Crank bearing inspection | GTAMotorcycle.com

Crank bearing inspection

timtune

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I'm starting to rebuild my old 750-4 and have a about 4 old motors (SOHC) with unknown history. I want to use the best parts to build one engine. Decades back in shop class we used Plastigage to check the bearing/crank gap. Is that the only way? Tearing each down to the bottom seams like a lot of work.
 
You don't need to tear them all down. Pick the one that looks the best and start there. If the clearance is good, the rest are parts engines, if not, move onto engine B.

If you really didn't want to use plastigauge, you might be able to do it with accurate oil pressure and flow rate but I have my doubts. Especially on a one off project. Figuring out the values of good vs bad engines will take more time than opening up a few engines (and a visual inspection of the bearings in the bottom end you are spending a lot of time and money on is never a bad thing).
 
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if the history is unknown
I'd suggest you do tear down to the bottom
it's possible to have crank/bearing damage
that won't show up on a clearance test

I mean you could dial indicator the ends
but that doesn't say much really
 
Didn't they have colour coded bearing parts so you know what the tolerance is that you are aiming for on that particular crank journal?
 
Can't recall. It was over 20 years ago that I built the first motor from the best of two. Back then I picked parts solely by visual and my best venier measurements.
 
You might want to watch Peter Anderson's channel on Youtube, not sure if it would be of any help, but he goes into a lot of detail rebuilding CB750's
 
Seeing how it's almost 50 years old, let's just assume you're going to do more than a cursory inspection.
Disassemble, clean, measure with a mic, then if they're in spec you would plastigage.
While you've got it apart you should clean all the oil passages and empty the traps in the crank.
I would also check the 50 year old crank, of unknown providence, for run out.

If it was me, I just assume everything is junk and replace it all... and YES, you'll have to measure and plastigage the new parts too.
 
My experience with vintage stuff has been that for something with unknown history, the best course of action is to treat your unknown-history engine as a "core", tear it down and replace all wear parts and all seals and gaskets. Even if someone claims the engine ran when parked ... you don't know why.
 
Here's the story on the last "take two engines and make one good out of them" that I was involved with. First engine (vintage bike, rare model, never sold in North America) spun a rod bearing at TMP. Tore that down, crank was toast. Underlying cause ... oil passage restrictor jets not screwed in all the way. (Seems that a few older Yamaha engines suffered from that malady.) Options (1) send the crank off to Marine Crankshaft in California to get reworked, or (2) get another crankshaft, or (3) get another engine. Owner got another engine. Suggestion, tear that down and build one good one. Did that. The replacement engine had an oil pan coated inside in black clutch-plate schmuck. And those oil passage restrictor jets were loose ... Built one good engine out of both ... and made sure that oil restrictor jet was never going to be a problem again.
 
I'm starting to rebuild my old 750-4 and have a about 4 old motors (SOHC) with unknown history. I want to use the best parts to build one engine. Decades back in shop class we used Plastigage to check the bearing/crank gap. Is that the only way? Tearing each down to the bottom seams like a lot of work.

Best advice I can give you is go here...

forums.sohc4.net


Sent from my iPhone using Tapatalk
 
Joined the sohc forum. Checked the manual - yep colour coded bearing inserts.

Funny note - I was reading through a hot rod magazine (Rodding & Restyling) from 1958. It details plastigaging the crank. Some things never change I guess.
 
Do it by the book, surgically cleaning everything and replacing all wear items and any suspect parts. You don't want to do it twice.
 
Do it by the book, surgically cleaning everything and replacing all wear items and any suspect parts. You don't want to do it twice.
You forgot make sure it turns over before bolting the motor back in the frame. Learned the hard way. Interesting side note the motor would turn over until I bolted the cases together and then stuck solid. Turns out in my mix & match tranny assembling I had mixed a gear set of K and F cogs. The difference in the number of teeth was enough to lock it solid.
 
You forgot make sure it turns over before bolting the motor back in the frame. Learned the hard way. Interesting side note the motor would turn over until I bolted the cases together and then stuck solid. Turns out in my mix & match tranny assembling I had mixed a gear set of K and F cogs. The difference in the number of teeth was enough to lock it solid.

That too (LOL) !
 

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