Do my connecting rods need reconditioning?

[jeremy3s]

I'm doing a rebuild on a spun rod bearing tt engine. This is my first rebuild on the 3s platform and, after reading about all the spun bearing rebuilds, I'm taking every necessary precaution to do a proper rebuild. Having said that, I don't want to pay for work that isn't necessary.

I'm using the '91 FSM as my guide. When it comes to checking the connecting rods, it leaves me wanting more info, but I may be overthinking the issue. I brought my block in to have it bored .020" over after thoroughly checking the bores and finding them to be out of spec. While I was talking to the machinist, he recommended having the big end of my rods reconditioned for another $100. While that isn't an unreasonable amount, he really didn't give me a satisfying reason as to why they should be redone. In any case, his words have me thinking hard about the issue and have caused me worry. That worry can only be fixed by either blindly having the rods redone or understanding how to properly check the big end of the rods and making a decision on reconditioning based on that correct understanding. I prefer the latter choice.

The machinist mentioned that even though only one rod was knocking, the knocking probably caused distortion in the other rods. The only related info about the issue I found in the FSM was how to check bearing clearance. I included a pic from the FSM that shows where to take the rod bearing measurements. Is the whole matter as simple as taking those measurements, comparing them to the rod journal, and then it's good to go as long as the measurements are within the bearing clearance numbers regardless of out of round and taper?

6745

I've been searching the web hi and lo to find out the "proper" way to do this. I found one engine building book that listed .0005" as the max out of round. None of my rod big ends have this much out of round. I couldn't find any info on taper. The most taper I measured was .0001". I checked the King Bearing website to find the acceptable range for the bearing housing (big end) and they all fit within that range. When checking measurements where the rod and cap meet, I found what seemed like an "abrupt" transition of as much as .0002" from one side of the mating surface to the other on a couple of the rods.

My gut says these are probably just fine, but I'm hoping someone with experience in this could give me some guidance.

One other thing I wanted to mention was that I didn't find anything in the FSM about torquing the rod bolts with any type of lube. I torqued them dry to check them. Is this the correct way?

[Chris@Rvengeperformance]

Yes, have them reconditioned. The conditions in the engine that caused one bearing to spin probably damaged the other rods as well, eventually they'd all spin. This is not a place to try to save $100 on.

If it were me I'd spend the 500 for aftermarket h-beam rods, like BC, 3sx, ninja, etc.

Sent from my RM-845_nam_vzw_100 using Tapatalk

[stealthee]

I'm with Chris. If I were rebuilding a spun bearing engine I would just replace all the rods.

[jeremy3s]

Thanks for your replies.

I do understand that getting the rods reconditioned/replaced is an easy way to ensure the rods are good to go.

What I am looking for though is some numbers that represent an acceptable tolerance of distortion. There must be some measurement that can be taken and some tolerances that could be referenced that a person could put their trust in.

I am confident the problem that caused the spun bearing was oil starvation. When I bought the car, I picked it up from the shop the owner left it at. He brought it in for an oil leak. When the shop was done with it, it had a spun bearing. The dipstick tube was loose at the block and there was oil all over the front of the motor. The car is in very nice bone stock condition otherwise and looks like it has been cared for over the years. The evidence suggests that the car saw little, if any, driving at it's limits.

I am in no way opposed to having the rods reconditioned, but I really want to understand why - in a quantifiable way - before I have it done.

[Keyan]

I have ebay rods for sale if you want them. One needs a new alignment dowel installed but I will sell them for cheapppppppppppp if you want it

[stealthee]

Look at it this way. Don't do it and spin another bearing you are out a lot more money than doing it the RIGHT way now.

[jeremy3s]

How exactly does one determine the right way? If I pay a machinist to have them resized, he is going to hone them to a certain known specification - within a tolerance. My point being, there is an acceptable known tolerance.

I bought a set of 6 rods/pistons from a local that were in good condition with 50k on them to replace my rods. The only problem was he removed the rod bolts from one of them so I couldn't use the whole set. I went through and started checking the bores in the 5 good rods from my engine to find the best one to use as the 6th rod. I started checking mine against the set that I bought only to find the tolerances in those that came from my engine were better than the set I bought.

Having said that, would you say rods coming from a good running engine, with 50k on them, should automatically be reconditioned/replaced?

My concern is that I'll have them redone and then I'll retorque them and the specs will be no better than they are now.

I've spent countless hours understanding each purchase I've made on this build to ensure I'm only spending as much money as absolutely necessary. $100 may be a small price to pay for one item, but on a rebuild, excess costs add up fast. Everything I've purchased is either direct oem or oem equivalent quality parts so I am not looking to cut corners...

[CoopKill]

I guess I don't understand your issue. Have you asked the machinist the tolerances he plans to use? Somewhat like line boring the crank, the rods need machined to be sure the opening is not eggshaped. It is not outlandish to assume deformation due to the pounding that ensues with the slop of a spun bearing.

Do you have all of the manuals.

[JasonY]

the stock forged rods fatigue with mileage and age, really high mileage cars break rods bone stock. Replace them with new. Even though the wrist pin bushings are of mediocre quality on the cheaper rods, they are fine for 500hp cars.

Jason

[jeremy3s]

I guess I don't understand your issue. Have you asked the machinist the tolerances he plans to use? Somewhat like line boring the crank, the rods need machined to be sure the opening is not eggshaped. It is not outlandish to assume deformation due to the pounding that ensues with the slop of a spun bearing.

Do you have all of the manuals.

I do have all the manuals. Basically I am just trying to figure out what specs are necessary to determine whether the rod needs machining or not. The fsm is clear about specs for cylinder bore, crankshaft, and heads, but not so clear about rod big end distortion.

[jeremy3s]

the stock forged rods fatigue with mileage and age, really high mileage cars break rods bone stock. Replace them with new. Even though the wrist pin bushings are of mediocre quality on the cheaper rods, they are fine for 500hp cars.

Jason

Broken connecting rods aren't something I've seen any discussion about...

[JasonY]

Because few people keep the stock rods when rebuilding an engine these days since "pretty good" alternatives are readily available and pretty damn cheap and way better than stock, especially high mileage and old stock rods.

Jason

[jeremy3s]

I just emailed the guy that wrote this - http://kingbearings.com/files/Engine_Bearing_Failures_and_How_to_Avoid_Them.pdf. If he can't give me a definitive answer, I'm probably asking the wrong questions.

[jeremy3s]

Case Closed.

From: jeremyswanson@comcast.net
Subject: Question to Dr. Dmitri
Message Body:
Hi. I am wondering what an acceptable amount of distortion is on a connecting rod big end. I am rebuilding a Mitsubishi 6g72 turbo engine that spun a bearing due to oil starvation. The machinist I’m having bore the cylinders tells me I need to have all the rod big ends resized due to the knocking from the spun bearing distorting the rest of the rod big ends. While he may be right, I’m not one to spend money without understanding that there is indeed a problem. My service manual includes specifications for twist, bend, and thrust clearance, but the only spec it lists for the big end is oil clearance. Is there an allowable tolerance for big end out of round and taper for the King Bearings? The maximum out of round I measured was .0003” and taper was .0001”. Those tolerances seem insignificant to me, but I have no experience with this subject so I’ll defer to your opinion. Thanks!

From: Ron Sledge [mailto:askdrdmitri@kingbearings.com]
Sent: Monday, May 12, 2014 1:34 PM
To: jeremyswanson@comcast.net
Subject: RE: Question to Dr. Dmitri from <jeremyswanson@comcast.net>

The rod that had the spun bearing should NOT affect the other connecting big end/rod bearings. Your measurement of .0003 out of round and .0001 taper are within the allowable tolerance spec. The cause of the spun bearing more than likely was not caused by the dimensions you reported. It is important however that the big end diameter be the right size in order for the bearing to have sufficient crush to keep it from spinning. What is it’s size?
Your out of round and taper #’s are fine.

From: Jeremy Swanson [mailto:jeremyswanson@comcast.net]
Sent: Monday, May 12, 2014 2:52 PM
To: 'Ron Sledge'
Subject: RE: Question to Dr. Dmitri from <jeremyswanson@comcast.net>

The sizes of the big ends range from about 2.0867 - 2.0871". For future reference, what are the acceptable maximum limits for out of round and taper?

From: Ron Sledge [mailto:askdrdmitri@kingbearings.com]
Sent: Monday, May 12, 2014 2:14 PM
To: 'Jeremy Swanson'
Subject: RE: Question to Dr. Dmitri from <jeremyswanson@comcast.net>

Acceptable out of round spec is .0000 to .0005.

[CoopKill]

Now we getting somewhere. I wonder what Merkel accepts.

[GTwizard]

Sending the rods out to machine shop does not mean all will be cut and re-sized.
The machinist will inspect for not only size, but squareness, twist, Fracturing and other things that could be detrimental to your build.
Some of your rods may not need reconditioning while others may. Even extreme high dollar rods should be checked and reconditioned when rebuilding.
It also depends on the intended performance levels you will be looking for. But even for a stock rebuild, I would not go with out the machinist inspection and recommendations.
He could vary well check them out and recommend not reworking them, but better replacing them. If you have a machinist you trust, have him make the call.

[jeremy3s]

I've checked dimensions, like length and twist, and haven't found any evidence of distortion. The big ends were the last unknown for me and now that issue is settled. I forgot about cracks though so I might have to buy some penetrant dye.

The bearings wore evenly in the rods before, besides the one that spun of course. I don't have the capability to check them as a machinist does, but I haven't seen ANY evidence to suggest there is a problem.

It is certainly possible I'm overlooking something, but I've done enough checking now that I'm comfortable using the rods without bringing them to a machinist.