Lotus Elan

Weber Installation - Thackeray Washers

PostPost by: Galwaylotus » Tue Dec 03, 2013 1:59 pm

I just refitted my DCOEs and experienced all the pain and aggro that goes with fitting the Thackeray washers. The bottom ones are particularly difficult and setting the gaps nearly impossible - until I thought about it a bit.
The gap between coils is supposed to be 0.040 inches. The thread on the studs is a 5/16-UNF with a pitch of 24 tpi. One 360? turn is 1/24 inches or 0.04167 inches in decimal form. If the nut is tightened just until the coils touch and then backed off one full turn (ok, less 14.4? :roll: ) the gap will be very close to spec. I use a mirror to see the lower studs and carefully tighten until I see the coil gap just close then back off one turn. Makes it a lot easier than trying to get a feeler gauge in there and I am more confident in the consistency of the result. :D
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PostPost by: billwill » Tue Dec 03, 2013 5:33 pm

You are reading too much into the specification of the gap in the coils of a thackray washer. It simply is not that critical.

All you have to ensure is that the o-rings in the inlet separatotors (or the combined rubber/alu separator) are lightly nipped from pure round and that when you rock the carburettors the rubber doesn't lift-off & make a leak.

Your practice of do it up tight and then back it off one turn is more than adequate as a procedure, without trying to see under there & stick feeler gauges in.

When you have them done up, look down fron the top of the carburettors and check that the faces of the carbs are parallel to the faces of the head and that the sepataror is nicely positioned midway between them. This checks that you haven't forgotten one & left it too loose or too tight.
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PostPost by: billwill » Tue Dec 03, 2013 5:56 pm

By way of seeing how NON-critical this setting is, the older Workshop manual says a clearance between coils of .040 " and the newer manual specifies .050"

The writer of the second manual was just being sadistic I think when he said that you should measure this with feeler guages. :)

~~~~

The older manual gets its priorities right, it says:

Page M9

"To ensure that the Rubber O-ring gaskets between the manifold and carburettor bodies are not flattened, care should be taken to ensure that there is vertical movement of the carburettor assemblies about their manofolds. This is represented by a total up and down movement of 1/8" at the trumpets and this is achieved by retaining a measurement of .040 " clearance between the coils of the spring washers at the manifold studs."


See: the IMPORTANT bit is the ability of the carburettors to flex a little, without leaking air of course, the use of the thackray washers and the gap in the coils is merely the way to achieve this.

And the writer even there has given spurious acuracy by putting in the second 0 implying it needs to be accurate to one thou, when in fact plus or minus 10 thou is OK in this context. ( it should have been written "by retaining a measurement of .04 " clearance".)
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PostPost by: billwill » Tue Dec 03, 2013 5:59 pm

PS: when you use the rubber bung versions of the Thackray washers, there ain't no coils there to stick feeler gauges in. :lol:
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PostPost by: Galwaylotus » Tue Dec 03, 2013 7:40 pm

True. But I wasn't aware of those when I ordered replacement Thackeray washers! I should also mention that I am using the MISAB plates rather than the double O-ring and spacer set-up. The manual does say not to tighten the latter system too much or the "O-rings will be flattened into the recesses of the plate." Not an issue with the MISAB plates.
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PostPost by: Galwaylotus » Tue Dec 03, 2013 7:42 pm

BTW, my reason for posting this topic was to save someone else's sanity when trying to achieve the 0.040" clearance! :wink:
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PostPost by: Esprit2 » Tue Dec 03, 2013 10:00 pm

In addition to the gap between the coils of the Thackeray washer, also be mindful of the gaps between the soft mount spacer and the manifold and carb flanges on either side of it... two gaps per joint.

The width of the gap on either side of the spacer will vary by brand, and more specifically with the thickness of the O-ring used. If the vintage Lotus OEM metal soft mount is used with original-spec O-rings, then the gaps should be .040" (1.0 mm), +/? ? more specifically, 0.35 - 0.50 inch (0.89 ? 1.27 mm). It's more important that the gaps be uniform on both sides of the spacer, all around the spacer, and spacer to spacer, than it is that the gap be dead-nuts one specific value. First be within the broad spec range, then be uniform in the use of whatever value you use. The book spec, .040 (1 mm) is a good mid-range, round-number value.

Various aftermarket plastic soft mounts each use different thickness O-rings and gaps, so read the instructions. Even the later Lotus OEM soft mount used in the carbureted Turbo 910 engine used much thicker O-rings that required a 0.080" (2 mm) gap. Use good judgment. If it appears you are crushing the crap out of the O-rings? then don't. You'll just ruin them, or they will squirt out of the joint, and cause air leaks and poor running.

SU and Zenith Strombergs each have their variations on the soft mount, different from the Weber/ Dellorto mounts. Using vintage-spec Lotus parts, the soft mount gap for Strombergs is .070" (1.78mm); but, follow the instructions that come with any modern-spec O-rings you use.

*~*~*
The Nyloc mounting nuts are first to be tightened until the gaps on either side of the soft mount are to spec. In my experience, the factory metal spacers fit quite well, but some aftermarket plastic spacers bind on the studs (incorrect hole spacing), and do not slide freely into position. Best advice... avoid using spacers that don't fit properly. But if you have plastic spacers, the odds are that the fit is less than well.

If that's the case, and you're intent upon using them, then forcing them into position is where the use of gap gauges (~.040" / 1mm shim or wire) or feeler gauges can be especially useful. Insert a shim into each gap on either side of the spacer, then tighten the nut only (no Thackeray or grommet) until the joint's stack of parts goes solid. Tighten snuggly, but not forcefully. That will force the stubborn soft mount spacer to slide into position without abusing the Thackerays or grommets. Then slack off the nuts and remove the spacer shims.

Again, the best advice is to avoid ill-fitting spacer plates.

Once the soft mount spacers are properly centered, with all gaps uniform and within the spec range, then during final assembly...

If Thackeray washers (springs) are used, then also limit nut adjustment to maintain a MINIMUM gap between the spring coils of 0.035" for the final setting. A larger gap of .040-.050 inch can be used as long as care is exercised to make the gap uniform all around. 0.35?0.50 inch (0.89?1.27 mm), but uniform.

If rubber grommets are used, then tighten the nuts until all clearance/ slack is taken up between the nut, cup washer and rubber grommet (ie, the stack-up just goes solid), then tighten the nut another 1 1/2 turns, ensuring that the "V" in all grommets is equal. Since the M8 nuts have a 1.25 pitch thread, 1 1/2 turns is equivalent to 1.88mm (0.074") of crush/ pre-load. Keep the setting uniform for all mounting nuts; and again, ensure that the "V" in all grommets is equal.

It may not be possible to achieve both a proper spacer gap and a proper Thackeray gap / grommet compression at the same time. If there's a conflict, then the spacer gaps are a direct reflection of O-ring compression, and take priority over the Thackeray/ grommet compression.

*~*~*
A newer type of soft mount consists of a thin, stamped metal plate with an integrally molded rubber "O-ring" around the center bore. It's a one piece part instead of the O-ring / plate / O-ring assembly. It's easier to handle, but thinner overall than the standard sandwich plate.

Overall, those mounts typically have less total rubber for compliance, and don't follow vintage assembly conventions. If you use them, ignore the Lotus gap specifications, and follow the instructions that come with those soft mounts.

*~*~*
Thackeray washers experience metal fatigue due to the vibration, and fracture. The upper ones are more prone to failure, while the lower ones tend to "last forever".

The rubber grommets are fuel resistant, but not fuel proof; so fuel leakage will eventually attack them. Since stuff runs down hill, the lower grommets tend to fail first, while the upper ones see little fuel and last forever.

A compromise solution is to use Thackeray washers on the bottom and grommets on the top for optimum soft service life. However, when you mix Thackerays and grommets, the clamping force may (most likely will) vary between the two types, so pay more attention to keeping the gaps between the soft mount spacer and the manifold/ carb flanges uniform.

Thackerays will hold their setting well over time, however the same can't be said for the rubber grommets. Any rubber or polymer elastomer will take a compression set, yielding to a load over time (it's called Creep). If you use grommets, then re-check the fittings periodically. Especially shortly after a new installation.

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PostPost by: AHM » Wed Dec 04, 2013 12:10 am

Nice method Galwaylotus - better than mine!

Bill,

Your post implies that the load on the thackery washers is set to ensure that the 'O' rings are not misshapen.

I believe that the load on the thackery washers is set to provide the correct amount of "float" to prevent fuel from frothing. With the 'O' rings providing damping and a seal to the flexible mount. ie they are sprung and damped to avoid a resonant frequency.

.040 is simply a nominal measurement. It is standard practice to write decimal inches to 3 decimal places.

Using a feeler gauge is a simple method to ensure that the gaps (and hence load) are accurately and evenly set, with a commonly available tool - Which is the purpose of a workshop manual!
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PostPost by: Esprit2 » Wed Dec 04, 2013 6:21 am

AHM wrote:I believe that the load on the thackery washers is set to provide the correct amount of "float" to prevent fuel from frothing. With the 'O' rings providing damping and a seal to the flexible mount. ie they are sprung and damped to avoid a resonant frequency.
Both the O-rings and the Thackerays/ grommets need to be correctly set. Both need something compliant to push against in order to hold the carbs firmly, but with compliance. If either the spacer gaps or the coil gaps go solid first, the whole compliance thing goes out the window, even if the other still has a gap.

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PostPost by: AHM » Wed Dec 04, 2013 10:57 am

Tim,

I Agree! What have I posted differently?

Setting the gap correctly ensures that neither are solid, or too loose.

Adjusting the gap will change the range of resonant frequencies affected.

If you set the gap correctly you can't get it wrong
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PostPost by: ecamiel » Wed Dec 04, 2013 12:32 pm

when putting it all back together don't forget to put a dab of silicone sealer on the backing plate nuts to keep them from falling of and getting ingested otto the motor with horrific results.
It's standard racing practice.
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PostPost by: Galwaylotus » Wed Dec 04, 2013 1:12 pm

I prefer a mild thread lock. Silicone sealer contains acetic acid which promotes corrosion.
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PostPost by: Esprit2 » Wed Dec 04, 2013 6:12 pm

Galwaylotus wrote:I prefer a mild thread lock. Silicone sealer contains acetic acid which promotes corrosion.
Some silicone sealants made for use around the home do contain acetic acid. If it smells of vinegar, then it contains acetic acid. The acetic acid can promote corrosion, but it will also poison the catalytic converter in a modern car.

Automotive grades of silicone sealant, such as the Permatex Ultra series, don't contain acetic acid in order to make them safe for use around cat converters.

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PostPost by: Esprit2 » Wed Dec 04, 2013 6:19 pm

AHM wrote:Tim,
I Agree! What have I posted differently?
Setting the gap correctly ensures that neither are solid, or too loose.
Adjusting the gap will change the range of resonant frequencies affected.
If you set the gap correctly you can't get it wrong
It sounded to me like you were saying the Thackeray coil gap controls all. In a perfect world, it will. But there are often conditions in which the proper coil gap does not automatically produce the correct gap on either side of the spacer plate, so it's important to pay attention to both. The spacer gaps reflect O-ring compression. The coil gap indicates clamping preload and ensures a measure of free-play for movement.

I was just trying to point out that it's important to check both gaps, spacer and coil.

Of course, with grommets there is no coil gap, making it more difficult to visually judge pre-load. So then, it becomes important to rely more on spacer gaps for feedback. IMHO.

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PostPost by: Esprit2 » Wed Dec 04, 2013 6:26 pm

ecamiel wrote:when putting it all back together don't forget to put a dab of silicone sealer on the backing plate nuts to keep them from falling of and getting ingested otto the motor with horrific results.
It's standard racing practice.
If the nuts are Nylocs, then don't use a Threadlocker. An anerobic Threadlocker will attack the Nylon patch in the Nyloc nuts. You'll lose more than you gain.

If you wish to use a Threadlocker, then use a regular nut, or an all-metal distorted-thread lock nut.

Regards,
Tim Engel
Last edited by Esprit2 on Wed Dec 04, 2013 9:45 pm, edited 1 time in total.
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