The Stovebolt Site Forums The Shop Area Engine Shop 261 Crankshaft Question

I just had my 261's crankshaft turned. Are the main journals dimensioned differently as you go toward the rear? The machinist who turned the shaft said they were (got bigger as you went aft) and quoted from the manual he had gotten the set up info from for his machine.

Is this so?

John, I believe I emailed you a page from my Federal-Mogul shop specifications manual that has the crankshaft dimensions listed. Your machinist is right about the main bearings having different diameters, with the largest one back by the flywheel. I can re-send that page if you need it- - - -it's also got info on oil clearances, etc.
Jerry

John, you've got the answer to your question but I thought I'd mention one other related item to save you from potentially making a mistake that I recently made. The two mid bearing caps have to be installed going in the correct direction. If you look very closely at the numbers cast into each of the caps, the front mid cap will have an F cast into it and the rear mid cap will have an R cast into it. These are stand-alone letters not associated with any parts numbers cast into the caps. The letters are on one side of the bearing cap. The F side of the front mid cap goes toward the front of the engine. The R side of the rear mid cap goes toward the rear of the engine. Sometimes the R and the F are cast very lightly and hard to read. If you marked your bearing caps when you removed them with which side goes toward the cam, you'll be all set, but I've been known to mark them and still mix them up! The R and the F are bullet proof ways to identify the correct way to install them.

The danger is that the caps will fit and can easily be installed facing the wrong direction. I recently did that with a rear mid cap by accident and couldn't figure out why I was getting binding even thought it passed inspection with Plastigauge.

Just in case you weren't aware, I thought I'd add this to your post to save you making the same mistake that I made...but I made it even though I knew better!

Matt

Thanks Matt -- good to know! Each cap is individually packaged in a zip lock baggie with notes as to position and orientation. Your info will add to the insurance!

Jerry -- you did and I have it right here. Since 6 pm, I know one heck of a lot more about my crankshaft then I did prior I didn't understand that info you sent so I set it aside. After standing at the crankshaft grinding machine with the machinist getting a detailed explanation of how it worked and what he did to this crankshaft and what the challenges were, all that info makes a lot more sense now. Yikes.

That crankshaft grinding machine is a pretty impressive piece of equipment! Unfortunately, I did not get to see it in operation. I bet that would be something to see. Maybe I'll try youtube ...

He did tell me that if I needed the block align bored that I would have to take it to someone else as he did not want that job for a 261... After learning about the mains, I understand his pain...

Are 235's like that, too? Or is that something unique to the 261?

Yes, all the Stovebolt's are that way as well as the GMC's,Straight 8 Buick's and Straight 8 Pontiac's, must have been a GM corporate thing in that era. Also, all 4 housing bores in the block are also a different inside diameter as well, just as the crankshaft has 4 different diameter main journals, making line boring a job most shops don't want to mess with.

A line bore job is a real PITA, as most shops don't actually "bore" anymore- - - -they hone the main bores. On a stovebolt that would require a very special, very expensive hone mandrel capable of having four diameters set at once. The shop where I ran the connecting rod department actually had a line bore machine, so we could set up a block with four cutters and do the whole job in one pass. It might be good to mention that the old boy who ran that machine was in his late 60's- - - - -before the turn of the 21st. Century rolled around! I doubt if there are very many machinists still working for a living who remember how to set up a machine like that!
Jerry

Also different main sizes: Oldsmobile sidevalve L6, L8; Pontiac sidevalve L6