8BA rod help
 

Got the 8BA rods back from the machine shop, and they are saying they measure anywhere from .6.935 to 6.945 center line to centerline from the big end to the small end. In the shortest one, that is .065 less than the stock Ford dimension of 7.000.

Checking them myself, using a 6" vernier dial caliper I get anywhere from 6.990 on the shortest to 6.998 on the longest.

Anyone seen 8BA rods .065 short of the factory spec of 7.000 ?

Any ideas on they discrepancy in measurements between the shops and mine ?

The shop operation is 1st class, with an Italian boring and decking machine that was purchased from Petty Enterprises right after they started sending all their NASCAR engines out to be built. This is the only machine I have seen locally that bores directly off the centerline of the crank versus the usual off the deck. The owner is a perfectionist, as he should be, so I must be missing something in the translation of numbers.

I worked as a machinist 50 years ago, and still have a Bridgeport & lathe so I am not actually a rookie.

Thanks Fordbarners.

Re: 8BA rod help
 

Re: 8BA rod help
 

You and me both. If that's what they really are, they aren't going to work in the flat motor I am building.

Re: 8BA rod help
 

If the rods could talk, I bet they would probably say the big ends have been resized more then once and out of different engines. I doubt that is manufacturing variance.

Re: 8BA rod help
 

I seriously doubt you will get any accurate numbers using a vernier?? Maybe, but I doubt it.

You could always make a quick fixture using your Bridgeport, this would be real easy. It MAY not (depending on the fixture) give you the true center-to-center, but it will easily give you the differences between all 8 rods!

Thanks, Gary in N.Y.

P.S. We have two tools to check these numbers, first is a "dedicated" Tobin-Arp pin machine and the other is an in-house one we made to check the numbers. This one works in conjunction with our Bridgeport! I have a photo handy of that one but not the T-A machine!

Re: 8BA rod help
 

Hi Gary

Thanks for the info and photo. It looks like the big end is on a mandrel or plug, and the quill has a dial indicator on it so by changing the rods you can see if there is difference in the indicator reading.

Re: 8BA rod help
 

DROs are great. It is not uncommon for machinists to shave a cap a bit in order to align bore the big ends. This could shorten a rod a bit.

Re: 8BA rod help
 

Hi Russ, correct, instead of the cutter in the photo you just substitute an indicator in the collet, simple!

This is not exactly true when "boring" the large ends on a machine like our T-A? When you "shrink' the holes by removing material from the caps (this is the only way to do this part of the rod reconditioning on Flatheads) you simply "dial-in" the desired "center-to-center" and the machine will always remove the material from the caps only if necessary!

It is how the machine is designed! Having said that, it can only correct to a certain point! I wouldn't attempt to bring any rods back to the 7.000" C/C that are already .065" too short! You will simply remove too much metal from the caps!

On the T/A you would make the setup from the pin ends and dial in the required 7.000" to the big ends.

Thanks, Gary in N.Y.

P.S. The rod length's CANNOT be altered if you "hone" the big ends to size, typically done on a Sunnen rod reconditioner, they can only be "corrected" on a boring tool. This type machine can take material from where needed, unlike a hone!

Re: 8BA rod help
 

I would find the best possible way to measure, and repeat your measurements. A good vernier should be fine. Just be careful with the math. I'm betting that rod is not really .065" off. Don't let him bore your block off the crank center line!

Re: 8BA rod help
 

Ditto! I had a similar reaction, see below for what happens if you deck it off the crank C-L

Re: 8BA rod help
 

Using
6" calipers for that measurement leaves a lot to desire. Take them back to the shop tell them what you measured and have them measured again to resolve the difference.

R

Re: 8BA rod help
 

Thank for all your ideas. My plan is to take the rods back to the shop, armed with a lot questions. I will ask them to show me how they came up with the OAL that was .065 off the stock dimension of 7.000. There are also numbers on the rods I do not understand. I posted a photo of the big end marked .400 and the small end marked .129 ? Any ideas on that means. The big end ID is 2.286 and the small end .750.

The machine they use bores off the crank centerline not the deck. The crank bearing surfaces are mounted in V-blocks so the bore is square to the crank. I believe that is the best way. While it's in the V-blocks, they skimmed a few thousands off the deck to clean it up also. I would love to video tape how the machine finds center of the bore with a built in dial indicator and digitally reads out where you are at. If I get a chance I will, and post it.

Re: 8BA rod help
 

Did you have it balanced? Looks like weights to me.

Re: 8BA rod help
 

The crank is offset in a flathead block. Don't bore to the crank centerline. You will ruin your block.

Re: 8BA rod help
 

There is more than one centerline to be concerned with when aligning the block for boring. The block most certainly should be positioned so the main housings are perpendicular with the cylinder bores. The best way to do that is by locating on the crank centerline. However, the deck must also be square with the boring bar.

At first I thought the numbers on the rods were weights, as Ross mentioned, but now I wonder if the numbers are millimeters? And, if so, do they relate to calculations for chordal distance??

If you have a Bridgeport you can measure the rods to a very close number with the handle dial calibrations in junction with your BestTest indicator. It will take a little playing to maintain the rod centerline or, if you move in only one direction,(to maintain lead screw contact) you can do chordal calculations as you would with a dimensional qualifying machine and maybe like the machine shop did.

It is my guess there is a misunderstanding of one full decimal and the rods are within .0065. However, the usual condition is that the rods get long rather than short after being used.

Re: 8BA rod help
 

If you checked with in .010 I would go by your measurements.

Re: 8BA rod help
 

The old V8 engine diagrams show the Desaxe offset of the centerline of the crankshaft. Due to this, a person would have to take care not to incorrectly "correct" some percieved deck plane problem. The photo above in post #10 by Ross F1 definitely indicates what can happen.

Re: 8BA rod help
 

I agree, the misalignment can result in what you see in post 10. But, what your seeing, is not necessarily the result of such. The same result often occurs with correct decking proceedures. The later 8BA blocks are vulnerable to that result due to valve angles and amount of the cut.

Thank you JWL for your explanation regarding the boring technicals. Would you comment on the post 10 photo please?

Re: 8BA rod help
 

I checked 8 8ba rods that i bought for my engine from ebay all but 2 had the same code.
The big end is 2 .312 dia and,If you measure with even a scale they are diferent the first one was .030 short.
They were as follows .030,.050, - .020 -.010 + .010 - .030-.030
The finish from factory should be 5.437 for 7"
Big end Not cap end to machine small hole should scale to 5.437
So if they are from the same lot +/- as long as they are about the same length.

Re: 8BA rod help
 

Hi George, could you clear that post up more? Unless I'm reading it wrong, on the 8BA rods the big ends shoud be 2.291" (nominal)??

The bored hole in the pin end should measure .8125", also nominal!

Thanks, Gary in N.Y.

P.S. Am missing the rest of your answer unless your measuring w/o the pin bushing installed!