Are the 270 rod nuts supposed to be a lock nut? If they were a lock nut they no longer have any grip on the bolts. They have slots cut through each flat. Do I dare try and tighten them back up or would I be much better off replacing them with new aftermarket rod nuts?
The bolts themselves aren't very tight in the rods and caps but finding new bolts seems to be difficult.
GMC 228, 248, 270 and 302 engines all used the same part number nut, part number 560545 and bolt, part number 2192821. There was no lock washer used.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
I install 3/8-24 Helicoil inserts in the rods and use recycled NASCAR rod bolts. There is approximately twice the thread engagement over a bolt/nut setup, and MUCH stronger bolts than original. Recondition the rod big ends after installing the upgraded bolts. Ignore the original torque specs, and use a bolt stretch gauge during installation.
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I install 3/8-24 Helicoil inserts in the rods and use recycled NASCAR rod bolts. There is approximately twice the thread engagement over a bolt/nut setup, and MUCH stronger bolts than original. Recondition the rod big ends after installing the upgraded bolts. Ignore the original torque specs, and use a bolt stretch gauge during installation.
Wow, that's an upgrade I wouldn't have thought of. Those bolts look like they have a reduced shank diameter...what do you use to align the rod and cap?
Do you stack a couple of Heli-coil inserts in the rod to get that much thread engagement?
Helicoil inserts are available in several lengths. I usually use the ones 5/8" long. They pretty much fill up the entire side of the rod. If the shoulder on the bolt doesn't center up completely in the cap, ream it out a little and press fit a short centering sleeve into the cap.
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
On the long stroke engines I build, the 12 point bolts that point upwards also provide a bit more clearance between the connecting rods and the camshaft, rather than having the end of the rod bolts and the nuts hanging off the bottom of the rod cap. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Those are slotted hex nuts, similar to a lock nut but not really a locking nut. They were used in vibration areas like A/C mounts, water pump mounting, P/S mounts, any place a nut could have vibrated loose. The slotted head bolts you see, called PLACE-BOLTS, https://www.aftfasteners.com/place-bolts-specifications-dimensions/ are pretty neat technology, the nuts may do the same thing. The slots likely act like teeth gripping just enough to not back off.
Pontiac built all their engines with the same rod bolts and nuts, 3/8-24 tall nut, non-locking, straight nut. From inline 6 and 8 and every size V8 made, all used the same nuts torqued to 45 ft lbs. My guess is the design of the bolt is what keeps the nuts tight, not the nut.
If you decide to change them, use a tall nut, or even better yet a new bolt and nut set, but you will need to have the rod resized if you go swapping bolts. You could get away with new nuts, but you need to check each rod carefully when assembling the engine. The bolts locate the caps, changing them will distort them. There is a video's on youtube where a guy changes rod bolts on a Ford engine without resizing the hole and shows how bad they go out of round.
Those are slotted hex nuts, similar to a lock nut but not really a locking nut. They were used in vibration areas like A/C mounts, water pump mounting, P/S mounts, any place a nut could have vibrated loose. The slotted head bolts you see, called PLACE-BOLTS, https://www.aftfasteners.com/place-bolts-specifications-dimensions/ are pretty neat technology, the nuts may do the same thing. The slots likely act like teeth gripping just enough to not back off.
Pontiac built all their engines with the same rod bolts and nuts, 3/8-24 tall nut, non-locking, straight nut. From inline 6 and 8 and every size V8 made, all used the same nuts torqued to 45 ft lbs. My guess is the design of the bolt is what keeps the nuts tight, not the nut.
If you decide to change them, use a tall nut, or even better yet a new bolt and nut set, but you will need to have the rod resized if you go swapping bolts. You could get away with new nuts, but you need to check each rod carefully when assembling the engine. The bolts locate the caps, changing them will distort them. There is a video's on youtube where a guy changes rod bolts on a Ford engine without resizing the hole and shows how bad they go out of round.
That's interesting....much like your bolt example, the nuts have a recess on the face so the only contact is on the outer edge. So when the nut is tightened the outer edge acts as a lever to tip the slotted end inward to grip the bolt. I've seen that style of fastener but hadn't really understood the mechanics of how they work.
I will probably just end up putting the originals back and not overthink it. They've worked for 80 years and will likely last many more.
I ran the connecting rod reconditioning department of a big machine shop in Nashville for a few years. We ALWAYS reconditioned rods when changing to new bolts. Grind a few thousandths off the face of the rod cap to shorten the diameter of the big end a little, and then hone it round again with a Sunnen hone. The process shortens the rod by a few thousandths, but as long as all the rods get the same treatment, no harm- - - -no foul. On a blueprinted racing engine, find the shortest rod, recondition all the others to the same length, and deck the block surface a little to get back to the minimum deck height. That, and about a dozen other procedures is the reason racing requires a deep-pocket sponsor!
Bolts and nuts stay tight because torque stretches the bolt- - - - -not the design of the nut or lock washer- - - -that's why the same size and thread hardware has different torque specs for grades 2, 5, and 8 material. A Grade 8 bolt tightened to Grade 5 torque will loosen up and come out. The tougher the bolt, the tighter it needs to be to get the proper amount of stretch.
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
