The Stovebolt Site Forums The Shop Area Engine Shop Wrist Pin Lubrication

I'm still putting together a '61 235 engine together. I noticed one of the rod bearing 1/2 shells was not lining up (about .030 off) with the other 1/2 shell in the cap (side to side). The cap lines up with the rod perfectly. The hole in the bearing was not lined up with the hole in the rod as well. All other bearings lined up ok. I tried swapping bearing sets but the problem stayed with the rod. Then I started thinking this hole in the connecting rod was an oil passage to the wrist pin. Nope, this hole stops a fraction of an inch into the rod. Inspection of the wrist pin end didn't have an opening for oil either. I'm thinking this misalignment is not a big deal but I am wondering why the bearing and rod has a hole in it.

The hole in the rod is angled toward the cylinder wall to lubricate the piston skirt as it squirts oil onto the cylinder when the oil passage in the crankshaft lines up with the hole in the bearing and the rod. The hole in the rod is very tiny, about .020" or so. I use a pin vise and a # 60 drill bit to clean the hole in the rod during an overhaul, as they tend to get clogged with sludge and/or carbon deposits. Wrist pins are gravity-oiled from angled holes in the top of the pin bosses in the pistons that catch some of the oil scraped off the cylinder walls by the rings.

GMC engines have gun-drilled rods and pressure-lubricated wrist pins.
Jerry

Thanks Jerry. I'm so thankful for this forum. Do you see any problems with the slight miss alignment of the bearing half shells?

They haven't quit making 235 connecting rods, or at least there are a few thousand of them still on the market. If you're concerned about it, (and I can't tell if that concern is justified without a hands-on examination of the part) I'd spend a few bucks for one reconditioned rod. Of course, I never build an engine without reconditioning all the rods.
Jerry

Thanks again Jerry. I measured the other 5 rods. Looks like none of them line up perfectly. They vary from 0,005 to .025 offset. Seems odd the position of the tang groove would vary that much from rod to rod. I had them reconditioned and balanced by the local machine shop. I'm not that concerned (they've made a lot of trips up and down without me worrying about it) but I may run them over to the machine shop for their opinion.

And yup, the hole in the bearing leads to a hole in the side of the rod. That would have really bugged me had you not been there to point that out. Thanks again.

Since the big end of the rod is usually a littler wider than the bearing insert, a few thousandths of misalignment in the top and bottom bearing shells isn't particularly a concern. The critical thing is the roundness of the crankshaft journal and the rod housing bore, as that definitely affects the oil clearance. A .0015" oil clearance is approximately the thickness of a strand of hair, so there's very little room for error. Most people pay a lot of attention to the roundness and smoothness of a crankshaft journal, but they ignore the rod bore altogether. Both measurements need to be round, straight, and the right diameter. for the bearing to do its job and the oil pressure to be right.
Jerry