Bearing clearances: Tight vs. Loose??

[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.

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.

Brian Ebert was one of them. Boy he blew up a lot of engines on that dyno.

[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.

Brian Ebert was one of them. Boy he blew up a lot of engines on that dyno.

LOL, now that's funny right there. Another true story. Not sure why your quote didn't show the initials, BE from my post. But, you nailed it

[mt-engines]

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.

Brian Ebert was one of them. Boy he blew up a lot of engines on that dyno.

LOL.. things I won't comment on a public forum.

[RDY4WAR]

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.

The issue was the 5W-30. I would've run a 0W-8.

[PackardV8]

The bad old days, loose was fast.

Today, a F1 engine is built so tight, it won't turn over at room temp. It has to have an hour of preheat of the oil and coolant to get all the assembly up to a pre-determined safe temperature to fire up.

[mt-engines]

The bad old days, loose was fast.

Today, a F1 engine is built so tight, it won't turn over at room temp. It has to have an hour of preheat of the oil and coolant to get all the assembly up to a pre-determined safe temperature to fire up.

Is he building a formula 1 engine? No.

[mt-engines]

Like mentioned before.. .003 mains and .0022 rods minimum.

Even if you have a center counterweight crank. Its long and has a big arm.. they flex. You won't gain anything tightening the clearance up. If you are worried about oil temps, throw an oil cooler on it.

Nothing wrong with old school thinking on this.. you can always sneak up on the clearance.. but to arbitrarily run it super tight is wrong.

Watch your oil temps.. and use a weight for the temps.

[BillyGman]

The bad old days, loose was fast.

Today, a F1 engine is built so tight, it won't turn over at room temp. It has to have an hour of preheat of the oil and coolant to get all the assembly up to a pre-determined safe temperature to fire up.

Yes, my understanding is that it's also the same with the real tight clearances they run with the bearings of Pro Stock engines too. That does broaden my view of things. But of course I'm pretty sure that its a safe guess on my part that the tolerances on the block saddles as well as on the crankshaft journals of my engine aren't as tight nor accurate as that of a Pro Stock nor of an F1 engine. That's interesting stuff though.

[BillyGman]

Many of you have brought up some great points. Things I never even thought of. One of them being the size and weight of the crankshaft in the 632cid in question, and how that alone might dictate at least a .0027" to .003" clearance on the main bearings. Another thing for me is that I lean towards the use of NON-synthetic oil. And NO, I don't care to turn this into a big oil debate. But with a hi-volume/Hi pressure oil pump that my engine came with, along with the typical non-synthetic 10W40 weight oil I usually use, I think a .0015" bearing tolerance is a mistake. Especially in light of everything that's been stated. Thanks again to everyone who chimed in on this for me. it's been an education.

[BillyGman]

.....ok this brings me to another topic, although IDK if it's really that much in-depth to require that I start another thread, so because it's related, (sort of) to this topic, I'll make mention of it here, and you guys can either pick it apart or agree and confirm the following.....(please read on)...

...Another thing that's in the back of my mind, is what I saw earlier this year when I paid a visit to a local engine builder's machine shop. He showed me why he uses COATED BEARINGS.

He directed my attention to two customers' BBC engines that he had apart and was working on. According to him, both engines were used for plenty of drag racing. One had standard non-coated bearings, and that engine had one race season on it. The other engine, had seven race seasons on it, according to the professional builder. He said that he told that customer to freshen up the engine every season, or at least every other race season, but he chose to go seven seasons before bringing the engine in to be rebuilt.

He showed me how the engine with the non-coated bearings which went one race season, showed normal but obvious signs of wear on the bearing shells themselves. However, to my amazement, he also showed me how the engine which had seven race seasons on it, showed hardly any signs of wear at all on the coated bearings it had. In fact, I had to look real long and closely at the coated bearings to notice any signs of wear at all. They almost looked brand new fresh out of the box. He told me that's why he uses coated bearings, and he said it was the King XP coated bearings that he uses, and recommends. And he was not even pushing them, because he never once asked me if I wanted to purchase a set from or through him, despite the fact that I was there with a main bearing in my hand that I took from my own engine to show him the score marks I found in most of the bearings in that engine of mine. I must admit that I was impressed by that little show and tell that the machine shop owner displayed for me using two of his drag racing customers' disassembled BBC engines.

That's why I'm planning on using the King XP coated bearings, and I'll be using the part numbers which will yield an extra .001" clearance, since I discovered during disassembly, that Bill Mitchell and company chose to use merely .0015" clearance on the mains along with a high volume/high pressure oil pump, while at the same time recommended 10W 40 oil or 20W50!!! (this happens to be an engine that I made the mistake of buying from World Products a number of years ago, and I let it sit for years inside the car and inside my temperature controlled garage. And the engine has zero miles/zero runs on it. In fact, it was only fired up 5 times and run for no longer than 20-25 minutes each time, which includes the two times it was dynoed).

So I'm thinking that it's no wonder that my oil pressure was 79 PSI even at an 900 rpm idle. I also have to wonder if this had something to do with the oil leak I had, which is precisely why I ended up having to yank the engine out of the car in the first place. When I went to replace the rear main seal while I was in there, I discovered that most of the main and rod bearings were heavily scored, along with metal filings inside. I also noticed that they ground down part of the head of one of the #8 connecting rod bolts to make clearance room for the oil pump housing. my guess is that this was done with the bolt still in the engine. But I digress....

What do you guys think about the King XP coated bearings? Have any of you ever used them??

[rgalajda]

I don't know where in either of those videos where the opposite was stated. You never stated how you got to .0015"


Mains : 2.74" .0025 >.0033"

Rods: 2.20" .0022 >.0027"

There is always an exception but old school extra clearance has been out the door for years. Along with heavy weight oils. Ever since oil progressed in quality .


"Today, a F1 engine is built so tight, it won't turn over at room temp." Is this a bit of an overstatement. Makes you wonder how they did the assembly.

[LotusElise]

Like rebelrauser mentioned, it depends on many things. Just to name a few: bending of bearing axis through oscillation of load, torsion and their dynamic (vibrational) and precision of manufacturing. I run several oil endurance tests on race engines with 30 weight oils, revving up to 10,000 rpm. I go that far to say the oil household (temperature, flow flux, additives, aging, engine speed, ...) plays a bigger role in that game too.

I don't want to overcomplicate it, but without testing OEM clearances are the bare minimum, while bigger than this compensate exceeding limits regarding the minimal oil film height when engine speed and or or load or temp or ...is higher then stock clearances. Too loose is also the wrong way. Therefore I would follow simple but tested approaches in the field of that specific engine when no better experience is available.

On the Honda K-series forged crankshaft I run less then 0.002", which would be the upper limit of the stock clearances. These crankshafts have very small tolerances, e.g. coaxiality is smaller than 4 % of the max. clearance and these crankshaft-block-systems can stand at least 6 times of the rated OEM power and torque at 0.002" and 40 weight oils without any seizure. Means their forging lead to amazing overload capacity results.

[Tom68]

Seems the modern engines can use thinner oils because of modern clearances being tighter BS migrating into Old Wives Tales fantasy World.

Check the clearances in a 1960's workshop manual, I'm betting size for size shafts will have the same clearance.
Steel strut cast pistons were fitted tighter then than anything being used now.

Don't get confused by comparing Aluminium block to Cast Iron block clearances.

[allencr267]

...he uses COATED BEARINGS.
...hardly any signs of wear at all on the coated bearings... ...my own engine... ...discovered that most of the main and rod bearings were heavily scored...

I think it shows that the coated bearing engine had a straight crank & block, clearance & right viscosity & pre-lubed & warmed up & wasn't overloaded/abused/lugged/detonated, AND most important, wasn't assembled with trash in it. It was NOT the coating, a sacrificial prophylactic.

[miniv8]

This is one of those things in engine building where you have to look carefully into what there is to gain, compared to the risks involved.

What are the benefits of the tightest bearing clearances? If any?

Is it bragging rights? Do you guys think a big bearing bottom end is somehow better built or more of a Swiss Watch engineering marvel if it is assembled with same bearing clearances used in tiny japanese econo shit boxes or a tiny formula one engine?

I cant think of a single benefit of constantly aiming at the tightest clearances possible and impossible.