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.

Re: Bearing clearance
 

I run the old standard performance clearances of .001 per inch of journal size. This gets you in the ballpark. Depending on the application, I may run more in the rear main - like closer to .003. If you're racing and the clutch is getting really hot, then the heat transfers to the rear of the crankshaft - taking up some of the clearance.

If the crank is already ground, then sometimes I need to buy multiple sets of main bearings to get the exact clearances I'm looking for -- sometimes they vary by .001 or so . . . just the way it is. We don't have the luxury of .009 or .011 bearings (for a .010 crank) - to 'tune' the clearances. The best way to do it is to buy your bearings FIRST - then use a very accurate dial bore gauge to measure them in the block, then give the numbers to the crank grinder. This method is by far the BEST - and should give you the exact clearances you desire (if the machine and operator are top notch).

With all this babble, I will say that I'd rather have it a bit "loose", than too tight . . . loose is fast, tight is death! LOL

Re: Bearing clearance
 

That's why I have dozens of old used bearings, I can usually find what I need. A loose bearing runs cooler. but there is a limit on how loose is loose..

Re: Bearing clearance
 

Solid point Ron . . . I've had a couple beers this evening and I think the same thing might apply to some of my relationships! LOL

Re: Bearing clearance
 

thanks, I don't build or own high pro engines, but these make sense. When talking about thousands of a inch, ya have to wonder how much is to little. Oil flow is good.

Re: Bearing clearance
 

Rods and Babbitt don't seem to do much expanding.

Herm.

Re: Bearing clearance
 

I'm not a expert by any means. but we all want high compression heads... But if you put a blower on a flathead, you want to reduce head compression on average. Not talking about a nitro/drag motor. But Flow...

Re: Bearing clearance
 

So for .002 clearance on floating rod bearings, do you run .002 on the inside of the bearing and also .002 on the outside of the bearing? Or just .002 total difference in dimensions?

Re: Bearing clearance
 

Yes, for floaters, I run about .002 on both the rods and crank. I open the rods to 2.221 and the crank to 1.997. OR find bearings that are .1o8" thick. That's why fitting the bearings are important. Again, I have dozens of used bearigs to choose from

Re: Bearing clearance
 

The spec from ford says .0015 to .0035 total with max for a used engine being .005.
Thoose copper bearing shells seems to outlive the crank...probably the lack of babbit at start before the oilpressure is up that makes the difference...one way or the other they are tough on the cranks...

Re: Bearing clearance
 

I tried to use Cadmium/silver, when we were using the high oil pressure (60/80 lbs) but the bearings were flaking. Looked like they had chicken pox. Also the filit radius of the crank had a slice it it caused by cavation erosion. This was cleared up by increasing the side clearance and reducing the pressure to 40/50 lbs. The oil we used back then was Penzoil 40W . Track racing is much differant than drag or straightaway racing. the engine doesn't run long enough to build allot od bearing heat. Except when under Hi boost or nitro/nitros, Not sure how to keep one of them togher

This business of bearings is a long and frustration joinery . How do the NASCAR bearings stand up to 500 miles of FULL POWER runs?? I'm building an oil cooler for the roadster.

Re: Bearing clearance
 

Found a very informative/interesting site about bearings written by an engineer at King Bearing Co. It covers a lot of topics that have been discussed here. it's at:
substech.com/dokuwiki/doku.php?id=enginebearings then click on: Engine bearings [Subs Tech]

Re: Bearing clearance
 

I just pulled out of brand new set of NOS federal Mogul fullfloating bearings. They are .030 under size. They measure .125 thick.

Re: Bearing clearance
 

There are some copper Moly bearing materials for race engines that are pretty amazing...they will likely never cross the path of the flathead bearing unless someon does a custom order to King Bearings for a few thousand sets...

Re: Bearing clearance
 

The stock bearing is .110 thick. subtract that from the .125 and you get .015 X 2 = .030 Using floaters requires allot of math.

Re: Bearing clearance
 

Ahhh, I see! Now it makes perfect sense, the.125 was throwing me as I was thinking .125 minus .030 was .095 which is way below the .108 every ones talking about.
So I have to think in twos, 2x.125=.250 minus .030(bearing size) equals .220 divide by two and viola! .110!

Re: Bearing clearance
 

On aside note here, we use a high number of coated brgs on all our higher HP builds! When they come back for "freshening-up" most have absolutely no wear on the surfaces of either the cam, rods or the mains.

The coatings are really unnecessary on these Flathead units UNLESS you're chasing brg clearances, then the coating can help some, you can change the clearances by about .0005" (1/2 thou), sometimes even more!

Having said that we do recommend using coated cam brgs on the Flathead builds, this is one area you are VERY limited with size-correction options unlike the rods and mains! Most flathead builds can afford to be a bit "tighter" between the brg's and the journals.

The cost is slightly more than the uncoated pieces but in the camshaft area we feel it is justifiable!

Thanks, Gary in N.Y.

P.S. Here's a shot of some Flathead coated/uncoated brgs, we use "Calico" most times for coating these! As I stated above we have the option of working with the hole sizing on the rods/mains to do any minor size corrections, not so in the cam brg area!

Re: Bearing clearance
 

More info on bearing clearance. The machine shop that checked and assembled the crank and main bearings measured the OD of the crank with a mike, and the ID of the installed bearing & cap with bore gauge. They said the bore gauge was more accurate than plastigauge. Bore gauges are too costly to invest in where plastigauge is good enough.

Re: Bearing clearance
 

Don't forget about thrust clearance, also critical. I have a NOS '40 motor I am going to open up. Wonder what kinda clearances Henry had. I will find out.

Phil Swanson

Re: Bearing clearance
 

true, but my thinking is price now of a pizza price of beer price of going to eat out; all this and the money and food is gone. So for a little more than
a $100 you get a bore gauge which is not only used for bearings but a lot of other machine shop duty's. If I added up all the plastigauge over the years that would buy a bore gauge and we all know a strip of plasti doesn't go far especially doing shimmed Babbitt jobs. My father pounded me that the engine blood pressure tells the story. My personal ride has floater rod bearings and the only way is the bore gauge and rods were not standard, so
all rods were resized and honed to standard and turned and polished crank to .010 ac-cross the board. So with 1000 miles my blood pressure is 60 and
40- 45 idle hot and as soon as you move it goes to 60lbs all day and thats at
0015 clearance on all. I think this is personal preference, although back in
my oval track racing days they sounded like knockin on the curves loose as a goose never lost a engine may have had 20 lbs oil psi at 4 grand Oh the smell of Nitro methane and hamburgs / onions /beer at the track. Now thinking back a Chevy overhead couldn't cut it and also nobody ran a 8BAs either all
59ABs now ya all know why 2drs and business coupes went--the tracks...sam just ramblin again

Re: Bearing clearance
 

A good quality dial bore gauge is expensive and quite a bit different than a 2-point inside barrel mic. Plastigauge is a crude way to measure clearances and really is just to validate that you're in the ball park (it works in a pinch).

The dial bore gauges that are typically used are extremely accurate (if calibrated) and have 3 or 4 touch points (not like a 2-point barrel inside mic). They cost around $1300 - for a regular length one (sometimes you can find a used one for much less - but it may need calibration). If you have one with an extended length shaft (to reach into the various main bearings of a block), the price goes up. I simply cannot use a 2-point inside mic and accurately measure bearing clearances, taper, out of round, etc. (anywhere close to what a dial bore gauge can do) - also it is easy to scratch the bearing shells with all the fussing around.

Here is a picture of a Starrett dial bore gauge . . . not sure the size/range of it:

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