Checking Rod Bore with ARP Bolts
 

Hi Everyone,

A while back I had my 3.0L rods rebushed and resized using the old stock bolts. I then later decided to purchase ARP bolts instead of OE replacements. From the reading I have done here it seems the ARPs provide a higher clamping force than the stock bolts so I will have to re-check the bore of the large end using the ARP bolts in place to see if it is within spec. If not I will have to resize using the ARPs.

I would like to confirm the necessary steps so I do not damage the ARPs during the checking process and am able to re-use them for the final assembly:

1. Assemble rod and cap with ARP bolts (no bearings in place).
2. Tighten nuts to ARP recommended stretch (same full stretch as in final install?) per ARP procedure, using ARP stretch gauge.
3. Measure bore/inner diameter of large end with dial bore gauge.
4. If bore/ID is within spec, disassemble and check ARP bolt length to make sure they have not permanently stretched.
5. If bolts length is ok proceed to next assembly step (check bearings, crank, etc.)

6. If bore/ID is out of spec, send rods to get resized again (with fully stretched ARP bolts in place?).
7. Receive resized rods with ARP bolts and check bore/ID with fully stretched bolts in place.
8. If within spec, disassemble and check ARP bolt length to make sure they have not permanently stretched.
9. If bolts length is ok, proceed to next assembly step (check bearings, crank, etc.)

I will be using a freshly calibrated dial bore gauge and brand new ARP stretch gauge.

Thanks in advance.

You have the approach summarized well. I would add that in order to measure your bolts accurately, before stretching and after removal, you should use a micrometer with point style tips. A mic with flats doesn't produce repeatable results and is basically frustrating/pointless (no pun intended!). Plus a mic with point tips is performing the bolt length measurement in the same manner that the bolt stretch is measured with your ARP gauge- points placed in the bolt dimples. You can buy the points separately too. I got a set of points from McMaster-Carr that are held on the mic anvils with rubber "boots."

A few things worth noting in my limited experience. If you did need to resize your rod big ends, I would be sure to ask your machinist to stretch the bolts and not torque them. I say this because i've found that the amount of stretch is quite different when comparing a bolt tightened to the recommended torque and calling it good, vs. actually stretching the bolt to a target amount regardless of torque. Basically I found the bolts were under-stretched when just using the recommended torque amount. This is based on using ARP bolts with their special ARP lubricant on the bolt threads and under the nut flange.

But I don't know if that deficiency in stretch has a negative affect on the bore size of the rod. I do know that you're not preloading the bolts as desired if you're under-stretched.

I looked at the notes I took for the last time I did stretching, which was for bolts going on a 930 3.3L crank assembly. The ARP recommended torque spec is 40 ft-lb. for the M9 bolts. The actual stretch spec for these bolts is 0.0100" to 0.0105" I use a nice quality Precision Instruments (same company that makes Snap On torque wrenches) 10-75 ft-lb clicker style wrench to tighten the nuts. Using the torque wrench I can of course see how much torque it's taking to get to whatever stretch I measure.

The wrench goes in 0.5 lb increments and I usually go up by 2 or 2.5 lbs after working my way up to 40 lbs. I like to work my way up to 40 incrementally, going in increments of 10 on each side (do one side to 10, then other side, back to the first side now up to 20, then the other side to 20, back to the other side now to 30 etc.), to avoid loading only one side of the cap with a high torque amount with the other side completely unloaded. After 40 I go up by a little bit like 2 or 2.5 because I of course want to ensure i'm overcoming the friction to start the nut turning and I don't want to go past my target stretch amount.

Only reason i'm bothering with a measure of torque is because I like to know how much each bolt is taking to get to my stretch, even though I don't really care that much about what the torque actually is. That is, unless something is way off. I just use the torque observation as a way of checking my progress toward the desired stretch amount. But if I had a bolt that had way too much stretch at 40, i'd be concerned I had a bad bolt which has yielded somewhere. Similarly i'd be concerned if I had to turn a nut a bunch to hit a torque increment, or I couldn't get it to click (and stretch wasn't progressing), which would lead me to look for damaged threads.

Anyhow that last set I did ranged from 42.5 ft-lb to 48 ft-lb to achieve the desired stretch on all 12 bolts. Also worth noting that on the same rod I wouldn't always have the same amount of torque put into each bolt. Four of the six rods had different ultimate torques for the bolts. For example, one hit the stretch at 42.5 and the other at 47.5.

I like to get the bolts to have the same amount of stretch because that seems like the obvious symmetric thing to do and bolt stretch is w/out question a better measure of preload. That means I have to pay attention to the difference in length between the two bolts on each rod, since the stretch gauge isn't a direct-reading gauge. My stretch is a comparative measure based on what bolt length I have held as the zeroed unstretched length on the dial indicator.

Obviously you can tell it gets a bit tedious when stretching the bolts vs. just torquing the bolts to 40 and assuming it's good. Unfortunately I didn't make note of what the actual stretch amount was for each bolt at 40. But I do know for certain it was less than the spec'd stretch amount, otherwise I wouldn't have gone past 40! :D I also recall that the bolts which took 48 ft-lbs to hit the desired stretch had a 40 ft-lb stretch amount that was much less than desired.

So my point of all that is stretch is most important so the machinist ought to be doing that when loading your rods and honing them to size. Along those same lines, I always feel concerned when I hear people say they did the rod bolts with the case assembled, which means they just torqued them to 40. They had to go by only torque because you can't fit a stretch gauge inside the case bore!

Kevin,

I agree with you. I read many of the threads here discussing the various issues with torquing instead of stretching and I also have ARP's catalog with technical articles that basically says the same thing. Stretch for a reliable clamping force measurement.

I will look for the pointed mic tips like you recommended.

Regarding the machinist, is it good enough to give them the rods and caps assembled with bolts already stretched to spec? This way they dont have to mess with them. Or will the machinist have to disassemble them anyway to remove material off the flats on the caps and re-assemble themselves?

I would just give them the rods assembled with the bolts installed and snugged to keep everything together. Yep they will need to split them anyway if the big ends need resizing. They machine the parting line on the cap and the rod and then hone the big back to round and bore small end to reestablish the center-to-center length

https://www.youtube.com/watch?v=PE3EdP20gyA

After that they should balance them get the big end weights the same and of course weigh them. It's pretty cool how a rod is balanced. https://www.youtube.com/watch?v=QLpiF0E0EJU

Thanks for the detailed experience Kevin. So the rod bolts get stretched when the rods are reconditioned, then get disassembled, then get assembled to the crank and stretched again?

I expect i would only cycle them those two times.....with any luck. Does ARP recommend replacement after so many cycles?

Tom - plenty of information on this subject, but the bottom line is that if a bolt is not stretched beyond its elastic limit, it can be stretched pretty much as often as you want. Certainly the 10 or so times one might expect to be doing this on a particular set of rods in an engine's lifetime. I believe ARP gives a maximum amount by which a used bolt's untensioned length has increased over what it was before it was installed and used. It is a very small figure - maybe a thousandth of an inch?

The stretch amount for proper tightening of these bolts is well below their elastic limit.

So why not use new stock rod bolts?

Is it beacuse ARP has a higher clamping force?

interestingly enough I just went through some of this.
I had rods that were done and sized fine then half installed and realized the bearings were junk so ordered GT3 bearings and ARP bolts installed to stretch or .09-.10

I installed them all with no issues and called Ted at German Precision and asked him what his thoughts were. He said as long as they are smooth and fall freely they should be good to go.
I was half tempted to tear them back down and take them to him but he assured me they are ok.

Draco, only reason is one of those "while you are in there" reliability upgrades I suppose, not that there was anything wrong with using stock ones. Got GT3 bearings too.

Thanks Kevin and Crash. Time to get to it and see where these bad boys are. Will report back soon.

Thank You Jose,

Yes, I figured that it would be extra insurance. Good enough for me, so for my 3.2 that might go 3.4 what is the ARP part number for the rod bolts?

For some strange reason ARP's website does not list a 3.2 as a selection choice, very strange, only M9 or M10.

TIA,

Jim

P.S. for that matter can someone please confirm the ARP part# for the Head Studs?

3.2s use the M9 rod bolts. 204-6005
Head studs are 204-4206 ('65-'98).

Pretty cheap on amazon.
https://www.amazon.com/ARP-204-6005-Rod-Bolt-Porsche/dp/B004XJR016/
https://www.amazon.com/ARP-204-4206-Head-Porsche-911-930/dp/B0057Y7A48/

Use lots of the ARP assembly lube. I accidentally ruined a rod bolt by fitting one dry and having the threads seize.

Oh thank you Master Graham :D

If you are willing to accept some pro advice, you have missed a couple of really important steps. A competent shop will do this and be sure you know that they know what they are doing.

1. Straight check the rods.
2. Clean the rods.
3. Magnuflux the rods.
4. Clean parting faces with 400 - 600 paper on flat surface.
5. Fit ARP bolts and stretch. You can set the stretch Gage to zero before stretching and then stretch
to proper length. Always use 1 motion and ARP ultra grease on threads and under head.
6. Measure bore diameter. Probably will be longer in the 12 - 6 direction.
7. If they are out of spec, then they need re sizing. Be careful here. The split pin bushings that you buy do not allow for CCL to be adjusted. So dress the beam size with 400 paper only and cut the cap side only. This way the CCL will stay the same.
Otherwise you need custom bushings made.
8. Pin bushing will need honing also. Use factory clearances unless you need more oil film. Max would be 0.002" on a pin that size.
9. Disassemble and really clean well. Fit bearings and measure the bearing ID and calculate your clearances.

Well, for me sending journal and rods out for service and have them install my ARP bolts at Ollie's

We appreciate the advice from pros. So no need to preface your comments with "If you are willing to accept some pro advice" as if somehow people's responses here are telling the professionals to get lost?

With all due respect, your #1, 2, and 3 weren't asked about. it's not like the original poster didn't mention these because he thought they're not worthwhile to do. I think he was simply keeping the question isolated to stretching procedure.

In #5 when you say "always use one motion," are you referring to the movement of the torque wrench to click? I agree with that 100% to ensure the torque is not influenced by overcoming the initial frictional resistance to get the nut rotating. Otherwise I don't see how the singular motion of the tool is critical. As long as you impart enough torque, even in multiple applications, to stretch the bolt to the desired stretch, it's been stretched accordingly, no?

Thanks for the clarification in #7 on the treatment of the big end. I follow what you're saying in that just brushing the beam side with paper maintains that center point and then the cap gets cut to re-establish the bore, which of course is then honed to size.

Agree 100% that one should fully clean the rods when they come back from machining. You can find that machining grit is left over from honing the bores. Most notably, clean the exposed threads above the nuts before disassembling so you don't run the nuts over the grit as you remove them.

I posted all of the expected procedures in case some are not aware of all. Was not inferring these were forgotten.

As for the single motion, again, just good assembly practices so that no out side influences occur.

As for using paper to "brush" the beam side of the parting line, care has to be taken to ensure the parting face is flat and without inclusions. If any are then they all have to be dressed/cut and any loss of CCL has to be accepted. In a street engine this probably will make little to no difference. Will only increase Piston clearances. As for lowering the static CR, it will but by how much would have to be calculated. What I have seen in the past, the CR is often compromised when the valve seats are cut really deep.

Thanks for the advice so far.

Is the center point of the big end coincident with the flat mating faces of the beam side of the large end?

May not be, but hopefully, but they need to be checked. The only way to know for sure is to check. Changing the CCL is not a huge deal with a street engine and in a lot of cases where the beam and cap are cut, the CCL is probably never checked. To rebuild the rods correctly, the CCL should be held to spec. If both parts of the rod are cut the CCL should be adjusted by boring the pin bushing off set to hold the CCL. However, the split bushings sold will not allow this unless it is a very small amount. Bushings with more wall thickness are required.

Typically, the amount removed from the beam will be very small, maybe a few thousands of an inch, and the rod length will change this amount. But it is important to check each rod before and after rebuilding, so the amount removed will net the same length rods in the set. Dressing the beam side with paper is Ok as long as the parting faces are flat, and parallel to the pin end. Once dressed the bore radius should be checked to ensure they are all the same.

This all may sound like a lot of work, but this is the correct way to rebuild the rods. This will make the piston deck heights all the same, if the cylinder heights are the same, lower the chance of any cylinder detonating in the worse case, and make all of the cylinders have the same CR number. It starts with good machining practices.

$ 20.00 per rod rebuilding costs do not check this and they hope you don't either.

Awesome. Yep checking the rod length was the last item on my list before I do final assembly on the crank. I will take care of that this week.

I have finished checking the rod bore with stretched ARP bolts, bolt length before and after stretching, crank journal check, as well as the clearance check on the crank with plastigauge, which were all thankfully ok. I have everything documented for future reference.

To m42racer's point about same CR number in each cylinder, also worth noting are the cylinder heads themselves. The heads as cast from the factory are not really consistent in terms of chamber volume. So what you end up doing, from an economical/simple standpoint, is taking the head that has the greatest volume and using that for your baseline. The other heads can be massaged around the spark plug port (ports if twin plugged) to increase their volumes to match the baseline head.

Checking the rod length itself is a little tedious for the average DIY guy. You need to bore gauge the two bores, take half of them, then measure the rod length from "apex" to apex with something like a 0.001" caliper. Add that length to your two half bores and you'll get your center to center length. Not sure how accurate that is, but that's how i've done it and got reasonably repeatable measurements.