The Stovebolt Site Forums The Shop Area Engine Shop Dumb question #1 re: pilot bearings

Hello all. I need to clear up a discussion I'm having with my dad about pilot bearings. I understand that the pilot bearing in the rear end of the crankshaft is different for manual vs. automatic transmissions. Here's my question: if you have a motor that is mated to an automatic, to swap over to a manual transmission do you have to change the crankshaft to get the correct size pilot bearing installed, or can you just change the bearing? I've watched videos that show how to change a pilot bearing without removing the crank, but they're always dealing with automatic transmissions only.

Brian

I always thought engines that are mated to a auto trans don’t have a pilot bushing therefore if you put a Manuel trans in you have to makesure there is a bushing in the end of the crank.

Brian, the following is based on my admitted limited experience and does not necessarily apply to today's engines, only what's now commonly referred to as the classics. Many crankshafts were drilled for a pilot shaft/bearing/bushing regardless of automatic or manual transmission.

I do know some manufactures (Mopar big blocks) have a hub hole in the crank for the torque converter that can be used with a special bearing and shortened pilot shaft without any additional drilling. Again with Mopar's, if they came with a manual transmission then drilled for a pilotshaft. If originally a automatic car, might be drilled for a pilotshaft, or not. However it's possible to drill the crank while it's in the car if required to accommodate a standard transmission input shaft. Not hard to do with a good appropriately sized bit and drill. Fancy folks use a jig to insure perfection.

Good Luck,

RonR

Most cranks come drilled for a pilot bushing or bearing. Drilling a crank in the block is not acceptable, crank would need to be set up, not a normal procedure. Automatics do not come with pilot bushings, torque converter registers in flywheel since they always turn.

Pilot bushings, bronze or brass are easily made or bought. Pilot bearings are easily bought, however, when they fail, they frequently damage the input shaft which may or may not be replaceable, especially on an older vehicle.

If one is changing from automatic trans. to manual, one must always check to make crank is drilled, might be necessary to get another crank if not drilled.

Ed

Thank you all for the replies. I really appreciate the input (no pun intended)! I'm venturing into uncharted territories with my project, but really enjoying learning new stuff.

It had not ocurred to me that the crankshaft (in an older motor, like a 235) would be drilled for a pilot bushing regardless of which transmission it was mated to.

In my case, I have a later model 250 (non running) mated to a Turbo 350 in my '51 Chevy project. I acquired a 4-speed SM 420 a while ago to replace the auto. I haven't pulled the motor and trans out of the truck yet so I haven't seen the rear end of the crank. My dad commented that I may have to replace the crankshaft. I was planning to rebuild the 250 but just discovered it has freeze damage so now I'm looking at a replacement. I wanted to be sure, when I go look at another motor, that I wasn't getting one that needed a new crankshaft. Long story short, as long as the crank is drilled for a pilot bushing I'm ok.

Many thanks again, and Merry Christmas!

Brian

I have machined a crankshaft for an AMC V8 while it was installed in the engine, but we stood the engine up on the harmonic balancer, leveled it in all directions, and used a precision boring head in a radial drill press to machine an accurate hole. That one was set up for a different torque converter since the year of the engine didn't match the vehicle it was being mated with. AMC had swapped from a Borg-Warner transmission to a Torqueflite, and the TF had a different converter snout dimension.
Jerry

The 250/292 crank is very likely drilled for the bushing/bearing even though it may have an automatic trans hooked to it. When machining the crankshafts, they operators didn't know what transmission it was going to be hooked to. Only the special HP engines used special crankshafts, the other 99.9% of engines used standard pieces.

The one thing I will add here is that with GM motors, for a little while GM used two different crankshafts. One for auto and one for stick. A pilot bearing can be installed in either crank but the back of the crank was machined at two different sizes. I have come across this more often with early small block chevys, never with a 235. Not sure if the 230-292 motors ever had that difference but its no problem though as pilot bushings are made for both size holes. Order the standard bushing. It should require a good bit of press to install it. If it just slides or pushes in, you need the larger bearing.

In terms of material for the pilot bushing, the cheap auto part bushings are frequently steel or some other ferrous material. Run away. Other inexpensive versions are bronze but just plain old bronze. Hold out for a real Oilite bushing. I am pretty sure I have this right, the Oilite material was developed back in the 30s and remains the gold standard for pilot bushing material for our old motors. It is an oil impregnated sintered bronze. It oozes oil and lubricates the input shaft tip. Do not grease the inside of this bearing. Do not grease the tip of the input shaft. You seal up the pores and it wont ooze oil as it should. I make it a point to obtain the real McCoy and over 40 years later, no pilot bushing failures, and I am not easy on my motors or transmissions.

Other then that, I think you are good to go!

Thanks again for the added information. Mike, I read up on the different pilot bushings/bearings this morning and learned about the Oilite bronze bushings as you mentioned. I can see how that would be the right choice.

Earlier today I met up with a guy selling a motor on Offer Up near me. He had it listed as a 265, though I suspected he meant it was either a 261 or 235. As it turned out, according to the block number it is a '50-'52 235, a rather appropriate choice for my '51 truck. The best part is, upon a close inspection (removed the valve cover and looked in the fuel pump hole) the motor appears to have been remanufactured in the past and then mothballed. All of the openings (spark plug holes, etc.) have purpose-made plugs. The valve springs look new; everything is super clean. Other than a light film, there is no oil on anything. It appears I got a zero mileage motor for $100. It even has a new clutch pack on the flywheel. The pilot bearing appears to be a later style roller bearing, not a basic scintered bronze bushing. I plan to get the motor onto a stand Saturday so I can inspect it thoroughly.

Post a photo of that bearing if you can. If it is a true roller pilot bearing in the crank, you will want to remove it and replace it with a regular bushing. Those roller pilot bearings are hardened and can really damage and/or wear out the tip of the input shaft. Really designed for road racing.

Agreed. After doing some research, and hearing the advice on this forum, I plan to replace it with an Oilite scintered bronze bushing.