal. They were used mainly in circle track racing.
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 46,875


    I've never select fit a main bearing to .0005, and never saw the need to.
    jimmy six, 57 Fargo and Deuces like this.
  7. Ericnova72
    Joined: May 1, 2007
    Posts: 429


    It's a common thing with race and high performance engine builders who want to really blueprint clearances.

    Never more than .001" difference between shells don't put an +.001" shell with a -.001" shell, even though that would seem to make just a standard clearance again....the difference in thickness at the parting line can turn too much of a step difference into a oil squeegee, and the result won't be good
    Hnstray, Deuces and Budget36 like this.
  8. Budget36
    Joined: Nov 29, 2014
    Posts: 4,265


    The idea I gathered in the article was that you just had the block machined, align bored, etc. you measure and have .003 clearance, but for you needs, you want .0025-just an example. I don’t blueprint engines nor stay at HIE’s, I was looking at feasibility of it and if it was just a magazine claim or actually done in the real world
  9. Dick Stevens
    Joined: Aug 7, 2012
    Posts: 2,925

    Dick Stevens

    I've never done it, but I wouldn't hesitate doing it in the case that you mention.
  10. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,608

    seb fontana
    from ct

    Ford engines of the 60'-70's came with a tighter fitting [different p/n] front main top half to better support the accessory belt load.
    Hnstray likes this.
  11. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 2,738

    from illinois

    SOP for factory engines. Putting in 1 shell standard & 1 +.001 makes the bearing assembly ,.0005 different ..Last 2 aftermarket cranks I've been involved with ,this was necessary..
    Budget36 likes this.
  12. Glenn Thoreson
    Joined: Aug 13, 2010
    Posts: 98

    Glenn Thoreson
    from SW Wyoming

    Seems to me that you would need the same bearing size on all top halves or it would put bending stress on the crank. The bottom halves would need to be selective fit to get the clearance you want, which would need to be exactly the same on all bearings or, again, bending stress and perhaps a broken crankshaft. I was always under the assumption that all-out race race engines were set up with a .001"
    more clearance and use a heavier oil. More cushion and less surface to surface friction. A good machine shop can resize bearing shells to get exact results. But I'm not up on modern race engines and the idea of .0005" clearance sounds strange to me. Just my thoughts on this.
    2OLD2FAST likes this.
  13. lippy
    Joined: Sep 27, 2006
    Posts: 4,987

    from Ks

  14. Budget36
    Joined: Nov 29, 2014
    Posts: 4,265


    The article did mention that, went on to say doesn't matter if the same ones are on top or bottom, but all the same size needed to go on the same side.

    I'll see if I can dig it back up and put the link here.
  15. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 2,738

    from illinois

    Correct . If your max recommended clearance is .0025 & you're reading .003 ,a+ .001 over half shell will put you there .
    Last edited: Oct 12, 2020
  16. Budget36
    Joined: Nov 29, 2014
    Posts: 4,265


    Here's the excerpt, and the link:

    Mix and Match for Optimal Clearances
    Generally speaking, if you want to go a little loose, subtract 0.0005-inch and if you want to tighten up the clearance, add 0.0005-inch. The twist to that is bearings aren't typically sold in such increments. They're offered in standard size, undersized by 0.001-inch or oversized by 0.001-inch. You go with the undersized bearing to loosen the clearance and the oversized bearing to tighten it.

    Start with the standard bearings and if you find a clearance adjustment is required, move up or down, as necessary. And because 0.001-inch in difference can be more than desired, you can mix bearing sets to achieve the 0.0005-inch desired adjustment. Simply mix one of the shell halves of a standard bearing with a shell half from an over- or undersized bearing. Yes, it requires the purchase of two sets of bearing sets, but that's the price to pay for clearance optimization.

    One more thing: When mixing the bearings, make sure all shell halves are aligned. That means install all the standard halves on the block side and all the undersized halves on the cap side or vice versa. It doesn't matter which side they go on, only that the same sizes are on the same sides of the components. CHP
  17. Dave G in Gansevoort
    Joined: Mar 28, 2019
    Posts: 152

    Dave G in Gansevoort
    from Upstate NY

    Back in the day (1970s) engine builders I knew would use Scotchbrite pads and rub the bearing shells to get the clearances they wanted. Didn't sound like a good idea to me, but then again it worked for them. Besides most of the motors talked about were dirt track engines, which clearanced themselves due to weekly dirt ingestion, and were freshened every 6 to 8 race nights. Rings, bearings, valve andseat grind, and new gaskets/seals was considered sufficient. A race season was 18 to 20 nights per track, and most of us only raced 1 track a week most of the season, so 2 freshen ups a season. A winter rebuild would be an overbore, and new pistons, polish the crank, new rod bolts, heads redone, probably with guide liners as well as seat and valve grind. New valve springs and good to start the season.
  18. anthony myrick
    Joined: Sep 4, 2009
    Posts: 6,171

    anthony myrick

    Lots of foreign engines have select fit bearings.
    The clearances are checked and bearings installed to achieved the desired oil clearance.
    Joined: May 8, 2010
    Posts: 317


    Just about every engine I put together for the street (some 10 now) I have never even used plastigauge, a few of these engines are still going strong after 20 years.
    I am pretty sure most engines that were assemble new in the 50's, 60's and 70's (only ones I have done) all had numbers stamped on the sump mounting rail, I was lead to believe that these were for bearing and piston variations so the guy putting the engine together got the right set of "standard" units + or - off the rack.
  20. I've used the oversize bearings in the past, to get the clearance I wanted, but never mixed shells. I'm not agreeing or disagreeing with it, just have no experience with it.
  21. big john d
    Joined: Nov 24, 2011
    Posts: 163

    big john d
    from ma

    a good friend uses shifiroff (sp) engines and when freshening it up he use the same size mixed bearings as the engine came with needs to purchase two sets of bearings
  22. SS Pete
    Joined: Jan 13, 2017
    Posts: 23

    SS Pete

    I worked in machine shops 10 yrs. This was done on a regular basis. I first saw factory (GM) in a 283 in the 60s, and in 90s . common in upper end shops. some "kits" would have .0035 on mains which would result in low oil pressure.. the good bearings would be slightly chamfered at the parting line to facilitate this practice.
    Budget36 likes this.
  23. The chamfer at the parting line is to accommodate bearing crush, when the cap is tightened the bearing shells are squeezed together to keep them from spinning, the chamfer is so there is not a high spot at the parting line

    Sent from my iPhone using H.A.M.B.
    Deuces likes this.
  24. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,457

    from Brooks Ky

    Have done it and it works well. If you have too much clearance, then you have too much don't just ignore it....... Obviously too much clearance will not usually cause an immediate problem, its just that it may be the one thats a problem in the future.
    Budget36 likes this.
  25. This thread is an example of "if you haven't tried it, it can't be good". Racers are famous for inventive ways to achieve the end goal. I know people that won't do anything unless it's in a factory service manual. I know people that do stuff that nobody else can get away with. There are very few hard and fast rules in life. Someone can make it work better than anybody else.
    RidgeRunner, X-cpe and Budget36 like this.

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