Lotus Elan

Timing chain tensioner pivot bolt

PostPost by: 2cams70 » Wed Aug 30, 2017 1:19 pm

Did Lotus change the thread type on the timing chain tensioner pivot bolt at any stage? I bought a new one for my engine and it doesn't fit. At some stage in the past the corresponding thread in the head has been repaired with a Helicoil but I don't know whether or not they restored it to the original size.
The thread on the original pivot bolt is different to the replacement and also the hexagonal head on the original is bigger. Is there enough meat left in the aluminium to install yet another helicoil to match the new thread? Are pivots with the alternative thread still available from anywhere?
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img_19441.jpg and
Thread in head Helicoiled
img_19431.jpg and
Original LHS, Replacement RHS
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PostPost by: alan.barker » Wed Aug 30, 2017 4:20 pm

Have a look at past posts. If you bought one from burton i think it maybe wrong.
The correct thread on the pivot is BSPF (british standard pipe fine).
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PostPost by: promotor » Wed Aug 30, 2017 5:37 pm

I've never seen or heard of a different thread fitment supplied by Lotus - I'm going to guess your original one is a custom made jobbie to get around the Helicoil size availability as it might have been difficult to obtain when it was needed.
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PostPost by: alan.barker » Wed Aug 30, 2017 6:11 pm

Wire thread Inserts are available in BSPF no problem in a kit with Tap and Pilot Drill + Insert tool.
ARMSTRONG "ARMACOIL" G1/4 BSPF 19 tpi from Hillcliff tools ltd UK ?59.57 + vat.
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PostPost by: Craven » Wed Aug 30, 2017 6:50 pm

Just had a look at 4 old one?s they are all the fine thread, RHS pic
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PostPost by: 2cams70 » Wed Aug 30, 2017 11:26 pm

Thanks Guys. I think I'll leave it to my machine shop to replace the Helicoil with the correct type. Hopefully the head casting hasn't been bored out too far.
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PostPost by: rcraven » Thu Aug 31, 2017 12:09 pm

Whilst the right hand thread in the illustration in certainly finer than the left (i.e. more TPI), I don't think the F in BSPF means "fine". The abbreviation often used for the parallel BSP thread is just BSP or BSPP, but where BSPF is used the F just seems to stand for "fitting" or "form" and I don't think it means anything different from plain BSP or BSPP. As far as I'm aware there aren't coarse and fine versions of the BSP thread in the same diameters. All the charts I've looked at say 1/4" BSP is invariably 19 TPI.
So if ( :)) I'm right, one of those threads just isn't BSP.
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PostPost by: alan.barker » Thu Aug 31, 2017 1:28 pm

Yes you're right. I know when i repaired and fitted a Wire Thread Insert in my +2S it was for sure a BSPF thread
19 tpi.
I still have the repair kit which was not cheap but it was cheaper than getting a machine shop to do the job :D
Such an easy job to do yourself no problem.
On my +2S Engine the thread got damaged when i unscrewed the pivot screw. I didn't know the idiot who assembled the Engine had put Loctite on the thread. When i unscrewed it the thread tore out with the pivot.
I was very happy and used some of Miles Wilkins naughty words :roll: .
When you assemble there should be a special washer under the pivot head or you can use a Dowty washer which has a lip seal bonded to the inside dia.
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PostPost by: 2cams70 » Wed Sep 13, 2017 5:51 am

This bodge is proving to be a bit of a PITA. Because it has already been rethreaded incorrectly with a spring insert to 1/2" UNC it is not a simple matter to just replace the existing insert with a correct 1/4" BSP one. The fitting thread has to match the insert so to properly do this one would need to remove the existing insert and also drill out the fitting threads making the hole oversize. The 1/4" BSP insert would then need to be a solid thick wall type to take up the clearance. The problem is of course that there are no solid wall 1/4" BSP inserts available off the shelf. It has to be custom made. With a lot of google searching I've found that a "Full-Torque" brand FFT type insert custom threaded to 1/4"BSP would be ideal. I've shot a "Custom Made Special Insert Order Form" through to them so lets see what they come back with. I know I could have another pin made up with 1/2" UNC thread or simply reuse the existing one but to my mind this is not ideal.

In the meantime I've spoken to a local engineering firm "Metalock" in Dandenong who have said they would be willing to do the work. They do a lot of vintage and industrial repair and reclamation work and seemed to have a good attitude so I'm seriously considering handing the job over to them too.

I must say I was a little disappointed when I approached my regular well known engine reconditioner with whom I've already spent almost $6,000 on machining about it. Too hard, we don't do small jobs plus we are really busy was the general response. They recommended getting someone (not them) to make a new pin or find someone to hard chrome plate the existing pin and re-machine it.
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img_1957.jpg and
img_1955.jpg and
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PostPost by: rgh0 » Wed Sep 13, 2017 9:23 am

Another option would be to weld up the oversize hole and then redrill and retap it. Orger Engines in Bayswater should be able to that for you i would have thought.

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PostPost by: 2cams70 » Wed Sep 13, 2017 9:31 am

You said the name Rohan. The other thing they said was "our business focus has changed" Sounds ominous whatever that means. The concern with welding it up and redrill was lining it up correctly with the location spigot on the other side. Maybe the weld aluminium would be a bit soft too - not having had the same heat treatment as the parent metal ??
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PostPost by: rgh0 » Wed Sep 13, 2017 10:21 am

I know Orgers had changed hands but I have not had head work done with them for a while. It would be a shame if they are no long interested in restoring twin cam heads. The welding they have done on heads for me previously has been better than the original cast parent material. Maybe their welder had gone?, this sort of aluminium welding requires a lot of practice and skill especially when it involves rebuilding the entire combustion chamber when a valve is dropped. Building up a hole in the front of the head for machining and tapping is far simpler.

I hope I dont need to find a new engine machine shop as finding people who do good quality work in accordance with detailed specifications is not easy :(

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PostPost by: rgh0 » Wed Sep 13, 2017 10:26 am

To weld it up I would rebuild it with welding and then roughly drill it out. Then you use the hole behind as a pilot hole to bore the front hole to be concentric and the correct diameter for tapping. The bottom face of the head and sides of the head are used to ensure the hole is bored parallel with the head and pilot hole as well as concentric. Not that hard a setup on a multi axis milling machine.

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PostPost by: 2cams70 » Sat Sep 16, 2017 8:21 am

Thanks Rohan. If Orger's don't want to do it would you have any other recommendations who else around Melbourne might be able to? A long time ago I used to use Crankshaft Rebuilders for machining but when I checked them out as candidates for possibly doing the Lotus engine I wasn't too confident. Staff didn't seem to know what it was. I'm at a loss as to whom else I might entrust this type of work to. Also that thread - it is definately 1/4" BSP PARALLEL isn't it? Taper is more common and the pin itself is parallel. Why the heck would Lotus choose a BSP type thread when neither the thread in the head or on the pin itself is tapered for sealing? Surely if you didn't want the thread to seal you would use a standard UNF or UNC thread? - But then again, this is Lotus and BSP is definitely British unlike the American UNC/UNF used on all the Ford parts of the engine!
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PostPost by: rgh0 » Sat Sep 16, 2017 9:25 am

The thread is so short it is hard to tell if it was tapered or parallel and probably makes no difference as long as the bolt threads all the way in and compresses the aluminum washer which acts as the seal. I will have a look tomorrow and see if I can detect a taper.

I have used Orgers for all my engine machining work for the last 30+ years so dont really have an alternative I could recommend. Hopefully they are still interested in doing Lotus engine machining

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