116E rods in Lotus Twin Cam

[garrathj]

Am I interpreting correctly and are you saying the current piston wrist pins you have are 20.40mm versus the new rods small end at 20.62mm?

cheers
Rohan

Thanks Rohan,
No, the pins (old and new), old rods and both sets of pistons are 20.62, it's only the new rods that are 20.40 so as you originally said they need reaming, just annoying since they don't match the spec they provide.

Cheers

[rgh0]

Not that unusual to get rods that need final reaming especially if inexpensive (i.e. cheap) like the Maxspeeding ones, though they should make this clear in their advertising specifications .

Most specialist engine machining places will be able to do what's needed to ream match the wrist pins at 20.62mm. I would give them a sample wrist pin to ensure they match it correctly.

cheers
Rohan

[garrathj]

Not that unusual to get rods that need final reaming especially if inexpensive (i.e. cheap) like the Maxspeeding ones, though they should make this clear in their advertising specifications .

Most specialist engine machining places will be able to do what's needed to ream match the wrist pins at 20.62mm. I would give them a sample wrist pin to ensure they match it correctly.

cheers
Rohan

Yes, agreed, just frustrating since I need to get it back to my shop to get the extra work done and can't start my dummy build.

Definitely will take a pin for them.

Cheers

[2cams70]

Piston pin to small end clearance is the most tightly toleranced dimension in the whole engine. The small end bushes should be reamed and selectively fitted to each pin. Ford originally had piston pins graded in 0.0001" diameter increments.

[garrathj]

Piston pin to small end clearance is the most tightly toleranced dimension in the whole engine. The small end bushes should be reamed and selectively fitted to each pin. Ford originally had piston pins graded in 0.0001" diameter increments.

OK, thanks for that.

[Quart Meg Miles]

I am currently working on a final report for my engineering materials science course at college that examines the 116E connecting rod assembly more closely and (hopefully) numerically defines what the weak link is, and to what engine speed they are safe to. I'll hopefully have it finished and publish it here at the end of the term (in about 6 weeks). Stay tuned.
-Ben

Still listening Ben, how was it for you?

[benymazz]

I am currently working on a final report for my engineering materials science course at college that examines the 116E connecting rod assembly more closely and (hopefully) numerically defines what the weak link is, and to what engine speed they are safe to. I'll hopefully have it finished and publish it here at the end of the term (in about 6 weeks). Stay tuned.
-Ben

Still listening Ben, how was it for you?

I hate to disappoint but I came up short handed. There really are too many variables to come up with an exact speed they are safe to. Maybe if I got into it a little bit more and did simulations I could come up with an answer I was confident in but I'm not sure.

So, while I can't give an exact safe RPM, based on my research here's what I can confidently say:
Based on numerous documented failures, the bolts of the 116E rods are the weak point. There are essentially two likely ways for the bolts to cause a rod failure and although the causes are different the end result is the same

1. Engine over-revved, endurance limit of bolts exceeded. In this scenario, the engine is run at an RPM where the stress in the connecting rod bolts exceeds the endurance limit of the material they are made from. This causes them to weaken and makes them likely to fail at a later date at a lower stress level (possibly below redline).
2. Engine over-revved, clamping load of bolts insufficient In this scenario, the bolts are physically not rigid enough to keep the bearing cap clamped firmly to the rods at high RPMs. Eventually the bolts loosen and a failure occurs.

Now, let's compare. Paying attention to only the bolts, what are the differences between the 116E and 125E rods? 116E rods have a 11/32dia thread, while 125E rods have a 3/8dia thread. I also found that stock 125E rod bolts were significantly harder than stock 116E rod bolts (36 HRC vs 29 HRC). The workshop manual specification for the tightening torques is also hugely different (20-25 lbft vs 40-45lbft). The latter two differences translate to the clamping force on 125E rods being much higher (the endurance limit is increased as well but not to the extent that the clamping load is increased).

The astute observer will also note that 116E rods have the square "tab washers" under their heads whose only logical purpose is to prevent the bolts from loosening. As these washers are made from a softer material than the bolt and rod I suppose it is possible that they made the problem worse by flattening out under cyclic loads and reducing the clamping force even further, but to be completely honest I'm not sure if this effect is significant or not.

Based on the above information, one could make the conjecture that the change to and/or design of the 125E rods was made after observing a pattern of 116E rod bolts loosening... or one could chock it all up to confirmation bias...

So, to summarize: Are the 116E rods (not counting the bolts) weaker than 125E rods? Fundamentally, yes. The 125E rods are clearly thicker in just about every area. But based on what I've found it's my belief that the 116E rods fail due to bolt-related failures, not failures of the main "beam" of the rod itself. With better bolts tightened to produce a sufficient clamping load, I think that 116E rods would be just fine for a stock spec street engine.

Criticism welcome,
-Ben

[rgh0]

Hi Ben
This looks like a good analysis. If I was using 116E rods I would certainly use new ARP bolts and not the original bolts which are pretty poor quality for a rod bolt at that hardness.

Whether a washer is required under the bolt heads depends on the bolt head radius versus any radius on the rod cap itself. A modern rod bolt should never need a locking tab style washer given its torque and load requirements so it is strange that Ford used it ???.

You certainly don't want to use a soft tab washer under a high tensile bolt, especially with cyclic loading like a rod bolt. The bolt stretch on the original bolts will only be 2 or 3 thou at the specified torque loading and you could easily get this sort of flattening of a soft tab washer overtime resulting in loss of tensile loading and then bolt fatigue failure.

I have seen similar failures on stub shafts on large rotating machinery where the designers used tab washers under the bolts which in this case was a 600 mm diameter ring of about 20 high tensile bolts each 30 mm diameter. The bolts see cyclic loading as the shaft rotates and the soft tab washers ultimately distorted sufficiently for the bolts to lose tension and start to see cyclic stress and fatigue failure resulted .

cheers
Rohan

[lance54]

viewtopic.php?f=33&t=50189

[Quart Meg Miles]

Thanks Ben and Rohan for such comprehensive analysis, given the evidence. I've never understood why tab washers are specified so often, like on the flywheel.

One might argue that the 116 rod with ARPs and no washer might be the better race rod as it lighter and less likely to stress the bearing and piston at high revs. 4623's (Ben's car) early failure was a bolt, I wonder if all of them were.

Has anyone had a 116E rod failure that wasn't the bolt?

[nomad]

Thanks Ben and Rohan for such comprehensive analysis, given the evidence. I've never understood why tab washers are specified so often, like on the flywheel.

One might argue that the 116 rod with ARPs and no washer might be the better race rod as it lighter and less likely to stress the bearing and piston at high revs. 4623's (Ben's car) early failure was a bolt, I wonder if all of them were.

Has anyone had a 116E rod failure that wasn't the bolt?

My S1 engine on tear down produced two straight 116E rods and two bent ones???? I never ran the engine, that is the way it came from the PO. Quite sure the engine had never been apart from its build date.

Kurt