The Stovebolt Site Forums The Shop Area Engine Shop Main Bearing and Connecting Rod Clearances

After reading the Chevy Truck Shop Manual, I'm still not sure what the proper clearance is for the main bearings and the connecting rods. I'm working with a 216 engine, newly machined crankshaft, main bearing inserts (to match crank journal size), and re-babbitted connecting rods (also to match crank journal size). Thought I read somewhere that all clearances should be between 0.001 and 0.003" but when the manuals says "selective fit" I can't be sure.

So, here's the questions:
1. What is the proper clearance range for the main bearings?
2. What is the proper clearance range for the connecting rods?
3. What is "selective fit"?

Thanks for all the great input from the group. I've posted before and usually get very good information.

what year is this a 1949 as your Handel suggests ? i have some spec's .i can scan and e-mail you the page if you send your address.

It was common practice to adjust the rod bearings so that you could tap it back and forth on the journal with a very light tap with a 8 oz ball pein hammer. But that it would not move easily by hand. I think the 001 to 003 measured with plastigage for the mains would be OK. But if you can acheive 001 to 002 would be better. These are my opinions I'm sure others will respond with there opinions.
Cleon

If you look at the adjusting method for the main shims, you remove shims until drag is created and then replace one shim on one side. The shims are .002" and so that translates to a minimum of .001" clearance. The GMC engine which is different but similar calls for .001" to .003" clearance on a new assembly.

For the rods, a similar method was used. There were .002" shims and .001" shims so in theory you could be talking a minimum clearance of .005" on the rods. I don't think that you want a rod to have a clearance greater than .003".

I shimmed my rods before I installed the pistons on them and this way I was able to get a handle on the drag.

The side clearance is of huge import as is the wrist pin fit in the piston end of the rod. I was surprised when I did the last engine by the taper on the wrist pin bushings when miked as opposed to the feel method.

As for the rod fit with babbitt, is was common to use prussian blue and rotate the crank then reinspect for tight spots where the blue was thin. A scraper is used to fit the babbitt onto the journal and it is checked again fot round and fit. This is really important if you are refitting runin rods. Hopefully the rebabbitted rods are true but when torqued, things distort and this can result in warp of the fitting surfaces. All of this is best done before you hang the crank. It will work with the mains as well. The end play on the crank is also important and there is a thrust plate which I had to replace on the 248 GMC motor which I just did. This plate was worn beyond tolerance and I got it from Patrick in Arizona. If you call him, he will share alot of experience with you regarding your project. On my last 216 the rocker oiler pipe was a nuisance and I suggest you follow the oil path and clean all of that out while you are apart.

As usual, lots of great info to keep my project moving forward. I'll take a look at the rocker oiler pipe too - good idea.