Rod bolts fell out!

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jcmeyer5

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Disassembled my dads 340 today and had the bolts fall out of two rods. The engine has very few miles on it since rebuild. I am taking it apart to clean it up, check the work, and paint it. So the rod bolts. They fell out when I very lightly tapped the cap with a hammer to get it off.

I am guessing I’d be an idiot to try to reuse them?
 
I'd be pulling the pistons and being that they are clipped in,, I at least have all the rods magged/resized..........new bolts and at that point a set of new rods...
 
Not to hijack, but rebuilding our 70 340, and I didnt resize or replace rod bolts.....everything so far within spec. Should I just loctite them? Dont want to pull pistons to resize rods.
Opinions?
 
I wouldn't reuse them, but it's really not too uncommon for them to be loose or even fall out. Where can they go once torqued down?
 
Not to hijack, but rebuilding our 70 340, and I didnt resize or replace rod bolts.....everything so far within spec. Should I just loctite them? Dont want to pull pistons to resize rods.
Opinions?
Millions of stock rebuilds have been done reusing the rod bolts. If you resize, install new or don't install new bolts without resizing the rods. If it's just a refreshing up and you're not going to abuse it bad the old bolts should work fine.
 
Did you get your nuts off first
I would not put loctite on the bolt before putting it in the rod - you would have to torque before it hardens
just torque to spec with the correct lube which most likely was 30 weight EO
do not overtorque
you could locctitethe nuts but were they easy to take off?
if they felt right taking them off you are good to go
you do definitely not want to use arp or other HP bolt without redoing the rods
 
I’m not worried about replacing the bolts. I was more concerned that the bolts aren’t press fit. The only reason I would reuse the rods is because the engine is supposedly blueprinted and balanced. It shows signs of it... some of the pistons have been drilled.

Just curious what work are you checking? And how are you checking it, just a visual or measuring clearances etc... ?

Plastigage the clearances. The other reason to pull it apart is all the crud in the water jackets.

Like I said, the engine was rebuilt previously. It has forged pistons and a new oil pump and pickup. The rods are stock. It has a “purple” cam but I don’t know which one. Not even sure if it’s hydro or solid. I’m guessing hydro cause the valve train doesn’t look adjustable. Solid push rods? That seems weird. Mains look perfect. Everything looks new.
 
Thanks guys, shoulda researched/read a little more before starting assembly.
Or at least threw loctite on them. Rods were not resized. Crank turned 0.010 on mains only.
May have to tear down to be safe.
Thanks.
I wouldn't reuse them, but it's really not too uncommon for them to be loose or even fall out. Where can they go once torqued down?

Millions of stock rebuilds have been done reusing the rod bolts. If you resize, install new or don't install new bolts without resizing the rods. If it's just a refreshing up and you're not going to abuse it bad the old bolts should work fine.
 
Jim has made a decision. Jim is going to disassemble pistons from rods and have new bolts installed. Due to the massive difference between aftermarket rod weight and stock rod weight (200 grams each), I am going to stick with the stock rods and get the bolts replaced and have them resized. While there are some stout parts in this engine, this is not a race engine and I am not trying to spend more than reasonable to make the engine run well. New rods and rebalancing will cost far more than just replacing the bolts and resizing the rods.
 
Disassembled my dads 340 today and had the bolts fall out of two rods. The engine has very few miles on it since rebuild. I am taking it apart to clean it up, check the work, and paint it. So the rod bolts. They fell out when I very lightly tapped the cap with a hammer to get it off.

I am guessing I’d be an idiot to try to reuse them?


The shoulder on the rod bolts is what locates the cap to the rod.. if the cap is not properly aligned to the rod and the bore for the crank is mis-matched, it can cause an interference with the crank which can cause the crank to be harder to turn or even lock it up...

Whenever I change rod bolts, I have the crank bore of the rod refinished so they are aligned properly...


I would check the holes for the rod bolts in the rod for size before trying to go any further to make sure they are not enlarged... If they are ok, then I would get new bolts and have the rods refinished and install new bolts and re-machine the crank bore...


Also, how many times have the bolts been tightened??? If they have been tightened more than 5 times, I would recommend replacing them and re-machine the crank bore.... Keep in mind that if the rod bolts have never been replaced, they come out of the factory with at least two clamp cycles on them ---> which gives you only three left... If the rods were reused, and plastic gauged for the last rebuild, then you already have 4 clamp cycles on them... If you use them again, to plastic gauge and then reassemble them that will give you a total of six clamp loads on those bolts.. There can be a chance that they would be over stretched and weaken them... I would not recommend using them with that many clamp cycles on them...

The best thing to do if you want the engine to hold together, is to replace the rod bolts and have the crank bore re-machined... Otherwise you risk throwing a rod....
 
@krazykuda We bought the car back in 1998 with the rebuilt engine. Judging from the lack of wear on the pistons, it hasn't been run much (we didn't drive it). Piston bores are good too.

Your suggestion is exactly what I am going to do. Bolts are ordered. Tomorrow I will remove the pistons from the rods.
 
Re-sizing the rods will not fix a loose bolt hole....??? I'd check the holes for size for sure; if they are enlarged, try to figure out why.

BTW as for SCAT rods.... they will not cost much more than the re-sizing of the stock rods. As for balance, they will weigh something like 150 grams under the stock rods as for total weight. The change in bobweight on the crank will be on the order of -250 grams + or -. That will mean 1-2 holes drilled in the end counterweights. Not a big effort.

If the pistons weights have been equalized (as it sounds); that is easy to confirm with a scale on your own. If that is true, and since SCAT rod will all be closely weight-equalized from the factory, that means you can do this with just touching the crank for a new bobweight computed value.. and touching nothing else for balance. I can go into the details more if that is of interest.
 
I was looking at Eagle rods (cheaper than SCAT, and have good reviews). First step is disassembly. We will know more after that.
 
I would not loctite rod bolts or nuts. Usually the rod bolts are pretty tight in the rods. Rod bolt/nut assemblies also differ in weight. As long as you can,t push the bolts all the way in with your fingers I would leave everything as is and reassemble it torquing everything to spec. Good rod bolts usually have identifying marks, some even have machined dimples for measuring stretch.
 
I was looking at Eagle rods (cheaper than SCAT, and have good reviews). First step is disassembly. We will know more after that.

Watch out for some of the aftermarket rods, some of them do not have oil squirt holes like the stock rods do... The oil squirt hole squirts oil on the opposite cylinder walls to help lubricate them... Some companies claim that you don't need them, but I'm not gonna be their guinnea pig and do their testing on my engine and my budget... Also make sure if you use rods with oil squirt holes, you get bearings with the notch for the oil squirt hole, some bearings do not have them...
 
I’m not worried about replacing the bolts. I was more concerned that the bolts aren’t press fit. The only reason I would reuse the rods is because the engine is supposedly blueprinted and balanced. It shows signs of it... some of the pistons have been drilled.



Plastigage the clearances. The other reason to pull it apart is all the crud in the water jackets.

Like I said, the engine was rebuilt previously. It has forged pistons and a new oil pump and pickup. The rods are stock. It has a “purple” cam but I don’t know which one. Not even sure if it’s hydro or solid. I’m guessing hydro cause the valve train doesn’t look adjustable. Solid push rods? That seems weird. Mains look perfect. Everything looks new.



I worked at Trenton engine during the 71-76 phase and worked on the rod line. I worked at all the jobs on that line from drilling the small end on the Natco's to the broaching of the rod journals to the buhrs that drilled the rod bolt holes and made the notch for the bearing locator. 80% of the rods were originally fine and had the bolts pressed in the others dropped in fine but came out easily (just by rapping lightly on the bolt end side and they would come out) when first made. (yes they passed QC) as long as they didn't wobble in the rod hole they were passed on. The only time the rod got pulled was if the small end (or the big end )was not correctly sized and then it went into a bin to get copper plating on it so it could be resized. that is on brand new motors. your choice on using the old ones , but if new bolts I'd get then resized
 
So we disassembled the pistons from the rods today. What I found was what @66Dvert was describing. The bolts stay in fine until you tap on them. From looking at the bolts, it doesn’t appear they are knurled at all. They do not wobble in the bores. I couldn’t just push them in with my fingers. I was able to lightly tap the 3 that came loose back into the rods. They stayed well enough. New bolts are on order.

We are cleaning the rods, rings, and pistons today. Funny thing. The rods are lightly magnetic.
 
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