Bearing clearance
 

Question for the engine builders. In the process of assembling the 59L block, 3 & 5/16" bore, Merc crank, ground 0.010 under, balanced rotating assembly. NOS Federal Mogul 0.010 under main bearings.
Vern Tardel's book, page 143, Main bearing cap clearance 0.001-0.001.5, using plastigage
Ford Motor Company V8 engine repair manual, 1948, Page 131a, Main bearing liner clearance 0.000-0.003
Ron Holleran's book, Nostalgia, Page 8., crank clearance 0.001-0.003, "grind or polish to the low side".


Opinions on main bearing clearance. Is their an advantage to having a little more clearance ? Thank you Fordbarners.

Re: Bearing clearance
 

I no expert here , but loose is power, tighter is longevity. if its too loose then you end up back to having a loss of oil pressure and thats was the need for "high volume" oil pumps . From memory mine was closer to .003 but felt nice and I have great oil pressure.

Re: Bearing clearance
 

I think Vern’s got it nailed down, also as RON says to low side.

Re: Bearing clearance
 

The low tolerance depends on how much the crankshaft flex under power...more power you need bigger clearance and thicker oil.
If youre main bearing bore in the block is good and straight you can go for tighter gaps....
Tighter bearings and lower grade oil is less powerloss...just have to find out when the bearing clearance is tight enough...or maybe you really don´t want to find out just that...
The babbit on a bearing shell is only .0005 thick and that´s what acts as a dry lube film before the oilpressure is up to tight clearance or misalignment in mainbore and that will be gone fast..
The old rule of thumb is .0007 to .001 clearance per inch of bearing diameter for a stock bearing.
So 1-1.5 thousands and a straight mainbore will be just fine...the .003 is the maximum clearance on a used engine when serviced or for racing.
There are 2 reasons to open up the clearance more oilflow to cool the bearings and to accept the crankshaft deformation under power.

Re: Bearing clearance
 

Thank you Fordbarners. Any other experiences to share like Talkwrench's "closer to 0.003 and great oil pressure. Very good point about more clearance equals better oil flow, which means better cooling. Ron says that in his book, that being the purpose of oil to cool the parts.

Re: Bearing clearance
 

Like most things, it is all a balancing act. Tighter clearances can use thinner oil viscosity, the greater the clearance the higher the viscosity the oil and/or pressure required. There are pros and cons to all of this. Another factor is engine life. A race engine will most likely have a shorter engine life in terms of the total number of engine cycles compared to a daily drive vehicle. When you start out with a larger clearance there is less margin until you are at a upper clearance limit.

Re: Bearing clearance
 

My two cents. A race engine operates at high rpm for extended periods and the higher clearances allow more oil to flow and cool the bearing. Because these are frequently torn down the wear is taken up when new bearings are installed. The tighter clearances will allow adequate oil for a street engine and as the bearings wear a bit they will still be within specs. I think mine was around 1 to 1.5 and it's still going after 35000 or so miles

Re: Bearing clearance
 

had my crank checked and main caps aligned honed crank is also a merc. .010 under. machinist said front and center bearings have .002 and rear is at .001. he said it was within spec but the engines he builds he likes to see at least .002. he said to try a different bearing to get the .002 Mark

Re: Bearing clearance
 

I wrote that book back in the late 80's, after spending most of my building time on racing engines. Published in 1992. Over the past 25 years, I've learned allot about engine building, you can't stay stagment in a field like this. The vast majority of these engines are now used in street applications. However, the bearing load factors have increased by larger displacements and compression ratios. What most people don't realize is the fact that tight bearing clearances create heat with high viscosity oils. Stock clearances of .001/.002 must have a thinner grade oil to prevent this. Today, Main bearing clearances are much tighter in performance engines, I try to get .0025 +/- .0002. Rod clearance depending on he type rod and bearing and following the manufactures recommend numbers . We also have to consider the advances made in the quality of the oil available today. Modern auto engines are small in displacement yet produce a great deal of power, and last hundreds of thousands of miles using a lubricant that runs like water at 200 F. I hope some of the new builders will look into this, there is no reason why these engines can't giver reliable service for 100,000 miles or more.

Re: Bearing clearance
 

On just about every Flattie we build we aim for .002"/.0025" on both the rods and mains! We consider this "ideal" for most street/strip applications.

One area we always check first are the main bore sizes AND the rod bore sizes. These numbers will have an effect on your actual clearances. If any of these holes are "out-of-spec" they need to addressed first, before attempting any bearing clearance checks!

The ride in my signature now has (I believe) over 40,000 miles on it and hasn't had a single issue to date. This unit has over 150 HP and over 260# Torque!

Read the P.S. below here, it is important when a crank needs grinding? We will NOT grind any cranks here that aren't included in an ongoing build UNLESS the customer gives us an accurate number, both rods AND mains.

Thanks, Gary in N.Y.

P.S. The one advantage you WILL have grinding the crank to any undersizes over using all new components, is you will be able to pre-measure the rod/main bearings in their respective locations THEN grind the desired clearances directly into the crank. Doesn't get any more accurate when done this way. It does take a very experienced grinder to get the sizes you need.

Re: Bearing clearance
 

Main and Rod clearance should be .001 per inch of crank. a 2 inch crank should have .002 thousandths clearance, or plus a half, .000-50 tops.

At .001, or .001-50, you will be pushing usable bearing material out of the way, and wasting it, on a 2" crank.

From cold to hot, a 2" crank will swell .002 thousandths.

Herm.

Re: Bearing clearance
 

Does this mean that at operating temperature .000" oil clearance is acceptable?

Re: Bearing clearance
 

The old rule of thumb says 0.0007-0.001 clearance per inch of bearing...
And remember all metal in the engine expand...not only the crank.
But im a youngster in this crowd...only owned and operating a crankgrinder for 20+ years...

Re: Bearing clearance
 

So the next question is: How much does the bore of the connecting rod expand at operating temperature? Is it more or less than the crankpin expands?

Re: Bearing clearance
 

It´s all about material and heat...like the difference between a steel rod and an aluminum rod....the aluminum rod expands much more then the steel.
If you like to play around with different bearing clearances and oil to get the optimum setup King Bearings has or atleast had a program called ENSIM.
Another thing is that the rotational effect creating an oil wedge in the bearing doesn´t work as good at low rews as high...so big bearing clearances below 2000rpm is bad while keeping up the rpm makes it work just fine.

Re: Bearing clearance
 

Thank you all. Learned a lot. Good points about expansion of the parts due to heat, measuring the mains and rods and grinding the crank to deliver the clearance you want, and oil viscosity vs clearance. The machine shop ended up getting a clearance of 0.002.5, on the mains, which I believe both myself and the engine will be happy with.

Re: Bearing clearance
 

This is a very fascinating subject. I think about this kind of thing when I lay awake at night. It's a good point to think of the big end of the rod and it's bearing in a linear fashion. 6" plus of strap heated does get a good bit longer. I argue with a friend who also does a LOT of work on old trucks. In every case he heats stuck bolts cherry red in his (usually successful) attempt to break them loose. I choose to heat the nut or surrounding area if practical. As for bearing clearances, the owner of my local napa store tells me he stocks plastigage for me, as I'm the only person who ever buys it. I'd be lost without it. Thanks all for the info on this thread.

Re: Bearing clearance
 

Funny you mention that about the plastigauge, I recently spoke to a fellow that rebuilt large industrial air compressors, he mentioned replacing the lower “rod” bearings and I said well make sure you check your clearances with plastigauge, and he didn’t know what it was. In his defense, he was not a mechanic, but this was an additional duty tacked onto his job.

Re: Bearing clearance
 

I am not understanding your Question, Mr. 40. I may be still not woke up!

But if you set a 2" crank clearance at .001, or .001-50, you will have a crank clearance at .000, as a Hot crank will be larger then that clearance. So what happens is, either the crank, or the babbitt in the insert has to give for size, and it won't be the crank!

Fortunately, after that, a hot crank will open up a bearing to what ever it needs for clearance, to survive.

So the whole idea of setting the correct clearance for any bearing, is to keep the bearing surface, as much as you can, from smearing, and wasting any measurable amount, of surface, for the most longevity.

One last thing, if say an engine in an Auto is pushed at high RPM's, much of the time, (not as bad in a pressure engine), but at high RPM's, the bearing clearance will be greater, then if it were driven at preferred normal speeds. This is more noticeable, in a splash engine.

Thanks,

Herm.

Re: Bearing clearance
 

Your .002-50 should be correct at that clearance. With oil, you will never hurt those bearings, at break in.

Herm.