Re-use Connecting Rod Bolts?

[mikeedler]

Really? Following the instructions in a factory manual is the dumbest thing you've read on an message forum? I don't know which internet you're on, but I'd like access, please.

factory manual??????????? If you would read an actual BMW factory manual, almost every single fastener that you remove from ANYTHING on a BMW is supposed to be replaced.

best of luck

[aventari]

Oh yeah you are correct sir, I withdraw my previous comment. I wasn't thinking about the other pistons.

I'm also not terribly worried about a 250,000 mile engine in a $300 car blowing up. I'm just throwing in new bearings while I have the pan dropped to eek a little more life out of it because it's going to be hard miles on the track from now until it lets go. The car is getting built for 24 Hours of LeMons / Chumpcar.

If it blows I'll drop in an $80 junkyard motor over a weekend, no big deal.

See you guys on the track :)


edit:

"the bentley doesn't say you can't" is about the dumbest thing I've read here in a long time.

Really? Following the instructions in a factory manual is the dumbest thing you've read on an message forum? I don't know which internet you're on, but I'd like access, please.

[Exodus_2pt0]

This is actually not true--the crank bears that force, but the rod bolts do take a lot of force slowing the piston/rod down on the upstroke.


Regardless I'm reusing them and I hope I don't have to post in here again in a few months

It is true... You are wrong.

Each piston is attached to the crank. When one piston is on its power stroke, another will be on its intake stroke. The force generated by the piston on its power stroke, will transfer to the piston/rod assembly that is on it's intake stroke, through the crank. Therefore, the connecting rod bolts attached to the cylinder that is on the intake stroke must bear the full force in order to pull that piston/rod down.

Get it yet? It is a tremendous mistake and misunderstanding to assume that connecting rod bolts aren't under extreme pressure.

If another person attempts to tell me i'm wrong again, I suggest you go and take some classes on Internal Combustion Engines.

[acolella76]

They can be reused just enough to torque down the bearings so you can measure bearing clearance. That is about all that I would reuse them for... after that buy new ones. There's nothing more depressing than spending a shit load of money rebuilding an engine and getting rod knock 100 miles later. Ask me how I know!

[reelizmpro]

You can and will probably get away with it but it's not advised and if one fails, you will have a big mess to deal with because you didn't take the extra precaution. That's too big of a gamble...just change them for peace of mind.

[nando]

okay, good luck buddy.

"the bentley doesn't say you can't" is about the dumbest thing I've read here in a long time.

[aventari]

This is very far from the truth. Rod bolts bear the full force of each piston firing.

This is actually not true--the crank bears that force, but the rod bolts do take a lot of force slowing the piston/rod down on the upstroke.


Regardless I'm reusing them and I hope I don't have to post in here again in a few months

[bmwstudent]

I've banged dirty chicks with no rubber before. Just because i've done that doesn't make it okay or safe. I don't' recommend it. Throwing a rod is like getting herpes, its permanent.

[Exodus_2pt0]

rod bolts have far less stress on them than head bolts do. ive re used connecting rod bolts before.

This is very far from the truth. Rod bolts bear the full force of each piston firing.

[aventari]

Are these pictures from E30 rod bolt failure after reuse?

[nando]

Connecting rod bolts are torqued far less than head bolts so its hard to compare the two. rod bolts have far less stress on them than head bolts do. ive re used connecting rod bolts before.

uh.... no.

also, there are lots of things that are flat out wrong in the Bentley manual for E30s that we've known about for years. it's a reference, not an absolute holy bible of E30s. E30 Rod bolts are TTY, they stretch by design, and must be replaced. End of story.

[ApexMotorsports]

If don't need to spend the extra $100 for bolts it'd be nice,

If you hate your engine, don't buy rod bolts. The connecting rod is the most stressed part of the engine. Therefore the fasteners that hold them together are critically important. If you had ARP rod bolts, I would consider it a possibility for reuse if you had documented the pre stretch length of each fastener and it's time and cycles 97% of engine builders do not do this. But even doing so I would only keep as a scary spare. But factory rod bolts go in the trash as soon as they come off the rod cap. Example, if head bolts stretch you end up pushing coolant due to a lifted head. If a rod bolt stretches beyond it's tensile strength you throw a rod {see photos below}... Or best case spin a bearing. Either way it's much cheaper to buy new bolts and have the peace of mind.

This loctite and reuse idea is wrong, unless the guy your building the engine for did something bad to your little sister and you want some payback. Rod bolt failure happens due to plastic deformation once the bolt has exceeded it's stretch threshold. Unless your using a rod bolt stretch gauge you won't know which bolt is stretching to much at your torque value. And the idea of using loctite on a rod bolt is not addressing the root of the problem. It's not that the rod bolt loosens and causes a failure. It STRETCHES and causes a failure.

Also using improper lubricant can cause fasteners to sheer torsionally {ie using loctite}. This is due to the fastener being designed for axial loading (torquing the fastener and the bolt stretching to the proper preload, ie stretch) and not for torsional loading. Proper lube for rod bolts, head studs, main studs etc are either engine oil if moly based lubes such as ARP lube isn't available. Don't install fasteners dry, but at the same time don't use loctite for crap sakes. Loctite on other components like cam fasteners, crank, flywheel hardware etc are highly recommended, but NOT for rods, main cap hardware, and head studs/ bolts unless they are head/ main studs for permanent installs or they need sealing due to the hole intruding into water jackets etc.

[Cabriolet]

ive reused them on 3 (2 I's and 1 eta)different motors and and a close friend has reused his twice. we run our cars pretty hard, no issue so far. always locktite them and torque them just a hair on the hightside of spec.

[Dj Buttchug]

Connecting rod bolts are torqued far less than head bolts so its hard to compare the two. rod bolts have far less stress on them than head bolts do. ive re used connecting rod bolts before.

op, your not going to have any issues here. make sure the fasteners are clean and the threads are quality. Use some sort of graphite thread lubricant (arp) makes some good stuff and make sure you get your torque settings right. you probably already knew that tho.

[aventari]

Resurrecting this thread to say that I'm replacing my rod and main bearings in a 250k mile 325is beater track car I'm building and I'm going to reuse the bolts.

Nowhere in the Bentley manual does it say they stretch and need to be replaced with each use so I'm not going to.


It's the same situation with watercooled VWs which I'm very familiar with. Everyone says you need to replace the head bolts _every time_ but the VW Bentley doesn't mention it and my father who is a metallurgist also says it's not necessary especially if the manual doesn't tell you to. So I've pulled the head on my turbo Scirocco at least 8 times and I only replace the head bolts after every 3rd or 4th use. Never had a single head bolt related issue even with the turbo.

If my e30 connecting rods fail after I reuse them I promise I will post here and eat my words.


I do have a question though: I'm replacing the rod and main bearings with the motor in the car, is that a difficult job after I drop the pan? Has anyone else done it? It's easy on VW's because there's no crossmember in the way, but on this car it may have space issues