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Question on bearings....

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sparkydave

sparkydave

Jedi Knight
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This is more of a generic question than a spridget-specific one, but how do you know when the bearings are getting worn out? And how do you know if a bearing has spun? I doubt mine is ready for bearings, since it only has 30,000 miles on it and the oil pressure stays above 20 at idle, but it might be nice to know for when the time comes. Better still, how would you know if you need oversize bearings? I know how to check on the notorious 1500 thrust washers, and mine don't show signs of excessive endfloat yet.

-Dave
 
Hello Dave,

loss of oil pressure, hot, is the first indication there may be a bearing problem. That and the engine getting a bit 'rumbly'. An engine has to get in a bad state for the bearing shells to spin and is not normally something that will be diagnosed unles you dismantle the engine.
Oversize bearings would be required when machining is carried out to remedy wear on the crank and the machinist would tell you to what size he has ground it to.

Alec
 
Dave,
I found out that my 1500 had oversize rod bearings when I dropped the pan for the first time. I pulled a rod cap and .010 was stamped on the bearing shell. I used a micrometer on the crank journal and the readings were pretty much what would be expected for a ground crank. Most repair manuals have tolerances listed for both the rod journals and the main journals on the crank. The only way to know for sure is to open it up and check with cars as old as ours are. I was and still am pretty sure that no one had been inside my engine but unless the crank came undersized from the factory...I guess someone was there before me. As Alec said, oil pressure is the best indicator for bearing condition. I smoked mine the other day when I let it get a little hot and the oil was a little low. Looks like I'll be pulling the engine as I don't think it is a good idea to try to change the main bearings with the engine in the car. A good rule of thumb is to change the rod bearings out when you change the thrust washers...or so they tell me anyway. The number of miles has less to do with bearing life than do factors like, over heating, religious oil changes, and high revs...
Hope this helps,
JC
 
You can check if you need oversized bearings by using a micrometer on the crank journals. Be careful however, because, although not common, sometimes all the journals have not been turned to the same dimension. This is especially true if the crank was turned due to a spun bearing. It's customary to turn everything down equally, but not manditory. Also, a crank could have been turned to say .10 under on the rods, but .20 under on the mains.

About a spun bearing... this usually occurs from over-revving an engine, or running an engine extremely hot. The bearing is held into the rod and cap by tabs on the bearing. When you spin a bearing, the locking tabs fail and either both halves will "cleve" together or the bearings will spin 90 degrees (or more) inside the rod and cap. In either case, you will hear a "knock" coming from the bottom of the engine (easily diagnosed by placing a steel rod against the oil pan with the engine idling). Also, a spun bearing does severe damage to the crank, usually requiring it to be turned if it is savable. It also usually destroys the rod and rod cap.
 
Thanks much for the info, it has been helpful. JC, somewhere I remember stumbling across a how-to on changing the mains, rod bearings, and thrust washers with the engine in the car. Then again it was written based on a Spit and the author didn't know if it was possible on a Midget. I don't doubt it would be considerably easier with the engine out.
 
Yes, oil pressure (especially after warmup) is a good indicator of bearing wear.

Also, another handy item is "Plastigage".
This stuff is little pieces of plastic that is calibrated to a specific diameter. Place the Platigage between the bearing and the crank and torque the rod cap down (Do not turn the engine). Remove the cap and the Platigage will be squished down. Use a special printed gage that come with the Plastigage to measure how wide the plastic has been been squished to. This will tell you the bearing clearance (typically about 0.001" to 0.002" in a good engine).
When I put the new crank and bearings in my racer a few weeks ago, I used this stuff to confirm that everything was OK.
Available for a few dollars at most auto parts stores.
 
Second the motion on the Plastigage. I've used it for years to confirm fit on almost every engine that I have built. As far as the notion of changing the main bearings with the engine in the car...I used that article to help me change the rod bearings. I think that it would be possible to change the mains as well but I don't think I want to attempt it. There is so much grit and grime down there and the space is so limited that I think it would be a booger to get the rear main apart and properly back together. After being in there, I think for me it would prove easier to just snatch out the engine. It is very difficult to do a good journal inspection with the crank in the block from underneath the car.
JC
 
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