Stretch bolt measurement with Carrillo Rods??
 

Hi everyone,

I've just received my Xmas present from me, in the shape of some CP Pistons and Carrillo Rods with Carrs bolts.

I've read up on how to use stretch bolts and how important it is that they're accurately measured for stretch rather than just going straight to the recommended max torque.

Well, I have aproblem.. the thread is in a blind hole in the Carillo Rods, so how can I possibly measure the stretch of each bolt in each rod??

I could easily make up a dummy rod from two blocks of steel and check the bolts that way, but I thought the whole point was measuring them in their final position in each rod?

I figure there must be engine building experts here who can help??

Thanks in anticipation of a million replies, and happy Xmas!!

I'm not an expert, but stretch bolts on rods are once-tightened only, stretch to measure, then if needed to be undone or retightened, you just use a new bolt and tighten and stretch to spec.

Higher grade bolts don't stretch like regular stretch bolts, and can be retightened more than once if I'm not mistaken, so you'd just torque them to spec, even if you have to do it more than one time.

I'd also find out for sure if they recommend using lube or torquing them dry, because that's very important as you'll get very different torques between lubed and dry.

Every bolt stretches. That's how they hold tension. I would call Carillo and ARP and see what they say. Im sure they've mocked up the bolts in a test jig to correlate torque vs. stretch with various lubricants. They probably have a torque spec with their specific lube since you can't get a stretch gauge on it.

One of the many engine builders should weigh in. Maybe one of the engineers that lurk around here.

I know in general for me it's torque + 90 degree rotation.

Hey Mike,


You are right and wrong here.. The stretch bolts you're thinking of are the kind used on cylinder heads. They do indeed stretch once and that's it, (Or the manufacturers have us believe this so we replace them every time we change gaskets). Once a bolt stretches beyond its elastic limit it's all over. You will sometimes feel this when tightening a bolt, it goes all "gooey".


The Carrs bolts I have are specc'd to be stretched .005" to .007" when tightened. They state "Torque not to exceed" 58ftlb. Which is where I have my problem. With these bolts you use stretch to determine correct clamping, not torque. I could go to, say 55ftlbs torque, but may have overstretched the bolt??


Thanks for your input Firestorm and Zooki, I'll wait for someone here to pop in I think. In the meantime I might drop Carrillo a line.


Thanks again guys.

My post was just from my experience with my Porsche engines where the rod bolts are once tightened only, to the stretch measurement.

When I upgraded to aftermarket rod bolts, they could be used multiple times (although I'd probably only use them once or twice again.)

On rods like yours I really don't know how you'd measure stretch.

If they're providing a stretch figure and there's no way to measure it then its a difficult one

torque is a poor method of bolt tension control , as its so dependent on lubricant and surface finish

a low torque figure ( 15ft/lb) to settle the joint then use an angle tighten is far better method