Bearing clearances

[Duke W.]

In some recent discussions with others on bearing clearances I stated that a one thousandth undersized or oversized bearing half could be combined with a standard size bearing half to adjust clearance up or down a half thou.

When I was asked the source of this information, I couldn't say as I dragged it out of my mental archives and cannot remember the source, and I never used the technique on any of my engines that I rebuilt as the clearances were where I wanted them with standard sized bearings.

So can anyone comment on whether this is a valid technique? Anyone ever used it? Is there a preferential half of the rod or main bearing saddle - upper or lower - where the oversized or undersized half should be used?

Duke

[Michael H.]

Re: Bearing clearances

Duke, Pretty sure you and I discussed this about a year or two ago. The use of one std and one .001" undersize main bearing shell on the same journal was common in production. The additional .0005 was for alignment, not clearance. It's a bit confusing because most will confuse the .001 undersize bearing as being thicker than a standard bearing but it was actually the other way around. Technically, this bearing should be refered to as an oversize, although it's actually thinner than a standatd bearing. (pretty confusing)

As far as I know, the .001 thinner shells were never available in service. I still have one or two that I removed from brand new partial engines in the 60's/70's. They're stamped .001" on the back but there's no mention of over or under std.

I don't recall this practice ever being used for rod bearings.

[Scott Marzahl]

Re: Bearing clearances

They are readily available from Clevite etc. I just thought they were used when a standard size crank is polished to the point where an oversize bearing is required. with that said, I recently came across a magazine article which mentioned the builder combined over/undersize bearing halves to achive the clearances he wanted. I can not find any mention in any Chevy Power Manual or engine building references that I have.

How about Clem, any input???

[Robert Jorjorian]

Re: Bearing clearances

Duke your advice of mixing bearing sizes is also offered by Smokey Yunick in his Power Secrets book on pg 25.

[Michael H.]

Re: Bearing clearances

Scott,

GM/Chevrolet always sold main bearings that were .001" UNDERSIZE/thicker but never sold any that were OVERSIZE/thinner. The description is confusing. An undersize bearing shell is thicker than the std shell.

GM sold these thicker inserts in .001", .002", .010", .020" and .030".

[Scott Marzahl]

Thanks Michael *NM*

[Clem Z.]

i did it all the time and GM even did this

on production engines that i have taken apart. they even has special sizes like .0007/.012 under as only .0005 were available over the counter. i may even have some around here that i removed from new short blocks or engines.GM once had a "knock" in the front main because of the load on that bearing due to the load of A/C,power steering and etc. they had all kinds of unsizes to fix this problem in the field

[Dick W.]

Re: Bearing clearances

I have been in several NASCAR engine rooms where they routinely mixed bearing halves to get the desired clearances

[Wayne W.]

Re: Bearing clearances

Henry Leland would turn over in his grave. I never ran across the proceedure in 60s engines, but those terrible seventies was a different story. I just chalked it up as crappy machining.

[Michael H.]

Re: Bearing clearances

Wayne,

Actually, selective bearings turned out to be a very efficient method of setting/adjusting bearing clearances. The range of bearing clearance was still maintained and it eliminated having to machine C/shaft journals to tight exact specifications. (not cost efficient in production) It was easier and cheaper to allow a slight plus/minus and make up the dimension with the odd bearing when necessary.

Because bearing inserts are slightly tapered, thinner at the ends, the center is the only part that makes contact with the journal. The ends of each insert don't actually make full contact with the journal. Because of this, the actual contact area of the bearing to C/shaft journal is unchanged, or nearly unchanged.

[Duke W.]

Re: Bearing clearances

Production bearing clearance specs cover a very wide range. For example, the 1963 Corvette Shop manual calls out .0008-.0034" for the mains and .0007-.0028" for the rods.

My preference for a road engine is about .0015" on the mains and .002" for the rods, and about half a thou higher for a racing engine.

So it looks like mixing halfs from a standard bearing with a one over or under bearing is okay for dialing the clearance in to where one wants if the crank journals are standard size.

I'm not sure if plus or minus one from the standard "oversizes" (actually undersize), like .010", for a ground crank are available, but I'll cross that bridge if I get there.

Thanks for the responses.

Duke

[Michael H.]

Re: Bearing clearances

Duke,

Yes, I see absolutely no problem with mixing upper/lower bearing sizes up .001" or slightly more. That gives you a better range on clearance if needed. (likely not necessary though)

I don't think any of the undersize bearings in the .010 and beyond range are available in the .001" additional over/under size but I'm not sure. I know that GM never offered any but I suppose aftermarket may have something.

Is the C/shaft that you are refering to a forged crank? If so, taking .010" off the surface may require a heat treat process, depending on the actual original process. (tuftride/nitride) If cast, no heat treat is required. I know a lot of forged C/shafts are ground and not retreated but it's a gamble at the very minimum.

[Clem Z.]

i have had 350 cranks offset ground .100 under

on the throws to make 3.562 strokers without any reheat treat for race engines. these cranks ran all year without a problem. if you check the FM catalog i still think they still sell these 3.562 stroker pistons. you use the 2.000 rods with this setup and .125 radius in the corners of the crank journals which use a special beveled bearing inserts

[Duke W.]

Re: Bearing clearances

Questions relate to general thinking or maybe a secret project.

Since the non-SHP/FI 327 forged cranks weren't Tufftrided, grinding them should not have a significant effect on durability, but I always advise to only grind cranks if absolutely necessary.

A Tufftrided SHP/FI crank will lose durability if ground - to no better than a ground non-SHP/FI crank, but I don't think it will result in failure on a street engine, even if it is occasionally revved to 7000.

Duke

[Michael H.]

Re: Bearing clearances

Duke,

I agree, it's not absolutely necessary to retreat the shaft after grinding but a forged shaft will actually have a softer journal surface after grinding than a cast C/shaft journal. The journal surface on a cast crank work hardens almost immediately on startup but a forged journal does not.

Most older forged c/shafts were induction hardened at each journal but later, the entire forging was either tuftride or nitride treated.

I have some GM documents on this, somewhere. If/when I find it, I'll send it.