View Full Version : Connecting Rod Bolts

11-16-2008, 08:54 PM
The book has two different torque figures for these depending on whether they are color-dyed or phosphate. I am going to post a picture of my bolts in hopes that someone can definitively tell me which I have.

11-17-2008, 12:43 AM
Ok, here are a couple of pics of one of the rod bolts. Can some one tell me if these are "color dyed" or "phosphate?" They are out of a stock 1979 Spitfire 1500 motor, if that helps. The book gives two different torque amounts depending on which type bolt it is, but no info on how to tell which is which.

11-17-2008, 12:38 PM
Basil hello;..Those standard factory bolts were the highest standard OEM
bolts for the time & black oxide coated.
I recommend you replace all those bolts with GRADE 8740 CHROME ALLOY MOLY BOLTS.
If they are original they are old & not trustworthy.
The bolt size you need is;3/8x1.65UNF AT 47FT# TORQUE
Good Luck

11-17-2008, 02:53 PM
I agree with replacing the rod bolts with better parts. Look into the ARP components.

11-17-2008, 03:13 PM
To answer the original question, those look like black phosphate (aka parkerized) to me.

"Color dyed" typically means zinc, cadmium or dichromium plating. How different is the torque spec? I'd guess about 55 lb/ft?. Does the spec call for oiled or dry threads?

Those are well engineered bolts, the shoulders are raised and radiused, and there's an integral washer under the head and the caps are dimpled. Can't tell if the threads are cut or rolled though. The real question is how many times they have been torqued.

Trevor Jessie
11-17-2008, 06:49 PM
If he switches con-rod bolts, then it would seem advisable to recondition the big ends since they may torque up differently. I dunno, I'm not a machinist nor an engine builder.

11-17-2008, 07:01 PM

That is not unreasonable to suggest. The question is whether or not the rods are still apart from the pistons or if Basil had them reconditioned prior to posing this question.

But if he did, I would assume that the machine shop would have given him new bolts to put it together, or at the least suggested that he get them.

Trevor Jessie
11-18-2008, 08:49 AM
My point was... I'm not sure if I'd take the advice of simply switching the rod bolts for stronger ones without consulting a machinist or engine builder. I would go back with the bolts that were there.

11-18-2008, 09:19 PM
Possibly, but not without checking for stretch.

11-19-2008, 10:53 AM
just had to stick my nose in this one. Change the bolts, period. If you are going to spin this motor more than stock revs or you are a "hard" driver and you are going to keep this car, either magnaflux(do this anyways) and re-machine the rods or put in new, aftermarket steel rods.
One rod failure will ruin the entire engine. Most stressed item in the entire engine-rod bolts, then rods.
Arp bolts-yes. ARP 2000 bolts, better (but if stock engine, not worth it).

let's see, new engine $5000 +. Rod bolts look cheap now don't they.

11-19-2008, 11:03 AM
one other point.
Never over torque the bolts unles you have the rods sent to a machine shop to have them machined with the new torque spec. same goes for the mains on the crank. you would need to line bore the block with the new bolts and torque. It is the distortion created by the additional torque that has to be machined out.

11-19-2008, 12:34 PM
more again, (will i ever shut up?)
ARP bolts will weigh less than the stock bolts. This is good, but if you are picky, you should re-balance your crank, any time weight is changed (pistons,rods,pins,bolts) you should re-balance.

11-19-2008, 12:59 PM
When I purchased my TR6, the first thing I did was remove the pan and check the bearings and journals conditions. Satisfied with the inspection, I began to reassemble the lower end. The first pair of rod bolts did not torque up. What the heck? I pull them out and checked the length. My goodness. They had stretched. I threw them out and bought another set of stock one. Everything went back together nicely. I have since replaced the motor.

However, the new motor received ARP rod bolts. I wanted no Tom Foolery with the new motor. I knew that I was going to drive this motor harder so I made preparations for that.


New ARP rod bolts. Priceless.