What's the best way to measure the Main, Rod, and Cam bearing clearances? It's been so long since I rebuilt an engine, all I'm familiar with is Plastigage, but I seem to remember Jim W. or somebody talking about measuring without the crank installed. Exactly how does that work, and how can clearances be adjusted? James
I'm no expert here but what you are referring to with measuring WITHOUT crank installed is as follows: Install main caps with main bearings in place and torque main caps down to specs...now measure the inside diameter of each "hole" with a inside micrometer or dial bore guage. On the crank-with it out of the engine you would use an outside micrometer to measure each main journal. The difference in the above two measurements is your main bearing clearance. Same thing can be done on rods---however I don't think it's a good idea to torque down the rod caps (with bearings installed) without a special rod holding fixture---or by holding the rods in a particular way in a bench vise. Others should pipe in here that do this on a regular basis but this should at least answer how Jim was referring to getting main bearing clearance measurements. Patton
How many points, and where would you measure, or does it make a difference? Seems like not having the crank installed, you wouldn't have the same bearing 'crush'...(?) So putting the big end of the rod in a vise (obviously not cranking it down) and torquing down the bolts is bad? Why is that? How else to do this? Thanks for the info, I'm learning! James
Like I said James---I put this answer up to get you some info--however don't take it all as gospel---there may be addtnl steps that need to be done (I don't believe so however on the main bearing clearance checking. As far as I know the main bearing method I mentioned (with crank OUT of the motor) is the most accurate method---you sure as heck can't do it w/a feeler guage if you are thinking there is some way other than plastigage to measure main bearing clearance with the crank installed. On the rods I am preaching caution as rods can be bent easier than you think ---sure they are metal but you would not believe how easy it is to tweak one of these things. Someone here that does this on a regular basis should be able to tell you exactly how they clamp down the rods prior to torquing the bolts. Also--are you using aftermarket bolts?--if you have changed to those then you will probably want to get the big end of the rods resized also (and prior to doing your checking). Ooops--didn't read your whole original question. To "adjust" things---if crank fits too tight then the crank needs to be machined to make it fit looser. If crank fits too loose then you are looking at align honing or align boring-which resizes the crank bore holes to whatever size you need. Be aware that when align boring you are moving the crank axis closer to the cam axis--thus the timing chain will fit looser. At some point you need to consider an undersized timing chain. As you can tell many parts of a motor can be affected when certain machining operations are done. "foot bone is connected to the ankle bone, ankle bone is connected to the shin bone, etc etc" Not sure if there are special "between size" bearings available to make up for minor differences in clearances. Patton
Bearing clearance question The difference in the above two measurements is your main bearing clearance. I thought this was divided by 2. Am I wrong?
you are probably right Skip----I will check and come back tomorrow. I was hoping one of the engine build experts on this site would have come in by now and bailed me out before I cause too much trouble! Thnx. Pattono No:
to check brg clearances w/ engine disassembled, measure brg journals (crank,crank pin,or cam) w/ an outside micrometer and record. Install and torque to spec main & rod brgs and using an inside micrometer or snap gauge w/ the outside micrometer, measure brg PERPENDICULAR to parting line and record. Subtract your journal measurement from the brg measurement and this result will be the total clearance-do not divide by two. The cam brg doesn't have the parting line but I still like to take measurement on the vertical.with it installed. Hope this helps. Jay
I measure clearances daily at the shop. Torque the main caps, and measure the inside diameter. I ussually take 4 measurements. I go top to bottom, then with the parting line, and the diagonals. If the Journal is out of round by more than a thou, I'd strongly recomend an align hone. I also measure the bearing itself, along with engine blocks, no two bearings are exactly te same :Smarty: I then Mic the crank. Any descent machine shop should have polished each journal in the crank within 2 ten-thous. Some simple math at this point. First thing I woul do would be check the mains. an out of round main journal will kill any attempts to set clearances. as for the rods, the process is about the same. Except a rod sizeing fixture is used. It measures the journal and checks for roundness at the same time. Again, anything more than 1 thou should be resized. 1 thou is the absolute limit for me. I recomend an align hone on anyblock, as ussually re-size the rods too. But hey, thats just me. o No: Lastly, I would leave Meausureing to the pros. it takes a touch to accuratly measure to a ten-thousandth of an inch. it is an aquired skill. Good luck :beer Joe
Thanks everybody for the good info! Joe, if I've got this right, you're measuring the caps and block (without bearings in place) so ensure roundness, then doing it all again with the bearings in place to check for clearances? If the clearances are close but aren't "dead on", since the bearings vary some, can the bearing halves be swapped around to get the desired clearances? How much variance in bearings do you see? As far as align honing or boring, why does this move the crank centerline toward the cam? Don't they cut a slight bit off the main cap mating surface and then hone/bore the hole back to round along the original axis?
James- The bearing thickness can vary about .0001-.0003. I've seen them off by as much as .0009. You have too multiply each of those numbers by 2, since there are 2 bearings. which can make our .010 bearings, .0118, and suddenly your crank won't turn. Ussually the bearings are right in spec, and uniform to each other. but I always double check their thickness becuase I have ssen some poorly machined bearings. I measure the mains torqued with out bearings to ensure roundness, and to make sure they are properly sized and within .0005 of each other. I then add the bearing thickness, and voila, I have the final Journal size. :bglasses: I mic the crank to make sure it is round as well, and then determine what needs to be ground off it to get the desired clearance. When align honeing, about 1 thou is cut off of each cap. When the machinist cuts the cap, He determines the appropriate amount to cut off based on the measurements he should have already takenfrom the journals. So the crank at most is "moving" in the block a thou or 2 at most. As far as mixing and matching bearings, I would partner the bearings with one that most closely matches its own size. for th most part, they are ussually within .0002 to each other. :beer Joe
Hi All, Any thoughts/comments on mixing bearings of say std. size and .001 undersize (on the same journal) in order to obtain the correct clearance. I have seen guys do this for years, but never knew if one would obtain a uniform bearing crush in this fashion. Regards, Craig
bearing crush Using different size clearance on bearing has no affect on bearing crush. Crush is a function of the bearing outer circumference which should remain constant with a .001, .010, or .020 undersize bearings. I think using a standard lower half with a .001 upper half is entirely acceptable and sometimes used by the Buick factory.
Wow, this is why I love this board! Getting a BPG membership for Christmas, that'll make my life complete...:laugh: Thanks everybody for all the good info, I'm starting to think I might know something about what I'm going to be doing... One more question: what exactly is bearing crush? Do the bearings really get smaller, or are they just getting clamped into place?
