Bearing clearances: Tight vs. Loose??

[BillyGman]

This is one of those topics that I thought I was at least somewhat well informed on, since two professional engine builders told me that the .0015" clearance on the main and rod bearings that I was seeing with my build, wouldn't be enough for my street/strip 900 HP pump gas 632cid engine, (which has an iron block BTW, steel 6.7" rods, and flat top forged pistons). This engine will be used to drag race at the drag strip, and also driven about 2,000 miles per year on the street. The engine will be going in a heavy car, ( about 3,900 lbs).

I was told by these two professional engine builders that .0025" to even .003" of bearing clearance should be my target goal, and I thought I was all set with that advice. That is until I watched these two vids, which seem to state the opposite. Here are those vids which I speak of, and one of them shows an interview with a rep from the King bearings company, so that's what makes me take notice. I was planning on using coated King bearings which should yield about .0025" of clearance. But after watching these vids, I'm not sure what's best in the way of clearances, foy my application. I intend to use non-synthetic oil if that matters.

What are your thoughts? I understand that bearing clearances are, or can be, application specific, which is precisely why I've made mention of some of the details, as well as the intended use of the engine I have. Please shed some light on this bearing clearance topic for me if you can. Thanks in advance.

https://youtu.be/Zr-gGOa2Jnk?t=512

https://youtu.be/aB-lf4kokSQ?t=156

[panic]

The OEM rod and main clearances have started at .001" per 1.00" of journal diameter when Woodrow Wilson was president.
2.75" main = .028" clearance
2.25" rod = .023" clearance

[1972ho]

Wow .0015 that’s tight that only gives about .00075 radial running clearance not a lot room for oil to flow through and you always have to run 0w10 oil.

[lance flake]

I am a firm believer in the .001 to much nobody knows .001 not enough everybody knows. You do not need excessive clearance but .0015 at 900hp may get you in trouble with the slightest margin or error.

[novafornow]

JMHO, but with that much stroke, I would target .003" min.

[rebelrouser]

Depends on a lot of things. First lots of new production engines have really tight bearing clearance, but they recommend very thin oil.
I am old school, bigger clearance and thick oil. The main reason I like bigger bearing clearances especially on the mains is that a typical V-8 engine block was built to handle around 400HP from the factory, most performance builds are going way over that, so the block flexes some, if the block moves around and causes a main bearing to lose oil clearances, then boom, it blows up. If you have a really good aftermarket block stiff enough to handle you power levels then tight clearances with thin oil will work, look at NASCAR combinations. Or again old school bigger bearing clearances and thicker oil are a safer way to go, especially if you are not trying to get every HP possible.

[RDY4WAR]

The tighter the bearing clearance, the more precise the machining has to be, and the oil has to be a lower dynamic viscosity though with a higher pressure-viscosity coefficient. The only crank that I would trust at .0015" would be a Bryant crank for a cup engine. The oil would most definitely not be conventional for such a case.

The above recommendation of .001" per 1" journal diameter is a good standard and what you should follow. It doesn't have to be exact, but ballpark.

Mains - .0025-.0029"
Rods - .0020-.0024"

That's what I would aim for. I still wouldn't recommend conventional though. There's just no application where it's the better option, especially in a 632 with forged pistons. The above clearances would be happy with a 30 grade so something like VR1 Synthetic 10W-30, HPL Bad Ass 5W-30, Driven LS30 5W-30, or Red Line HP 5W/10W-30 would be where I'd look. Any of those would go the whole year with ease as well.

[Walter R. Malik]

As tight as you can make them without ever experiencing metal to metal contact would be best.

Finding those clearances without destroying a lot of equipment is usually difficult so, it is a definitely a slow learning process with every combination.
Most times, to be set-up on the safe side with wider clearance than necessary is best and read the bearings on times the engine is being refreshed.

[modok]

.001 per inch minimum assumes a round hole. Bearings with some wall eccentricity allow going tighter, tighter in the tightest spot anyway, the average clearance all around still needs to be near the rule.

[Belgian1979]

https://www.mahle-aftermarket.com/media ... 1-205r.pdf

That's what Mahle writes.

.00075' to .001' per inch crank diameter and add .0005' to that for a perf. application.
The general consensus seems to be .001" per inch crank diameter.

[BillyGman]

thank you for the advice guys.

[Tartilla]

BMW is having massive issues with rod bearings on their V8 engines. The rod bearing tolerances are way too tight.... at around 0.001". There are mechanics that specialize in replacing rod bearings in the M BMWs to prevent the inevitable rod bearing failure.

As likely mentioned, bearing clearances need to be part of the overall engine plan and of course, the oil used is very much part of that.

Engine oils are always being pared down in their additive packages, as OEMs chase emissions parts longevity vs total engine life.

Sometimes having 2 full sets of bearings is the solution to match up the tolerance you want.

[1980RS]

I read someplace that as soon as you run an engine and warm it up you will loose .0005 clearance from heat so if you are at .001 clearance not much oil film strength there to be comfortable for me. Back in the 90's I had a S/G buddy who had his 496 done by this "punk hot shot" engine builder that said he could run tighter clearances and use 5W30 oil, LOL. After two races the 496 spun 3 bearings and the crank journals looked like the side of a quarter Sammy went back to GM specs on the bearings and never looked back.

[Bigchief632]

I would run .003 minimum on the mains, and no less than .0025 on the rods. That is of course, if you don't want problems. .0015, aaaaah, no. I wouldn't run that clearance on a stone stock 325hp 396. As said above, if they are a little loose, no one knows, if they are a little to tight, everyone knows. Seriously, what do you think you would gain by running tight clearances?

I would bet though, if you ran "standard" ACL race series H bearings, in my opinion, the best bearings to run, and a good crank, with housing bores sized in the "middle" you'll see .0035 on the mains and .0028-.003 on rods. Which is perfect.

[Bigchief632]

I read someplace that as soon as you run an engine and warm it up you will loose .0005 clearance from heat so if you are at .001 clearance not much oil film strength there to be comfortable for me. Back in the 90's I had a S/G buddy who had his 496 done by this "punk hot shot" engine builder that said he could run tighter clearances and use 5W30 oil, LOL. After two races the 496 spun 3 bearings and the crank journals looked like the side of a quarter Sammy went back to GM specs on the bearings and never looked back.

There you go. True story. I don't know the guy, but that's EXACTLY what I would have predicted, and would have bet a tidy some of money that would have been the result. I bet I can guess the hot shots name. There's 2 that come to mind. I am from MN too by the way. Intitals, RF or BE?