Resize Rods with New ARP Bolts

[porschedude996]

I purchased a set of ARP Rod Bolts and in the package on a small slip of paper, was the statement to resize connecting rod when changing rod bolts. I can understand the reasoning, but what are some thoughts on the application?

[Neil Harvey]

Yes, its very important. Not sure the condition of your rods but here is what I suggest.

Tools required will be bore gauge, micrometer, wet and dry paper, Bolt stretch gage/micrometer.

Remove the old bolts. If no chamfer is present at the parting line, now is the time to add one. Small chamfer at the parting line and the bore on each side of the cap and beam. This is done so you don't scrap the back side of the bearing shell when installing into the cap and beam. 0.020" x 45 is all you need. Just to remove that sharp edge.

Lap the cap and beam parting faces on some wet and dry paper. 400g is good. Just to make sure the parting faces are clean. If you do this and the faces show low spots, you will need both parts dressed and now you are into a full BE rebuild. If the faces are good, clean and fit the new bolts.

Torque the nuts to get the stretch ARP suggest with their grease. Now measure the BE bore. It may have changed some as new ARP bolts typically will add more clamping load than old factory bolts. If you measure the BE bore and its within spec, you are good to go. If not, you are into rod recon work.

Some additional info that may be helpful. Often with rods that have been rebuilt before, the parting line is not in the center of the BE bore. This changes the rod weights end for end. The CCL is not effected by this if the pin end was bored on the CCL length. But typically it never is ands you can end up with rods at different lengths.

[mepstein]

I just send the bolts, rods and crank to our machine shop and they come back all fitted up.
And credit to Neil’s description of what to do. It reminded me why we don’t do it in house.

[michaelmount123]

I agree that this is a machine shop operation and not for the home workshop. Torquing to the stretch method is always best and if the result is oversize, undersize or out of round big ends, what do you do? Only a proper engine machine shop will have the equipment to properly cut the rod cap, then hone to big ends to size. They can also adjust rod oil clearance to spec. It's not an expensive operation and it's one you want done correctly.

[porschedude996]

Well here’s great news for me. I bought a bore gauge and a ARP Bolt Stretch dial indicator and measured both the minimum stated torque value, and the the stretched value. The bore was concentric as measured three places. One less thing to worry about, and spend money.

[icarp]

Is your Bore Gage a Sunnen .0001" graduations , one rotation of the dial = .0010"
It is essential to be able to accurately measure the tenth of one thousandth of an inch with regards
to rod end sizing, as well as other parts in the engine .

send the rods out unless you have the best tools for the job

A new Sunnen gage is about $3,000.00

Ian

[Zuffenwerker]

You don’t need a $3,000 bore gauge to accurately measure .0001”. The sunnen bore gauge is the best as well as the bore gauge that is attached to the hone. But you can use a regular "peacock" style bore gauge with a good indicator….you must have measuring skills.

The rods can be a few tenths out of round anything over that they need to be reconditioned.

[zztot]

So I am not the guy that will be doing this on my build but I am trying to understand but I’m missing something in the explanation. First my understanding is rod bolts are used once ……as once the torquing is done …….the bolts have been stretched. ( perhaps this is a misunderstanding).
But how do you fit by torquing then measuring which seems as though they would need to be taken apart to measure.
Sorry for dumb question but trying to have everything ready (parts and tools) so it can be done.
Thanks

[targa72e]

ARP (as well as many 911 rod bolts) are not permanently stretched when torqued to spec. You can verify this by measuring bolt before torquing. Then torque to spec and undo, if bolt is same length it did not permanently stretch. ARP bolts come with a recommended stretch length vs just a torque spec.

john

[zztot]

Makes sense…….now I get it!

[porschedude996]

Originally Posted by Zuffenwerker

You don’t need a $3,000 bore gauge to accurately measure .0001”. The sunnen bore gauge is the best as well as the bore gauge that is attached to the hone. But you can use a regular "peacock" style bore gauge with a good indicator….you must have measuring skills.

The rods can be a few tenths out of round anything over that they need to be reconditioned.

I agree. I can read .00025 on a .001” increments. But if it is dead nuts on and dial is zeroed, and repeatable, then it round.

[PeteKz]

From what I've measured, the OE rod bolts on SC engines don't stretch either. I don't know about other engines. I don't see a good engineering reason to replace SC rod bolts that are not stretched, however, the factory says to replace them anyway. Maybe they are just staying well on the side of safety margin. Maybe "just because" that's the way Porsche does it.

If I was on a tight DIY budget, I would measure the original rod bolt lengths, retorque the rods with them, check for BE roundness, then remove the OE bolts and remeasure them. If they are the same lengths as before, they have not stretched. Reuse them.

If you are building a race engine, forget about "budget". Just buy the ARP bolts and reuse them multiple times, because you will have the engine apart many times.

[porschedude996]

Originally Posted by PeteKz

From what I've measured, the OE rod bolts on SC engines don't stretch either. I don't know about other engines. I don't see a good engineering reason to replace SC rod bolts that are not stretched, however, the factory says to replace them anyway. Maybe they are just staying well on the side of safety margin. Maybe "just because" that's the way Porsche does it.

If I was on a tight DIY budget, I would measure the original rod bolt lengths, retorque the rods with them, check for BE roundness, then remove the OE bolts and remeasure them. If they are the same lengths as before, they have not stretched. Reuse them.

If you are building a race engine, forget about "budget". Just buy the ARP bolts and reuse them multiple times, because you will have the engine apart many times.

Yes, I agree, but i’m wasn’t sure if the bolts were stretched from the factory torqued. I retired the little guys. Maybe make some steam-punk art.

I like you caption below your name. The last car I had was a Ferrari 360. Rarely drove it because I didn’t want anything to happen to it. It was Garage Art, just not on the wall.

[mikedsilva]

Originally Posted by icarp

Is your Bore Gage a Sunnen .0001" graduations , one rotation of the dial = .0010"
It is essential to be able to accurately measure the tenth of one thousandth of an inch with regards
to rod end sizing, as well as other parts in the engine .

send the rods out unless you have the best tools for the job

A new Sunnen gage is about $3,000.00

Ian

I wonder if all machine shops use these $3k sunnen bore gauges and use them only in a temp control room after the parts to be measured have reached the correct temp.

I doubt it.

[PeteKz]

Originally Posted by porschedude996

Yes, I agree, but i’m wasn’t sure if the bolts were stretched from the factory torqued. I retired the little guys. Maybe make some steam-punk art.

I like you caption below your name. The last car I had was a Ferrari 360. Rarely drove it because I didn’t want anything to happen to it. It was Garage Art, just not on the wall.

Heh, heh. My other fun car is a Ford GT. I have 80,000+ miles on it. It ain't garage art either.

[Neil Harvey]

Originally Posted by mikedsilva

I wonder if all machine shops use these $3k sunnen bore gauges and use them only in a temp control room after the parts to be measured have reached the correct temp.

I doubt it.

There is always the user tolerance to be considered. The most accurate tools can give the errors if not used correctly.

How do you set the gage to the number you need. Is it repeatable. We use setting rings we made years ago and these are used with the utmost care, not to be damaged in use.

Setting bore gages with mics is a "feel" thing that has chances of adding in errors. Measuring down in the tenths is not as easy as it may appear.

If you measure anything and it gives no issue later, can you consider this to be good to go? I would think so.

[cgarr]

I use master gauge rings to set my Sunnen AG-300 then there is no error. rods are honed 2 tenths oversize from minimum spec..