Main engine bearing clearances

I'm rebuilding a '75-'77 Buick 231 odd fire v6, and I had the crank shaft ground down 0.010 for the main/rod bearings. I've got a copy of the FSM for the 231 v6 which lists the specs for main bearing to journal clearance at 0.0004 - 0.0015. Upon checking it with plastigauge, I found I have just under 0.002 clearance. Probably 0.0018 to 0.0019 depending which one you look at.

I found a copy of the specs for a 225 v6 online:
http://www.teambuick.com/reference/225_v6_engine_specs.php

Both the 225 and the 231 list the same main bearing journal diameter (2.4995) and the Clevite main bearings are the same part # for both engines. The main bearing to journal clearance for the 225 is listed as 0.0005 - 0.0021. Would I be crazy to run the 231 with 0.002 clearance?

 

are you sure you torqued it to the right spec? I don't think that plastiguage is perfect. Personally I would go with it, but other guys will probably give better advice than me

 

Plastigage is really the only reasonably accurate way to check clearance like this, but that being said I do agree did you torque the caps correctly and have them in the right order and oriented correctly.

 

In my opinion , The 225's always seemed to like to run on the tight side and when loose were more prone to spun bearings. This may also be attributable to the poor oiling system...........Last time I looked at Plastiguage it only measured in thousands +/-..........not sure anyone could convert that to tenths and ever read that accurately? Chances are your crankshaft is under ground a few tenths and likewise your ability to measure tenths exactly is also very hard to see..........If your interested in knowing exactly what size it is get a Mitutoyo tenth mic and a set of snap gauges and then measure the components.............If you have no way of checking the main cap bores when torqued down.........your just chasing numbers.

 

Plastigage is a "get me close check because I don't have the more accurate equipment to do it" method. It works but won't get you dead accurate like a snap gauge and micrometer or a proper bore gauge and micrometer. Millions of engines have been assembled using plastigage successfully but but it is not a very accurate method if you're concerned about .0001" increments. The machine shop I worked in wouldn't allow the stuff in the door but that was a precision custom engine building shop.

 

Everything was torqued correctly with caps in the right order/direction. I just meant to say it was a hair under 0.002, not that I could accurately read ten thousandths with plastigage. I have a dial bore gauge and a set of micrometers, so I could get more accurate numbers if necessary. I guess where I'm at is my quick check with plastigage showed me being quite a ways off where I'd like to be.

I'm sure the engine would run fine for now with 0.002 clearance, the FSM says not to replace worn out bearings til you get above 0.003 clearance. After sleeping on it, I'm not sure it's smart to assemble an engine with all new machine work and parts that's already almost out of spec.

I'm 0.0005 above the specs for the 231, and at the extreme end of the specs for the 225. If I were to get 11 under bearings, it would put me at about 0.001 clearance, right in the middle of the 0.0004 - 0.0015 spec.

I'm new to this whole engine building thing, and I can't just type in summit racing and fine 11 under bearings for this engine. I'm off to google to look, but does anyone know of any good sources to find oddball bearings like that?

 

Take your crank to a crank shop and have them check it out and undersize it if required. They will machine it to fit the standard undersize bearings. I had some real problems with a set of main bearings once. They were stamped STD but were not. They were all different undersize. Federal Mogul too. could be that if you tried a different set of mains they might be more in spec. I know it sounds batty, but its my experience.

 

61CJ5.......and hello Lafayette , Callyfornya............You missed my point............and I'm not trying to be Anal.............but, if you want to know exactly what the clearance is you must first be able to measure the individual parts separately ..........meaning as I mentioned above , the clearance is the accumulative total of all related components........the first step would be to install a bearing half into each main cap and the other half in the block then assemble and torque the caps down to the correct Ft. lbs..........prior to doing so you should have in your possession the correct thread lubricant and of course a torque wrench that is known to be accurate..........Take a snap gauge and fit it into each bearing bore and then remove and measure with a .0001 mic............or as Nick mentioned above a dial bore gauge will also work.......but of course that devise would also need to be preset on a known sized fixture prior to it's use............All of this takes patience and a certain touch to get the right drag / feel on the snap gauges.

Once you know what the bores measure then take your tenth mic and measure the main journals on the crankshaft............with that info you can now determine where the tolerance is. You can go one step further and prior to loading the bearings , measure just the bare cap bores torqued in the block.....and see how concentric they are across all axis's.

Keep in mind all of the parts must be super clean.....no dirt, no dust.......any sharp edges on the caps at the mating surface should be slightly de-burred.

Do I think you need to go through all of this trouble? Probably not..........but if your interested then you should........It's doubtful that your engine will ever know the difference of a couple of tenths of clearance at the bearings. But knowing that it is loose is helpful if you feel the oil pressure is lacking.......Knowledge is always paramount!

Don't go buying any parts until you have a valid way of making these measurements.

 

I gotta ask where the heck did you guys here the term Snap Gauge, a snap gauge is totally different than telescope gauges.
A snap gage is a go no/go gage, not a telescope gage. And to be truthful most guys I have seen using Telescope gages, use them wrong because they did not have a good machinist teach them how to properly use them to start with.
This is a snap gage:

 

Sorry Mike , It must be a West of the Rockies phrase ...............Yes they are technically called Telescoping gauges and when released they snap open.............In our non PC World out here , most folks make the same analogy and call them a snap gauge when used to measure the ID of a Bore.
And you are also correct by saying that most folks do not have the feel nor touch to roll the gauges in and out of a bore while still getting repeatable measuring results..............which brings us to dial bore gauges whereby again most folks do not have a decent bore gauge nor a permanent setting fixture or an expensive ring gauge to set there dial bore gauge............

So being absent of all this when someone is trying to measure tenths with plastigage how does one proceed?

 

I just cannot see how plastiguage can possible be used to measure tenths. As it has been stated, plasiguage is just to make sure you're in the ballpark, but tenths is not in the ballpark, thats basically a magnitude more precise. I have no problem with plastiguage, but it is what it is, just a rough estimate. I am not a machinist and don't have training, but have made enough stuff to realize that a tenth is a tolerance that maybe by accident i could make once, but not repeatably and honestly in that sense that tolerance is not realistic in a engine even if the temperature changes in your assembly room. I don't even have measuring instruments that I trust to measure that precisely.

 

A Plastigage scale can be used just like a Vernier gauge on a pair of calipers or a micrometer. You can get within .0002-.0003 pretty easily if your patient.
I have built boat loads of molds that had tolerance's of plus .0000 -.0002, these molds had to be assembled in a temperature controlled room and we checked our parts with a .00005 indicator.
Your right you can check the motor parts and change the tolerance by changing the temperature, thats why I would never have an engine machined in a shop that did not have temp control in it and my parts, bearings and pistons were there for them to fit, not just bore and go. Then you control assembly by letting everything normalize to the same temp and assemble.

As far as tools that you trust to be that accurate I can understand that to a degree, thats why when you buy precision tools you buy quality like Mitutoyo, Starrett, Brown & Sharpe, Interapid and the best money can buy Tesa. I know not everyone can afford that but when its your livelihood you get what will last, I have my fathers Starrett and Mitutoyo micrometers and my Grandfathers Lufkin Micrometers and telescope gages along with other machinist tools, my grandfather retired in 1966 so most of his tools date from the 30's to the 50's and still work great today.
Being good at machining is a life long learning experience!!!
When you quit learning is when they throw dirt in your face.

 

A friend used to machine parts for nuclear bombs. Basicly no roon for any tolerance problems. I suppose that is why I had him do the machine work on my Harley.
I did get bored one night and calibrated a micrometer and then just held it in my hand for a while. The reading did change. Back in those days I was in a class and we were told that tools like these are stored, calibrated and used at the same temp.

 

I tend to agree. This isn't rocket surgery.

Not recommended but I've even used just vernier calipers and an eyeball when I had to.

 

Point taken. I Removed the crankshaft, oiled the bolts and torqued the main caps down with bearings loaded in them. I used a recently calibrated old school snap on torque wrench. I then used a micrometer that reads down to 0.0001 to both measure the crank journals and to set the dial bore gauge. I used a calibration block to check the micrometer before use. I used the dial bore gauge to measure the difference between the journal diameter and the I.D. of the torqued down bearings. I redid all measurements several times making sure my results were repeatable. All parts and tools were at room temperature. My dial bore gauge only reads in 0.0005 increments, but it's fairly easy to estimate a little bit closer than that.

Main #1 - 0.0017 clearance
Main #2 - 0.0018 clearance
Main #3 - 0.0016 clearance
Main #4 - 0.0019 clearance

So in this case the plastigage was not terribly far off, but it's nice to actually know. All of these are outside the specs in the FSM for the 231 (0.0004-0.0015) but are within the specs for the 225 (0.0005-0.0021). Seeing as the crankshaft and main bearings are the same between the 225 and 231, I don't feel using the specs for the 225 is out of line.

I wish it were tighter, and if I could find 11 under bearings I would use them, but I think this will be ok. It's a low revving Jeep engine that won't see many miles, and it will have a better than stock oil pump. Unless anyone has any objections, I'm going to continue on with assembly.

 

61CJ5.............Good for you!.........At least now you know and you got an education along the way. I would run it , should be fine............what about the connecting rods......were they ever re-sized? and how was the measurement on the Rod Journals on the crank........equally as important. Don't forget to also measure your Crank end play..............and clean out all the oil galleys..........Carry on!