[ejw]

What is bearing clearance range for main/rods I should be looking for
Na 455 strip only
Thankyou

[charlie66]

2.5 rods and 3 to 3.5 mains. Is safe..

[chiphead]

Most bearing MFGs say that the safe minimum clearance is .00075" per inch of journal for a street motor. So for a 455 Pontiac, that would be .00168" on rods and .0024" on mains. That is a bare minimum, don't go tighter than that ever on a street motor.

For a street/strip engine, I like .00095" per inch.

For all-out race, you can go .001" to .0011" per inch of journal, but that starts to cost oil pressure at low engine speeds.

So strip only would be .0025" on rods and .00325" to .0035" on 3.25" mains.

So I get the same answer as Charlie, but you see the reasoning behind it.

[ejw]

Thanks

[Torment]

At .001" per inch a 40 or 50 weight oil is in the ballpark. The tighter clearances have their place if you can hold them, and a lighter weight oil would be recommended.

Here's a link to some information on this by King Bearing: http://www.kingbearings.com/explore/faq/#faq216

FAQ #4... hope this helps!

[steve25]

How's your Rod side clearance?
If your running a cast Crank I would stick dead on .003" as there's no need for .0035!

[charlie66]

Originally Posted by steve25

How's your Rod side clearance?
If your running a cast Crank I would stick dead on .003" as there's no need for .0035!

How do you adjust that?

[Cliff R]

Rod and Main bearing clearances are important. With that said so is the fit and finish of the crankshaft, and the rod big ends.

If your machinist is doing his job the rod journals will not only be round, they will be smooth finish, as will the main journals, and the crank will be straight. Thrust surface should also be very smooth.

With a well prepared crankshaft, straight line bore, and rods correctly size you will be fine at .002" on the rods and .0025-.003" on the mains. I also like to see the thrust on the low side of the spec and NEVER over .008" for any reason. I broke that rule ONCE many years ago and it was the only Pontiac engine build where we killed the crank and thrust bearing in, so never repeated that deal again.

I like to see these numbers for all of our engines at any power level for N/A. We've done Pontiac builds to 750hp, a few are dedicated drag racing engines with .002" rod and .0025" main clearance and they survive constant thrashing, high rpms, and into the 9's over 135mph weekend after weekend, year after year with zero issues anywhere in that area.......Cliff

[slowbird]

Originally Posted by Cliff R

I like to see these numbers for all of our engines at any power level for N/A. We've done Pontiac builds to 750hp, a few are dedicated drag racing engines with .002" rod and .0025" main clearance and they survive constant thrashing, high rpms, and into the 9's over 135mph weekend after weekend, year after year with zero issues anywhere in that area.......Cliff

Sorry but if you run those clearance on a high hp na engine you'll have issues especially if the crank isn't fully counter weighted.

[Cliff R]

So what do you recommend instead? Please enlighten us.......Cliff

[steve25]

Come on now, granted He did not state what times He was looking to run at the strip, But theres a big difference between a motor making 750 hp and spinning no more then 7400 and a motor making enough power to run well into the low 8s and turning 8500 or better no?

Maybe a steel Crank with less than 2.750"mains will bend enough to need .0035" clearance, but sure as heck a cast Crank with 3.00 mains will not.

I do agree totally though that if I was looking to make over 780 hp with a factory block I would want a fully counter weighted Crank, period!

[slowbird]

Originally Posted by Cliff R

So what do you recommend instead? Please enlighten us.......Cliff

You made a blanket statement, those clearances no matter the na hp. I don't agree.
We found main clearance can be effect by the type of rod being run, alum or steel, rpm effects it and compression/cylinder pressure. Anything that causes the crank to flex/twist will require the main bearing clearance to increase.
We ran .0037 to .004 with stock 455 blocks, when we started making more power turning more rpm with aftermaket stuff we were up to .0045. Now that we're running fully counter weighted crank we're down to .0037 and it looks like it can be tighten up more, going to try .0032.
All im saying is there are way to many variables to just say run this clearance no matter what.

[Elarson]

This is way off track from the OP's question but just for amusement.....we couldn't keep main bearings alive in the Boss Bird until we got looser than .007.

So I agree that all the variables must be considered.

Eric

[Cliff R]

Understood, there are ALWAYS variables with this sort of thing. The OP wasn't overly specific, so I told him what we've done here with perfect success in N/A engines at pretty high power levels (750hp), and cast or forged cranks with high quality steel connecting rods.

IF he wanted specific details down the ten thousandth then maybe we need a bit more information?

I'd add this when it come to this topic, a stock Pontiac block has limitations that the aftermarket blocks don't. WAY back before the good blocks were available we started to see issues with stock blocks at high HP levels that indicated to us they were moving around some. This included dowel pins on the oil pan, discolored bearings, bearings loose in the caps, cracked webbing, and failing and failed lifter bores even when a brace was used.

In addition and I reflected on it in the first response, how good your machinist is will also be a player in what clearances you can get away with, followed closely by how good your parts are, and how straight and sturdy the block is......FWIW......Cliff

[slowbird]

Originally Posted by Cliff R

Understood, there are ALWAYS variables with this sort of thing. The OP wasn't overly specific, so I told him what we've done here with perfect success in N/A engines at pretty high power levels (750hp), and cast or forged cranks with high quality steel connecting rods.

IF he wanted specific details down the ten thousandth then maybe we need a bit more information?

I'd add this when it come to this topic, a stock Pontiac block has limitations that the aftermarket blocks don't. WAY back before the good blocks were available we started to see issues with stock blocks at high HP levels that indicated to us they were moving around some. This included dowel pins on the oil pan, discolored bearings, bearings loose in the caps, cracked webbing, and failing and failed lifter bores even when a brace was used.

In addition and I reflected on it in the first response, how good your machinist is will also be a player in what clearances you can get away with, followed closely by how good your parts are, and how straight and sturdy the block is......FWIW......Cliff

What is the compression in those 750hp engines?

[Cliff R]

Pretty high as the last one was 505cid with "high port" heads and we did that engine back in 2009, and have only freshened it up once a couple years ago. It's probably around 13-14 to 1 compression, but to be perfectly honest we did it so long ago I can't remember all the details.....Cliff

[Tom Vaught]

I think the best compromise to at least two of the knowledgeable answers above would be:
It depends. Let the engine tell you what it wants. I am sure that is what Eric and others are saying.

Lower horsepower engines (our mule 4.6L Boosted Ford engine made 750+ hp and ran STOCK FORD clearances). It depends on the selection of parts, how the parts were made, what the other variables were with the engine, etc.

Specifics on what you got away with are fine if you build an engine for someone else with the same level of parts and machining that you used. YOU KNOW THAT WORKS. When you have other combinations of parts a learning curve is required to see what that engine combination wants.

This is why we had a EBU (Engine Build Up) area at my old job,.... to find out "what a specific engine wanted" for a given power level under a given load. All good posts but the engine and its parts are the final decision maker.

Tom V.