Bearing Clearances?

[BuickTom]

I'm in the process of building a new engine, girdled 4.1 production block, fwiw. I'm curious about pros and cons of running either tight or loose main and rod clearances and what the general consensus is around here. I've seen some info saying running these motors on the loose side is about the worst thing you can do while others say there is nothing wrong with it. The guy building it likes to build motors looser, he's only done a couple turbo 6's supposedly quite successfully, but has done numerous other hi perf chevys, fords and mopars.

Its going to be used in a primarily street driven car, with frequent trips to the track, looking for easy mid 11's, maybe someday reaching for a high 10.

Thanks,
Tom

 

[TurboDave]

Well, opinions are like A** H****, everbody's got one, so here's mine.

Go with the clearences in the Buick Power Source. You'll see they're a little on the tight side as compared to the rest of the world of high performance. IMHO, forced induction engines need to be run on the tight side.

YOU CANNOT build Buick V6's or any other forced induction motor to the same specs as your typical HP Chevy small block. Too many engine builders have tried it, and the results are not pretty.

 

[Steve Wood]

There is no answer to that question.

I prefer nothing over .0015 on both, assuming rpm will not exceed 5500. The few engines I have checked from the factory were tighter.

Many very successful engine builders like .002 and have plenty of oil pressure which may be due to the care they take with the oiling system. That is outside of factory spec and may come from the factory suggestions of using larger clearances for Stage engines turning 8000+ rpm.

As rpm ranges increase, larger clearances should reduce oil temperatures and increase bearing life. I don't think it increases power much in the ranges we are speaking.

I think engines last longer built tighter as long as oil temps stay reasonable. That is a personal opinion and should not be construed as an absolute truth.

 

[Russ Merritt]

I like em tight too!!

 

[Buick Mark]

Its funny how Buick seems ahead of the times in alot of areas concerning engines. Todays engines run tight clearances, just as Buick has always done. Not to mention the deep skirt block of the Buick V-6 and 350, and cap screw connecting rods, sequential fuel injection, distributorless ignition, etc,etc. I remember reading an article about one of the new Vette motors talking about this all new technology, Im like whoopie, Buick has been doing that for years. Run the clearances Buick says to run and all will be good Mark

 

[Nick Micale]

Originally posted by Steve Wood

Many very successful engine builders like .002 and have plenty of oil pressure which may be due to the care they take with the oiling system. That is outside of factory spec and may come from the factory suggestions of using larger clearances for Stage engines turning 8000+ rpm.

Lots of things are different since the Buick info of the '79's and 80's especially with our turbo motors. My comments apply to you and others building a stock block to go 10's, or almost the limit.

We have found problems engines built to the .0015 spec that ended up much less than that. So this means almost no clearance and a ruined bearing or more. Just checking and re-checking during build, the tolerance may be .0007-8 difference on the bench as the parts deform or conform.

I would say 6000+ RPM is not unusual for a race motor and to do this often, you best have a good oiling system. I have not seen a bearing sieze because it is too loose, UNLESS it did not have an adaquate supply of oil. We have built engines for years and not lost a bottom end from being set at .002. As much time and care is spent in the oiling system as the assembly of the motor.

Now Steve and I are only .0005 apart on what constitutes loose or tight . What really counts is what does your builder think is loose? Like Dave said, you do not want to build it like a typical Chevy that may go .0035. I could maybe get Steve to go with .0017-18 and still be happy [at least on a strip motor]?

 

[Intercooler]

I have had them both ways, loose and tight. I actually had better oil pressure with the loose motor I did and it is still holding up to this day. When I say loose I mean .0025"-.003" on the mains and .002" on the rods. If I had to give my ideal clearance it would be .002" on the mains and .002" on the rods.

 

[tc86gn]

There was a really good article on the buildup of these turbo V-6's on gmhighperformance.com it said something about having tight clearances(no more than.0015)on the mains and rods because we produce a lot more power at a lower RPM than it would take say a 350. It would be revving higher to make the same HP. So they need more clearance. Not sure if I agree or disagree but the article is worth checking out.

Tom

 

[Steve Wood]

LOL

Yes, Nick, you might! (but not too many others could)

 

[Intercooler]

I don't. If you read the Powersource you can see as the HP level goes up so does the clearance on the engine builds. I think half the crap Merkel used in the write-up come from the Powersource book because they talk about opening up the main oil hole passages. Don't see a need in that either!

 

[tc86gn]

I don't see the need for that either but there are a few good points in the article related to bearing clearances. I prefer not to go more than .0015.

 

[Chuck Leeper]

Clearances

Nick has the same idea I have on the clearances. At a .0015, a brg bore on a rod, or main for that part, gets a bit tight... Now you are in DEEP do do...
Do this test... Take a FRESHLY REBUILT rod and check the bore. Put the rod in a ROD VISE and loosen the bolts and then re-torque.. I'm betting a steak dinner at the BOP, that it'll change!! I have seen this done and it took several "touchups" on the rod hone to where the rod stayed at spec...
Following that logic; the rod is in spec,[after the initial rebuild hone job] you put the engine together and now it's NOT and the engine has never had a load on it!! They most likely will "cave in" at the parting line.. that plus the loading during max power and the parting line added clearances go to pot and scrape the oil film off the crank and, well, you got the rest of the story..
I vote at least .002!! Let it knok when cold... won't hurt it... EVERYONE lets the engine warm up anyhow!!!

 

[Steve Wood]

Re: Clearances

Originally posted by Chuck Leeper
EVERYONE lets the engine warm up anyhow!!!

Everyone over the age of 50, anyway!

Several that I checked that were stock from the factory were around .0012.

When you look at the Stage engine tolerances in the Power Manual and then look at the rpm graphs, they are turning 8500 rpm. They also suggest oil pressures of 100 psi. I find it strange how often I see quotes on clearances from the PM but no one quotes the oil pressures. I think it is a case of pick what you like because it sounds like a race engine.

Factory specs are:

Mains: .008-0018

Rods: .005-.0026

Now, having typed this....it is the first time that I have noted that the rod range is both tighter and looser. I would gather that either the factory cannot hold tight tolerances on its rod machine or it just ain't very crucial. I assume this covers the distortions caused by assembly pointed out above.

Okay, Nick, you can build my new TA block with .0015 on the mains and .002 on the rods.

I still think the reason for long factory engine life is tight clearances but admit there may be (note the may be ) a difference in an engine that runs into the 11s at times and one that runs tens all the time. Differences other than about 100 hp, that is.

 

[BuickTom]

Thanks for all the replies guys. I think I'll probably go on the loose side and go 2 and 2 with it. After talking with the guy building the motor and the guy cutting the crank yesterday, their idea of loose was .003 - .0035 so we got that all cleared up anyway.

For those of you who recommended running 2 and 2, do you have any specific recommendations as far as oiling upgrades?
I had been thinking of staying with the stock front cover and gears, and following the recommendations about enlarging and radiusing the oil passages in teh power source manual. Its an 84 4.1 so it should have the same size main oil galley as the 86-87 turbo's, but i haven't actually measured it.

Thanks again,
Tom

 

[Intercooler]

Do the front cover mods, stick a 60-70 PSI pressure relief spring in there, std. volume pump, end clearance at .001"-.002" and go!

 

[Russ Merritt]

RIGHT!!!

 

[Chuck Leeper]

Crankz and brgs

Looking at the FM brg catalog.. Interesting info.
There's a whole section there telling how to polish different kinds of cranks.. even goes into what polish methods to use for different kinds of belts. It warns that improper polish direction will kill the brgs. It also recommends a 3 step polish. 280,320,400 grit polish belts be used. This includes the thrust surfaces!
The manual goes on to say that rod clearances from .0007 to .003 are within range. The ranges for the mains are from .0005 to .0029 and as high as .003 w/ the race brgs. [See my previous post on rod bores that don't return to specs]
The info includes a warning about brgs that have the "cosmetic coatings" on them..[tin plating] Under high loading such as racing, [I read "while detonating" into this too!!], the tin plating will migrate across the back side of the shell, causing high spots on the ID. These intrude into the oil clearance and become concentrated load areas...[Could this, plus a crank not polished correctly, a rod that closes up, cause a spun brg??? )] The CH series race brgs do not have this coating, but are not recommended for supercharged applications. They recommend the "tri-metal" brg construction [CP or P] for street and "moderate" performance use...