Lotus Elan

Fasteners for Joining Prop Shaft to Diff Input Flange

PostPost by: CBUEB1771 » Wed Sep 23, 2015 5:57 pm

Back in mid-June there was a brief discussion of the fasteners to be used for joining the prop shaft's flange yoke to the differential input flange. This was in Glen's very interesting thread on his S1 rebuild. I recently revisited this topic with the intent of using nuts and bolts with reduced size wrenching surfaces in order to use box wrenches (ring spanners) for assembly and disassembly. I found that MS21250 type bolts and corresponding nuts (H20) work well for this purpose. A 3/8" 12 point wrench is used for both the bolt and nut (5/16" nominal bolt diameter). Two special notes are made regarding the MS21250 bolts, 1) they have a large radius transition from the bottom face of the bolt head to the unthreaded shank and 2) they have a UNJF thread form. Regarding the first note, it is best to use chamfered washers (MS20002C) under the bolt head to give clearance for the large radius transition section. Regarding the second note, the UNJF thread form differs from the UNF form by having a relatively large radius at the root of the thread. Therefore nuts made for this thread form must be used to prevent damaging the root radius.

I found that bolts with either a 3/8" (MS21250-05006) or 1/2" (MS21250-05008) grip (unthreaded shank length) provide the correct fit with the unthreaded shank section passing through the mating plane of the two flanges. The shorter of the two is best with the small diameter flanges for the 3.9:1 CWPs and the longer is best for the larger diameter flanges for the 3.55:1 and 3.77:1 CWPs. The flange thickness are the same for both types but with the small flange there is not enough clearance from the bolt holes to the differential flange body to fit even a small diameter AN flat washer under the nut. Interestingly the the flange on the base of the H20-5 nut is just small enough to fit this location with no interferences. The photos show assembly of the large type flange yoke with an MS21250-05008 bolt and H20-5 nut. A MS2002C5 chamfered washer is under the bolt head and a thick (0.063") AN5 flat washer is under the nut. I sourced all of the hardware from Coast Fabrication: http://www.coastfab.com. The diff input flange is a new one from TTR with drillings for both the large and small flange bolt patterns. Ring spanners fit on both the bolt head and nut without difficulty for both large and small flanges. Also the bolt head does not foul the the weld yoke on the prop shaft even at full deflection of the u-joint.
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Russ Newton
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PostPost by: gjz30075 » Thu Sep 24, 2015 9:32 am

Excellent,Russ! Would a Gearwrench spanner fit on either side?
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PostPost by: Certified Lotus » Thu Sep 24, 2015 11:39 am

Russ, nice engineering solution! Now I know what to replace those very difficult to install bolts and nuts with. Thanks!
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PostPost by: CBUEB1771 » Thu Sep 24, 2015 2:08 pm

Greg,
I don't have any Gearwrench tools so I can't answer your question from first hand experience. I looked at the Gearwrench on-line catalog and I suspect that their 3/8" ratcheting box wrench would fit on the prop shaft side if you have the large diameter flange yoke for the 3.55 and 3.77 CWPs. My goal was to get away from standard 5/16" UNF fasteners with hex heads (1/2" across-flats). You can only get an open end wrench on these on the differential side and I find it annoying trying to apply reasonable torque to the nut. I have also added photos of the assembly with the small diameter flange yoke for the 3.9 CWPs. As mentioned above I needed to use a slightly shorter bolt in this case because there is not enough room for an AN washer under the nut on the differential side. Why I have a prop shaft with the small diameter flange yoke is another story. I bought a new prop shaft from TTR and they shipped it with the small diameter flange. I don't think that the +2 was ever supplied with a 3.9 CWP and the small flange, at least that is what the Service Parts List leads me to conclude.
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PostPost by: rgh0 » Fri Sep 25, 2015 12:45 pm

The original diff drive flange had a shroud that covered the diff input seal lip reducing dirt getting in there. Looks like the reproduction TTR one does not have that.

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PostPost by: CBUEB1771 » Fri Sep 25, 2015 12:55 pm

"The original diff drive flange had a shroud that covered the diff input seal lip reducing dirt getting in there. Looks like the reproduction TTR one does not have that."

Good observation, I hadn't compared this to the original one yet. Checking the Service Parts list I see that the shroud is a separate part (A036 R 6034), probably stamped sheet metal. Perhaps that could be fitted to the TTR flange.
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