Dellorto Flexible Setting.

[vincereynard]

I have been attempting to fit the carbs and found the 40thou gap a bit trying to set.
The bottom nuts are especially challenging.

However, being ever tempted to cheat, I reasoned that at the 5/16th UNF thread is 24 TPI.

That would mean that one complete turn would be 1000/24 = 41.66 thou.

Therefore, using a 6inch extension on a 1/4 socket, nip the bottom nuts up to no clearance and back off one complete turn should give 40 thou.

Is this nonsense? I've tried it on the accessible top nuts and it seems to work.

[William2]

Are you using Misab rubbers/washers?

[vincereynard]

Are you using Misab rubbers/washers?

No idea! Nylon spacers with built in O rings. Double coil washer things. "Misab" is a new term to me.

[john.p.clegg]

Vince

Your logic sounds good to me , go for it...I made the studs up so that when the nuts " bottomed " on the thread then the 0.040" was achieved...

Misab , I believe was the Scandinavian / Swedish manufacturer of the vibration absorbing carb mounting plates..

John

[vincereynard]

Vince

Your logic sounds good to me , go for it...I made the studs up so that when the nuts " bottomed " on the thread then the 0.040" was achieved...

Misab , I believe was the Scandinavian / Swedish manufacturer of the vibration absorbing carb mounting plates..

John

Cheers John. I imagine an even tightness is more important than a couple of thou anyway.
So that the carb. face is square to the O ring.

As it happens I first did the top ones the manual way, tweeking them up measuring with a feeler gauge.
Of course the top then influences the bottom and vice versa. What a daft system.

Nipping them up then back a turn will probably result in a more accurate result.

So you made the studs with smooth length = Compressed washer + .040 + plain washer?
Neat!

Vince

[alan.barker]

Imho i would not nip up the nuts to bottom out YOU RISK TO DAMAGE THE CARB FLANGES
Alan

[nigelrbfurness]

Be mindful of potential air leaks. I would back them off two turns and if there are no leaks, leave it - otherwise do them up a quarter turn at a time (both bottom ones together) until you have no leaks. I would treat the 40 thou figure as "guidance" rather than absolute where a flexible mounting is concerned.

[Esprit2]

Sorry about jumping into the conversation late.

Over-tightening the soft-mount joking connection will crush the O-ring, potentially damaging it to the point that it won't seal properly when the joint is backed-off to the correct gap. 'Solid' certainly qualifies as 'over-tightening'.

Instead, use feeler gauges, or cut a couple of lengths of 0.040" (1 mm) diameter music wire long enough to lay along the vertical sides of the joint, one on either side of the soft mount spacer plate. Cut them long enough to stick out far enough to grasp. Adjust one side, shift the wires to the other side, and adjust it. Alternatively, bend a 'square U' large enough to lay across the top and down the two sides of the flange. Just lay one in each gap on either side of the soft-mount spacer plate.

Tighten the nuts, tightening the gap until it lightly touches the wires. Pinch to a light slip fit. Remove the wires.

Regards,
Tim Engel

[Orsom Weels]

Sorry about jumping into the conversation late.

Over-tightening the soft-mount joking connection will crush the O-ring, potentially damaging it to the point that it won't seal properly when the joint is backed-off to the correct gap. 'Solid' certainly qualifies as 'over-tightening'.

Instead, use feeler gauges, or cut a couple of lengths of 0.040" (1 mm) diameter music wire long enough to lay along the vertical sides of the joint, one on either side of the soft mount spacer plate. Cut them long enough to stick out far enough to grasp. Adjust one side, shift the wires to the other side, and adjust it. Alternatively, bend a 'square U' large enough to lay across the top and down the two sides of the flange. Just lay one in each gap on either side of the soft-mount spacer plate.

Tighten the nuts, tightening the gap until it lightly touches the wires. Pinch to a light slip fit. Remove the wires.

Regards,
Tim Engel

But the clearance should be measured between the coils of the Thackary (double coil spring) washers, not the flange it's self. I think Vince is only referring to closing that gap up & backing off, not nipping the flange up to the point there is no gap. I'm not sure that would even be possible without damaging something. Personally, I have always done it by eye with a dentists mirror to see the 4 underneath. I make sure all studs are the same length, screw all nuts on until they are flush with the ends of the studs, then tighten each a 1/4 turn at a time until I get a uniform gap between the coils of all the washers. Been doing it that way for over 30 years & never had a problem. As Vince says, I think a bit of flex with a uniform gap on the flanges, with no leaks, is more important than a specific measurement.

Regards, Tim

[vincereynard]

There is a slight problem with my "tweak it up and back 1 turn" system - It is nonsense!

Firstly, the thackery things do not compress completely in the first place.

Secondly, when they are compressed so that the ends contact, the gap is seemingly random. On some the ends of the washers are bent inwards, holding the coils further apart.

Thirdly, they are all different thicknesses to begin with. I've measured all 8 loose and they
are 0.182 > 0.204 thou. Quite a range. Perhaps the smaller were originally on the bottom?

So that even if they are all set, (somehow?), at an accurate 40 thou it may be virtually meaningless.

Additionally, of course, the bottom one will be under compression from the weight of the carbs. the top opposite.

I shall get Sue to send a new set and see how they measure up. Although I don't hold out much expectations.

[Orsom Weels]

Additionally, of course, the bottom one will be under compression from the weight of the carbs. the top opposite.

Actually it's the top ones that have the weight of the carb's on them, but once all tightened to a certain degree, the 'springiness' of the Thackaries should overcome & compensate for that. With a matched set of Thackaries (by which I mean all from the same batch), it should be possible to achieve visually similar gaps in all the washers whilst keeping parallel & even gaps on the mounting flanges themselves.
As said, in over 30 years & many many thousands of miles, I've only ever set them visually & by 'feel' & it's one area of the twin-cam that has never given me any problems.

Regards, tim

[vincereynard]

Actually it's the top ones that have the weight of the carb's on them,
Regards, tim

Of course it is. My apologies - a bit of brain fade.

Anyway I'll check out the sizes of these new ones and ponder on Plan C.

[billwill]

The Lotus manual is over-precise on this matter, it really is NOT THAT CRITICAL.

You only have to make sure that the O-ring(s) are pressed slightly against the metal and that rocking the carbs does not let top (or bottom) of the O-rings lift-off the metal.


I never bother to measure mine, I can see by eye when the above conditions are correct.

[Esprit2]

Okay, I'll apologize for the abuse of bandwidth up front... but here's the full story on soft mounts as I see it.

When installing the carb soft mounts, all gaps are important. Don?t focus on just one. In a pecking order, the gaps on either side of the sandwich plate are priority #1. The gap between the coils of the Thackeray washer (coil spring), or the amount of crush in a rubber grommet (V-groove width) are important, but they?re secondary. Getting them ALL right is what makes the soft mount work properly.

The coil clearances and grommet crush are important with regards to proper application of the chosen hardware, but it?s the gaps on either side of the sandwich plate that make the soft mount work.

There are many brands of soft mounts out there, but there?s no standard thickness of O-ring, and hence, no standard gap. O-ring and gap sizes vary, so read the instructions that came with yours, and use the correct gaps for it.

If you?re using the vintage LOTUS OEM metal soft mount and O-rings, then the gaps on either side of the sandwich plate should be 0.040" (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... but 0.040" (1.0 mm) is a good gap for the LOTUS soft mount. First be within the broad spec range, then be uniform in the use of whatever value you use.

A much later (1980-87) Lotus OEM soft mount used in the carbureted Turbo 910 engine will also fit your vintage engines. However, it had much larger O-ring grooves, used much thicker O-rings that requires an 0.080" (2 mm) gap. Use good judgment? skinny O-rings/ small gap, fat O-rings, large gap. 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. NEVER tighten a soft mount until the gaps close down and go solid. If you do, start over and install new O-rings.

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.

*~*~*~*
In my experience, the Lotus OEM metal spacers fit quite well and slide right on, but some aftermarket plastic spacers bind on the studs (incorrect hole spacing), and need to be forced into position.

Molded plastic parts continue to shrink over the years, even if they?re aging on the shelf and have never been sold. Some plastic sandwich plates can become a tight ?stretch? fit over the mounting studs, and require considerable force to slide them into position. In that case, tightening the nuts enough to produce the correct gaps on either side of the sandwich plate may cause the Thackeray washer?s spring coils to go solid, or may over-crush the rubber grommets.

On the other hand, if you tighten the nuts only until the gap between the spring?s coils is to spec, then you may not have clamped a binding/ sticking plastic soft mount together enough to achieve the correct gaps, or to ensure a leak-free fit.

If the sandwich plates aren?t fitting well and don?t slide on freely, then do not simply crank down on the nuts and crush them home. Fix the fit problem first, then properly assemble the ?free fitting? parts. Soft mounts are NOT a brute force fit.

Properly assembled, the soft mounts should have the correct gap on either side of the sandwich plate, AND either the correct coil spacing in the Thackeray washers, or the correct crush in the rubber grommets. ?All? needs to be right, not your one favorite feature.

Avoid ill-fitting spacer plates.

*~*~*~*
If Thackeray washers (springs) are used, then limit nut adjustment to maintain a minimum gap between the spring coils of 0.035" for the final setting. The full spec range is 0.35?0.50 inch (0.89?1.27 mm), but you?ll often find that the minimum coil gap (tighter) is required in order to maintain the correct gap on either side of the soft mount?s sandwich plate. Make sure the coil gap is the same for all the Thackeray spring washers used (ie, all studs & nuts are equally tight).

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-groove" 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. I?ve also seen non-metallic versions of the same theme.

Those thin-plate 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 for long service life is to use Thackeray washers on the bottom and grommets on the top. 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 on either side of the soft mount sandwich plate uniform. That?s Job #1.

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 an applied load over time (it's called Creep). If you use grommets, then re-check the fittings & gaps periodically. Especially re-visit them shortly after a new installation, since new grommets will take an initial compression set.

Regards,
Tim Engel

[vincereynard]

Tim,

Could you be a bit more specific?

BTW what are the grommets you mention? Luckily the plastic spacers are a nice, sliding fit.

Vince