Connecting rod identification

[cwh]

I need to replace one of the connecting rods in my cabover. It looks like it spun a bearing at some point. The rod in question has 99 R8 cast in one side and A105 on the other side. A few of the rods have the same casting. Other rods read 99A and 99 R9. Can’t seem to find any information on these casting numbers. The truck is 1941 but I’m not sure this was the original engine anymore. Any help would be greatly appreciated.

Chris

[Bored&Stroked]

These are the standard large-journal full-floating rods used on Mercury engines from 1939 - 1942 and on Ford Truck/Industrial engines from the same time period.

The '99A' is associated with Mercury and truck engines between 39 - 42 (and also during the war). The other (more common) part number you can use is for later '29A' full-floater rods. These rods are usually in 1945 to 1948 engines and are readily available.

If you're going to replace one rod, you should consider replacing ALL of them as a balanced set. You should be able to find a NOS set of eBay for a decent price. Also, you should replace ALL of the rod-bearings and make sure you know how to tune/fit the bearings to the rods and crank.

These are called "full-floaters" for a reason - the bearings must float and easily turn inside the rod and on the rod journal.

Is the engine all apart? What does the crank look like?

[cwh]

Thanks for the explanation. That’ll get me back on track.

I dropped the pan and pulled the heads to hone and re-ring the engine. This engine was stuck prior to my ownership. And it came with a set of rings.

The rod journal in question was not damaged. The remainder of the rods bearings and journals are in great condition as well. Only other damage that I’ve found was a handful of broken piston rings.

I have done some reading and watching for new bearing setup. I’m just trying to keep this truck on the road for a bit longer before the engine gets a full rebuild.

Chris

[Bored&Stroked]

Sounds like a plan . . . you probably saw my YouTube video on bearing setup. If you're just trying to keep it on the road a bit, then maybe buy just one rod. Yes, it may be out of balance a bit, but in the short term that will probably be okay. One thing to do is to take a close look at good rods and mic the big-end bore. Then mic the crank journals. Sometimes folks would bore/hone the rods and use oversize bearings in both the OD direction as well as the journal direction.

[tubman]

Wouldn't it be better for him to re-balance the rods to match the lightest one? Small, accurate scales are really cheap these days, and a fixture to allow weighing of each end should be easy to make.

Probably be better than a set of eBay rods which have been through who knows what over the last 80+ years.

[Ronnieroadster]

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Ronnieroadster

[Bored&Stroked]

Originally Posted by tubman

Wouldn't it be better for him to re-balance the rods to match the lightest one? Small, accurate scales are really cheap these days, and a fixture to allow weighing of each end should be easy to make.

Probably be better than a set of eBay rods which have been through who knows what over the last 80+ years.

It is not as easy as you think - you have to balance each end of the rod separately and you have to have a fixture to hold one end and weigh the other and it has to be exactly repeatable each time you take the same rod on/off the scale.

I made a special fixture that has a big roller bearing for the big end, so I could accurately balance the small ends.

This should give you an idea:

2015-02-02 21.29.20.jpg

[tubman]

Originally Posted by Bored&Stroked

It is not as easy as you think - you have to balance each end of the rod separately and you have to have a fixture to hold one end and weigh the other and it has to be exactly repeatable each time you take the same rod on/off the scale.

I made a special fixture that has a big roller bearing for the big end, so I could accurately balance the small ends.

This should give you an idea:

Attachment 578800

I have to believe that a simple "knife edge" fixture would get you close enough that it wouldn't matter on a low rpm street engine. Actually, I would worry about the "big roller bearing" having enough friction to throw a monkey wrench into the process.

[cwh]

Really appreciate all the feedback. I’ve got some good direction now. Sounds like a set of matched rods would be a good investment in my situation. This engine was seized up prior to my purchase almost 15 years ago. It really hadn’t been driven much until this year. I put probably 1200 miles on it this summer. Hopefully these repairs get me another 1200 miles. �� I have some videos on YouTube of the truck. https://youtube.com/@hardinggarage?si=pmSW-Y_jlk737x5o

Chris

[Bored&Stroked]

Originally Posted by tubman

I have to believe that a simple "knife edge" fixture would get you close enough that it wouldn't matter on a low rpm street engine. Actually, I would worry about the "big roller bearing" having enough friction to throw a monkey wrench into the process.

The challenge is that the rod needs to pivot on the exact same fulcrum for there to be a repeatable weight on the other end. I started out with just a round piece of polished bar-stock for the big-end . . . couldn't get repeatable weighs . . . had to keep testing other methods.

Then I use another fixture that holds the small end, such that I can weigh the big end.

When you start screwing around with this stuff you rapidly find out that if you can't repeat the measurement within a gram - you're wasting your time.

[petehoovie]

Originally Posted by Bored&Stroked

It is not as easy as you think - you have to balance each end of the rod separately and you have to have a fixture to hold one end and weigh the other and it has to be exactly repeatable each time you take the same rod on/off the scale.

I made a special fixture that has a big roller bearing for the big end, so I could accurately balance the small ends.

This should give you an idea:

Attachment 578800