Lotus Elan

Sizing Helicoil inserts for the bonnet and headlight pods

PostPost by: JonB » Wed Feb 26, 2020 2:40 pm

Hi

I need to fix my bonnet bobbins, plus one headlight bobbin on my Plus 2. Thought I'd stick Helicoils in there, but what size do I need? I measure the bolt head as 7/16", 20 threads / inch, diameter .243"

Does this one look right? Hard to tell from the picture but it looks a bit big?

https://www.amazon.co.uk/V-Coil-16-inch ... B00EO4NYUQ
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PostPost by: JonB » Wed Feb 26, 2020 2:49 pm

mbell said 1/4 20 coils: viewtopic.php?t=46122&f=51&start=0#p330463

Presume that's 1/4" 20 threads per inch?
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PostPost by: mbell » Wed Feb 26, 2020 3:00 pm

JonB wrote:mbell said 1/4 20 coils: viewtopic.php?t=46122&f=51&start=0#p330463

Presume that's 1/4" 20 threads per inch?


Yes, that is just from memory, maybe it is 7/16"...

Maybe check the parts diagram to confirm the bolt size. The thread insert should match the bolt. So if it is 7/16"-20 that the insert size you should buy.
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PostPost by: pharriso » Wed Feb 26, 2020 5:13 pm

According to RDent's part diagrams the Bobbin bolts are XUCS0420, which translates to 1/4 UNC (20 tpi) x 1.25" long.
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PostPost by: mikealdren » Thu Feb 27, 2020 8:39 am

I tried helicoils for the bonnet but they were too short, make sure you get them to the full depth of the thread in the bobbin. I ended up replacing the bobbins, a time consuming job.
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PostPost by: alan.barker » Thu Feb 27, 2020 9:31 am

You can get Helicoils different lengths.
If the hole in the damaged Bobbin has worn ovale then you might need to use a Keyed Insert which are a larger outside dia. You may need to make a drilling plate to keep the tapping size Twist Drill central also.
Think twice and drill once :wink: :wink:
Sounds like you need a "Zeus Book" + Screw Pitch Gauge + Vernier Caliper
Good luck
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PostPost by: RogerFrench » Thu Feb 27, 2020 9:56 am

7/16" is the bolt head across flats.
The thread size is 1/4" diameter x 20 tpi, which is 1/4" UNC not UNF.
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PostPost by: nmauduit » Thu Feb 27, 2020 10:14 am

I'm not sure how well helicoils would last in the alloy the pods bobbins are made of if original (they are often replaced by steel equivalent on repro body parts) : I would use complete thread inserts like Timesert or similar, and glue them in place.
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PostPost by: alan.barker » Thu Feb 27, 2020 10:35 am

For sure the keyed insert will be more durable as they will be hinge pivots. Plus the outside thread on keyed insert need only a standard Tap.
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PostPost by: The Veg » Thu Feb 27, 2020 3:08 pm

Interestingly, 1/4-20 is also the standard thread for lampshade finials. Did someone at Lotus have a bit of a sense of humour? :mrgreen:
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PostPost by: awatkins » Thu Feb 27, 2020 9:32 pm

Just a longer term suggestion about determining thread sizes: next time you’re at the hardware store pick up a 1” long screw and nut in each thread size below, say 1/2”, fine and coarse, tag each, and keep in a bag in your shop. From then on you can immediately determine any thread’s size.
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PostPost by: alan.barker » Fri Feb 28, 2020 8:40 am

If you buy a Unified Tap and Die set it comes with a Screw Pitch Gauge. Not expensive.
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PostPost by: steveh » Fri Feb 28, 2020 12:48 pm

Have a look at Rivnut's Jon , they are available in 1/4" 20 tpi , you will need to drill a 3/8" hole and you will have to buy the tool or borrow one, it's a great bit of kit to have anyway.
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PostPost by: nmauduit » Fri Feb 28, 2020 1:10 pm

one quick word of caution : some jig (or possibly the pod itself) should be used to ensure the 2 threaded hinges are reasonably aligned for final installation in case some play has to be taken up, or there is a risk of high friction or binding in operation.
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PostPost by: alan.barker » Fri Feb 28, 2020 2:16 pm

Rivnuts not a bad idea i like it but i still prefer stainless steel keyed inserts
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