Rebuilding Model T connecting rods

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Model T Ford Forum: Forum 2016: Rebuilding Model T connecting rods
Top of pagePrevious messageNext messageBottom of page Link to this message  By Sam Romeo on Wednesday, September 07, 2016 - 06:22 pm:

I need some information on rebuilding Model T rods. With the advise of a friend and watching Mike Benders video's, I learned how to babbitt the block and caps. Another friend turned me loose with his Berco Line Boring Machine and now I have a nice looking block with no holidays in the babbitt and a crank that turns freely with .004 thrust. I fudged and modified the rear main saddle to accept a babbitted thrust surface like the rear main cap. It came out nice but I'm not sure it was worth the extra work (it was a pain in the *** setting the thrust). Now I'm stuck on the rods. I have 6 or 7 rods and maybe 9 or 10 caps to play with (none were matched sets). The mating surfaces were very uneven from filing and need to be ground flat before babbitting. Now a new problem jumped out at me. I need a machined surface on each side of the mated cap and rod to clamp the part into a cap grinder. It also needs to be square to the bore of the mismatched rod.I made a fixture to set my rod length and another one to center the freshly babbitted rod and cap in the mill. Can anyone tell me what the correct bore of an unbabbitted rod and cap should be? Also should the big end be resized with or without the shims in place?
I've bored several rods both ways so far and in several cases, it required taking a lot more from either the cap or the rod to get everything round. I know that the finished rod with babbitt is all that counts, but it seems that if I don't get this right to start with, I'm might machine thru the babbitt into the cap (I know the rod length is 7.000 inches, so the cap is the only part in danger). Before anyone asked, yes I did weigh the rods and caps and will have everything balanced before final assembly.
This has been a lot of fun learning about the old engines and how they were rebuilt. After this I have a 1913 Buick engine that will be next in line.

Thanks
Sam


Top of pagePrevious messageNext messageBottom of page Link to this message  By Trent Boggess on Wednesday, September 07, 2016 - 09:00 pm:

Sam,

I rebabbitted the rods in one of my cars myself, but I had an expert Model T engine builder (He has won the Montana 500 with his engines) by my elbow during the entire process.

Originally, as they came from Ford, Model T connecting rods had no shims in them. I used a laminated rod shim pack under each side when I bored the rods to size on a mill. It was cheap insurance if the rods still needed special fitting after being bored to size.

There is one thing shown on the factory drawing for the connecting rod that most people choose not to do for some reason. The rods had oil grooves broached into them at the points where the rod and caps joined. Also, the Babbitt was cut back on the collars of the Babbitt at these points to allow oil in to the broached grooves from from the sides. To me, this appears to be part of how oil would get to the interior bearing surface of the rod.

There are many people who do rod rebabbitting professionally who read this forum and comment frequently. They will probably have more to say about this. What I can say is that the rods I rebabbitted are still in service 7 years later, and that they still have all their shims in place.

Respectfully submitted,

Trent Boggess


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Wednesday, September 07, 2016 - 11:06 pm:

Why not lap the caps flat with emery cloth on a surface plate?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Sam Romeo on Thursday, September 08, 2016 - 10:06 am:

Trent - thanks for the info on using the shims. Did you bore each rod to a predetermined size or just until they were round?
Ted - I do have a surface plate but lapping the rods and caps on this wouldn't keep them square to the rod bolts / big end bore or parallel to the wrist pin. I realize the babbitted surface is what really counts, but not doing this spot on goes against my nature.
Thanks
Sam


Top of pagePrevious messageNext messageBottom of page Link to this message  By Trent Boggess on Thursday, September 08, 2016 - 02:53 pm:

Hi Sam,

The process began by selecting which rods and caps to rebabbitt. I started with a large pile of candidate rods and caps, and melted the Babbitt out of all of them. I then started weighing the rods and caps separately. I ended up with 5 rods and caps that each weighed about the same number of grams. I did this because I wanted the finished rods to be as close in final weight as possible. In the weighing process, the rods and caps were "mated", meaning that during the rest of the process that cap and rod would remain together.

Each rod and cap was thoroughly cleaned, and tinned. It takes some pretty potent soldering salts or liquid flux to get new Babbitt tinned to the bearing areas on the rods and caps.

The caps and rods were poured separately using one of Gene French's connecting rod molds. After knocking off the sprues, I mated the rods and caps beck together again, using a pack of rod shims on each side and a special set of rod bolts and nuts that were made to fit the connecting rod boring jig that was used to hold the rod and cap while boring on a Bridgeport mill table.

The crankshaft I used was an EE that had been magnifluxed and ground down on the mains and rods to .005 undersized. The rods were bored out to a finish size by multiple passes in the mill. We snuck up on the final size, rather than try to do it in one or two passes, which might have accidently resulted in boring the big end too big. The final size was about .0015 larger than the crank pin. Note that I selected 5 rods and caps, and poured 5 rods, and bored 5 rods. The 5th rod was "just in case". I didn't need it in the rebuild, but have kept it against ever having to replace a rod.

After boring I went to work with a round file to create the oil grooves I mentioned before. Then the rods were straightened. You would think that with the detail put into this process the rods would already be straight, but for some reason they don't all turn out that way, so they were straightened on a connecting rod jig.

Finally, the pretty much finished rods were weighed on the scale, both the big and little ends. There will be variations in the weights no matter how hard you try to keep them of uniform weight, so you weigh them, and grind away a bit on the heavier rods to bring them with in 5 grams of the lightest rod.

It was a lot of work, and wasn't done in an afternoon, but I ended up with 4 rods that were almost identical in weight. Since being installed 7 years ago, they have performed well, the car frequently is run down the road at 52 mph.

I also weighed and balance the pistons. Even a set of four brand new pistons can come through with weights that vary 10 to 20 grams.

As much as people complain about the cost of rebbabitted rods today, one thing I learned from my experience was given the amount of work I put into the rods, newly rebbabitted rods are a bargain. In fact, I am amazed that they sell them as inexpensively as they do.

Respectfully submitted,

Trent Boggess


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kohnke Rebabbitting, Clare, Iowa on Thursday, September 08, 2016 - 08:38 pm:

Agree, Trent.


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