For the other enginerd-types on JL!! If you can suffer through my thought process and math, I’ve got questions at the bottom!
Jeez, I can’t believe it’s been 8 months since I started this thread! Since then, I was able to locate and purchase some inner fulcrum bearing tubes that are the proper material and proper dimensions, except for the length. They are 1.385" long, so I’ll need to have them trimmed down by a local machine shop.
Samples of old and new inner fulcrum bearing tubes:
In preparation for having them cut, I was taking some measurements yesterday (measure twice, cut once) to ensure I will have them cut to the proper length. As I mentioned above, the OEM ones I removed were 1.145" long. That “OEM” length was supported by @angelw (Bill Angel) in post #16 on this UK E-type Forum thread on a similar topic:
https://forum.etypeuk.com/viewtopic.php?f=3&t=7711&hilit=inner+fulcrum+bearing+length
Anyway, while taking measurements I discovered that the width of my inner fulcrum arm flanges are not all equal. I was not expecting that. The two pictures below show the dimension I’m referring to, and the variances in the widths across the four flanges. “Opposing corners” are very close to measuring the same, which I find interesting, and I’m wondering if there could have been a variance in the original casting mold?
So, what’s the big deal? These dimensions, along with the inner fulcrum bearing tube lengths and inner thrust washer thicknesses are critical to ensuring the inner fulcrum bearing tubes are captured by the outer thrust washers and keep the bearing tubes from spinning (as discussed earlier). In the UK Forum thread I linked above, Bill calculated a 0.006" clearance between the inner and outer thrust washers, with a 1.145" long inner fulcrum bearing tube (meaning the tube is 0.006" longer than the sandwiched components and the tube is captured).
With my fulcrum arm flange widths varying by ~0.025" in width, that’s a huge relative difference when we’re talking ~0.006" clearances!
I took some measurements of my existing inner fulcrum hardware, and the new rubber sealing rings I purchased:
Calculating worst cases for my thick and thin flanges and inner thrust washers, with a 1.145" bearing tube:
Bearing tube length - (inner thrust + fulcrum flange + inner thrust) = clearance
Thick worst case: 1.145" - (0.118" + 0.897" + 0.118") = 0.012" clearance (tube is captured)
Thin worst case: 1.145" - (0.117" + 0.871" + 0.117") = 0.040" clearance (tube is captured)
So, while the tubes are captured with a 1.145" bearing tube, I was concerned that the resulting clearance may be too much and not actually compress the sealing ring at all, leaving the bearing assembly “open” for debris intrusion. If I did my math correctly:
[Bearing tube length - (sealing ring + fulcrum flange + sealing ring)] \ 2 = compression for each sealing ring
Thick worst case: [1.145" - (0.139" + 0.897" + 0.139")] / 2 = - 0.015" (each sealing ring compressed ~10.8%; seems reasonable)
Thin worst case: [1.145" - (0.139" + 0.871" + 0.139")] / 2 = - 0.002" (each sealing ring compressed ~1.4%; seems rather low)
Out of curiosity I calculated worst cases for my thick and thin flanges and inner thrust washers, with a 1.125" bearing tube offered by the usuals:
Bearing tube length - (inner thrust + fulcrum flange + inner thrust) = clearance
Thick worst case: 1.125" - (0.118" + 0.897" + 0.118") = - 0.008" clearance (tube is NOT captured)
Thin worst case: 1.125" - (0.117" + 0.871" + 0.117") = 0.020" clearance (tube is captured)
I’m wondering if the usuals maybe had some flanges that were on the thinner side when they decided to offer 1.125" long bearing tubes?
All of this makes me wonder if the variance in inner fulcrum flange thicknesses is common or not. Has anyone else measured?
Anyone have some on the bench they can measure (@BrentKeats perhaps)?
Do I have anomalous fulcrum arms?
I’ve got to verify my math and since I’m having then inner fulcrum bearings custom cut, figure out what length I want to use. I’ve thought about having them all cut to the same length and maybe adding some thin shims to balance things out if I need to, but I haven’t decided yet.