Connecting Rod Bolts

[digger]

So I guess unless you are pulling the rods to have them resized ARP bolts should never be used in any engine....

correct, most engines dont require shell changes in-situ and if you were required to do that it only makes sense to use the same type of bolts that were already there unless there was a known issue with the actual bolts

So I guess unless you are pulling the rods to have them resized ARP bolts should never be used in any engine....

So what do you guys think I should do now? I just got my car all put back together and running again. After replacing the rod bearings and using the arp m10 bolts stretched to 0.065-0.070 per ARP instructions. Should I tear it all back apart and replace with oem bolts?

Many many people have done what you did in both the s50 and s54 without resizing even if you are supposed to.

[eacmen]

ARP M10 bolts torqued using stretch gauge to the ARP spec. So far no issues, engine has not exploded yet!

[tdott]

Get stock. They're cheaper, have more clamping force, and create a rounder bore than ARP. They are a downgrade, not an upgrade. Only downside is they're a bit more of a PITA to torque.

Do you have any measurements to prove this?

BE did extensive measurements on S65s and found that the ARP bolt torqued to their spec is a good match in terms of eccentricity and clamping force.

[Obioban]

Do you have any measurements to prove this?

BE did extensive measurements on S65s and found that the ARP bolt torqued to their spec is a good match in terms of eccentricity and clamping force.

Lang racing compared both on M3F. It was lost to the forum going down.

[jbfrancis3]

Lang racing compared both on M3F. It was lost to the forum going down.

This has been carried forward but its more nuanced, at least for what they’ve publicly stated. Which is,for the ‘03+ they note ARP’s need to be torqued beyond ARP spec to achieve same bore shape. They do not note the method to achieve or go beyond spec. Plus BMW one-time bolts are cheaper as well as adequate. At customer request they will do ARP, which tells you its not necessarily a no-go. For the under ‘03, they recommend ARP.

https://www.langracing.com/s54-rod-bearing-and-bolt-replacement-with-arp-fasteners/

[Slideways]

It's all in post #8, but it is worth reposting:

S54 engines manufactured prior to 2003 come with larger rod bolt that are not supposed to be replaced according to BMW TIS. The later engines use a rod bolt that is smaller (torx head) and is one time use. On the early engines we prefer to use ARP fasteners because we don’t know the condition of the stock rod bolt. On the newer 2003+ engines we have found that the stock bolt is completely reliable and a bit cheaper than the ARP bolts so we will typically use those unless the customer requests ARP bolts. In our bench testing we found that the ARP bolt in the 2003 and newer engine needs to be stretched beyond the ARP specification to achieve the same bore shape as the stock BMW bolt.

A benefit of the ARP bolt design is that it allows us to use the rod stretch gauge to preload the fasteners. Because the rod bolt is arguably the most critical bolt in the engine, we like being able to use the most accurate method we can for tightening it. By using rod bolt stretch we have found that depending on conditions the torque can vary as much as 10ft lbs from fastener to fastener to achieve the same stretch. Using ARP hardware take slightly more time using the stretch method but we believe the peace of mind and longevity is worth it on the earlier engines. Another benefit of the ARP bolts is that they can be reused in the future instead of replacing them in the future. This is particularly useful on a track or race car that will be serviced frequently."

[eacmen]

Chris Fix posted a video building an M54. They compared stock vs ARP con rod bolts. Two major outcomes:

1. Don’t use ARP torque spec values. You must use stretch gauge.
2. ARP did not oval out the M54 con rods and for their test the ARP was the better fastener for their application.

Findings may not follow to S54 or all applications, but an interesting video nonetheless:

FF to around 17 minutes:

https://youtu.be/KVE322t-oqw

[Shonky]

Chris Fix posted a video building an M54. They compared stock vs ARP con rod bolts. Two major outcomes:

1. Don’t use ARP torque spec values. You must use stretch gauge.

Not quite. At 40 minutes he does final torque of the rod caps to the ARP torque only spec (36 ft lb) and then measures the stretch and says its right on the correct stretch.

[eacmen]

Not quite. At 40 minutes he does final torque of the rod caps to the ARP torque only spec (36 ft lb) and then measures the stretch and says its right on the correct stretch.

I stand corrected. So the guidance would be not to use the lube that ARP supplies?

[Thoglan]

Even the builder was surprised it didn't oval the bore. It's not like the rod bolts are a weak point in these so more clamping force seems unnecessary, unless you have the rods out and can re-hone/measure the bore to be sure. Seems like there is no downside to using OEM bolts in my eyes. I know my engine builder has claimed to have measured the bore oval'ing on one of his S54 racecars with ARP bolts.

[eacmen]

Even the builder was surprised it didn't oval the bore. It's not like the rod bolts are a weak point in these so more clamping force seems unnecessary, unless you have the rods out and can re-hone/measure the bore to be sure. Seems like there is no downside to using OEM bolts in my eyes. I know my engine builder has claimed to have measured the bore oval'ing on one of his S54 racecars with ARP bolts.

I would use ARP on the older con rod design over re-using the OEM bolts as they are not available to buy from BMW.

[digger]

I stand corrected. So the guidance would be not to use the lube that ARP supplies?

No just use the lube but cycle torque them a few times. In fact any critical fastener that is not torque to yield should be cycled to ensure consistency

If you use arp bolts and you have a failure it’s not going to be due to using torque method instead of stretch on these engines it will be user error

[Obioban]

Even the builder was surprised it didn't oval the bore. It's not like the rod bolts are a weak point in these so more clamping force seems unnecessary, unless you have the rods out and can re-hone/measure the bore to be sure. Seems like there is no downside to using OEM bolts in my eyes. I know my engine builder has claimed to have measured the bore oval'ing on one of his S54 racecars with ARP bolts.

The ARP s54 rod bolts also provide less clamping force than the stock bolts 😜

[bigjae46]

Nowadays there's no reason to not have a digital torque wrench with an angle gauge if you're going to do things like rod bearings. Then torque procedure isn't all that hard with the OE bolts.

[tlow98]

I would use ARP on the older con rod design over re-using the OEM bolts as they are not available to buy from BMW.

Yeah I’m tackling this decision now - I have an early car with the m11 bolts.

BMW says to reuse them.

Lang says they have never seen a stock bolt failure.

It’s kind of hard to move to an ARP bolt with slightly unknown results when doing it in-car and not being able to measure the bore distortion.

My OE non torque to yield bolts have held up great for the life of the motor and in a way are known quantities. Makes me want to reuse them. It seems to me that introducing ARP bolts introduces more variables, not less.

2 options:

1 - stay with stock ‘good’ bolts
2 - move to ARP bolts and have an unknown bore distortion

I kind of like my odds on number 1. That’s what BMW would do. This is what they did with all the recall cars and they have been proven to be very reliable.

actually, how can I find out if my rod bearings have been done once already? I don’t have documentation.