Lotus Elan

Timing chain tensioner pivot bolt

PostPost by: ceejay » Mon Jan 01, 2018 10:18 am

Well some interesting findings people. Rohan I think you might be on the right track
with threads. I have attached a close up photo of the tensioner pin and the thread form gauge held
up to it...note that it is the 18TPI gauge but it does not fit perfectly...almost right but not quite.

I then grabbed a main oil gallery plug which is the same size and looks to have the same thread,
but found that it isn't...it screws in about 1.5 turns into the head, then binds...I tried the 18TPI thread gauge on the gallery plug thread and it fits perfectly, so pipe thread the pin must be.
I could not fit or count the internal thread in the head.

The thread number of 19 was mentioned, the only 19 TPI thread is on the G series BSPF where G1/4 and G 3/8
have 19 TPI (parallel series)...i am not that familiar with these thread forms. That info was taken from the Sutton Tools guide Tapping drill sizes.

So I am still non the wiser for this. Why the odd ball thread size? it does not make sense?
All that I know is that the new pin that I have here screws in perfectly into the head...but if one was making a repair, I would need to grab a pipe thread form gauge which I don't have...and check...could be that it's 19TPI. Maybe contact Burtons or others to see what light they can shed on what the thread form is.
Sorry i cant be more helpfull.
timing-chest-tensioner-pin.jpg and
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PostPost by: rgh0 » Sun Jan 14, 2018 4:36 am

I did some more careful measuring of the pivot bolt thread today just for fun.
I measured the bolt thread pitch at 5.35 mm for 4 threads which is 19 Threads per inch (TPI) and matches BSP 1/4.

The original plugs from the oil gallery and oil drain drilling on the head also measure the same 19 TPI and these plugs also fit perfectly into the pivot bolt hole. These are parallel plugs with a flange and washer and are BSP like all the other plugs in the Lotus castings. The drilling for the threads in the head matches the BSP diameter and is the same as the pivot bolt. It is only the Ford castings that have NPT plug threads.

I am now convinced the original thread in the pivot bolt is 1/4 BSPF Parallel for both the bolt and for the head tapping.

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PostPost by: alan.barker » Sun Jan 14, 2018 9:04 am

alan.barker wrote: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.

Sorry boys,
but to quote myself back in August the thread is BSPF 19 TPI.
In a former life i was a QC Engineer and have measured all types of threads using Pitch Gauges, thread parellels, thread wires, optical equipement for angle, form crest, root form etc.
If the Pivots from Burton have a different pitch then Burton have got it wrong :shock:
I don't think i'd like to tell Burton :roll:
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