superdave269

I have some red .002-.006 plastigage ready but I can't find my specs for a stock Gen iv 454 rod clearance. I think it is 0025 but can remeber if thats the high or low side of what I will be looking for. Somebody must remember,thanks

Mooser

0.002-0.0025 for rods
0.0025 -0.003 for mains
A little tighter if the engine is stock
Rear main can go a little looser

HIH
Mooser

427Hotrod

.0022 would be *nominal*. You're looking for .001" per inch of journal diameter. BBC has a 2.200" rod journal.

.002"-.0035" will work. A little loose is better than a little tight.

JIM

superdave269

Thanks guys. I thought it was .002-.0025 but I wanted to be sure.
Happy Holidays!

7T1vette

My Chassis Service Manual (1971) has those clearances as:

Rod Bearing (cold) .0009-.0025" for both 454 engines

gcusmano74

Please allow me to respectfully disagree.
A too loose bearing allows the oil to leak away from the bearing without building enough hydrodynamic pressure.
(From the 1974 overhaul manual, 454 engine) Chevrolet calls out a clearance spec of 0.0013" to 0.0025" for crankshaft main bearings #1 to #4. For the #5 bearing, they say 0.0024" to 0.0040".
If you can turn the crank by hand (with only the crank installed), when the crank bearing clearances are inside of that spec, you will be fine. On a microscopic level, the crank probably bends anyway while the engine is running.
Rod clearance specs on the BBC are 0.0009" to 0.0025".

427Hotrod

No problem...but remember..the factory allows a wide tolerance stack up to allow fast assembly. Not that it's the best..just that it will probably run Ok in a normal mild waranty application. They'll gamble. And yes, things do flex in operation...BBC parts are heavy and with some RPM things move around. Very very few factory cranks are actually straight..especially the over the counter ones. The extra clearance allows them to work even though things aren't perfect. Rods pull inward in the center of the cap..that's why most bearings are eccentric..not round. They have more clearance at the parting line than in the 12-6 o'clock position.

As far as a hydrodynamic wedge...as long as I get oil pressure in there I'll have one. Bearings are cooled by oil flow...looser clearance allows more flow. While these will operate at below .002"..I'd certainly feel better on the looser side like .0025-.003 than I would at .0015-.001".
If things ever touch..you're done for.

But hey..that's just me!

JIM

7T1vette

Well, the Engineering department didn't really work that way. They first determined the 'optimum' tolerance for those bearings (initially from actual engine testing), then apportioned tolerances to the parts that were part of those 'stack-ups'. Since there are only a crankshaft journal, a bearing set, and a connecting rod involved, it's not rocket science to figure out what tolerances need to be in order to get the clearances you need.

And, GM didn't do 'dumb stuff' on critical operating clearances, when it would negatively impact warranty costs.

IMO, you can apply the factory GM engine tolerances to any general purpose engine of similar type, and it would serve you pretty well. If you want max power...at the expense of shorter life...make those tolerances larger so there will be less frictional drag. Otherwise, go with the factory specs.

Solid LT1

This rule has worked for me for over 30 years and I would guess this poster has as more experience building FAST motors that live long lives than I do.

I would want a minimum of 0.002" preferably 0.0022-0.0025" clearance for my build. For a Hi-perf build a minimum of 0.0025" to allow oil flow away from the rod journal at high RPMs, that will allow the hot oil to leave the journal in a timely manner to prevent bearing failure. For a 280HP production LS-5 hydraulic NCRS trailer queen a tight 0.002" would be acceptable.

L88Plus

".0022 would be *nominal*. You're looking for .001" per inch of journal diameter. BBC has a 2.200" rod journal.

.002"-.0035" will work. A little loose is better than a little tight.

JIM"

"This rule has worked for me for over 30 years and I would guess this poster has as more experience building FAST motors that live long lives than I do."

Yeah, but keep in mind that Jim uses a cylinder head for a piston stop to keep rod stretch to a minimum... LOL

As ususal 540HOTROD Jim has it spot-on. These are the same specs that pretty much every performance builder uses. My local guy wants .0025 - .0027 rods, similar on mains with .003 on the rear main.

With the wide variance that GM called "acceptable", it's no wonder that two different engines, built the same day would have 10psi or more difference in oil pressure. .0009 rods and .0013 mains is way to tight for me. An old fart builder once told me that if you build one too loose, you'll have lower oil pressure but you'll have plenty of room if you rev it. Build one too tight and it will loosen itself up for you...generally all over the track or ground below the engine.

superdave269

No wonder I couldn't remember if .0025 was the low or high side of the scale. It seems to changes with everyone's answer, but all seem to agree it's in the zone. I can't wait to pick up my bearings this afternoon and see what I measure. Thanks everyone.

MotorHead

It's 1 thou per 1 inch of journal, you can't go wrong by doing it this way. Plastigauge is not accurate enough and no engine builder worth his salt would use it but if you are doing it in the car then that is the only way.

427Hotrod

Certainly they established a range they felt they would operate in with no issues for normal operation. But they don't measure everything either. All the parts are made within a nominal range and when all thrown together they should end up somewhere in that range of tolerances. Some loose, some tight..some "perfect". And usually that works,..in fact if it doesn't there is something out of whack....but that's why you check them. I still go to OEM engine assembly plants every year. The ability to track tolerances is way more advanced than in the old days..and yet it's still pretty much the same deal. The good news is that most parts DO come out of the box pretty darn close...but you never know. There are spot ck's done, but not all of them. And the factory didn't know then, or now. Engines still have issues from time to time purely from tolerance stack up early in their warranty life.

And yes, cranks are routinely sold over the counter with some pretty healthy runout from GM. A little loose will let them get by.

As I mentioned, the engine will run fine at .002...heck might be fine at .001" if it truly is a stock 454 that is like an LS-4/5 deal that will never rev much over 5000 rpm..but the OP didn't give us those details. Could be a solid lifter LS-6 deal..or an L-88 for all I know. Wouldn't really matter.....0025-.003" will run in all the applications and not worry about hurting anything. I wouldn't expect any wear issues out there either. My bearings always look brand new when I've ck'd them.

JIM

7T1vette

Machining and gaging systems...particularly for critical operating tolerances....were much better in the 1970's than most folks know. Gaging for such processing operations was usually fully-automated and a 100% check.

If that were not the case, the Chevy V8 engines would not have the reputation of long life and reliability that they do.

superdave269

Thank you everyone for the helpful information. I enjoyed reading all of this. I put it together last night and believe my crank is a little out of round because the plastigage did not squish evenly. Looks like I have .002 on the outskirts of the crank and more in the middle. I chuckled to myself when I saw this and thought this should make eveyone happy because I think I have everyone's numbers. I did start it up (briefly as I did not want to fill garage up with exhaust) and had no knocks. Weather permitting I will test drive it today. Thanks again !