Lotus Elan

Valve Clearance Question: .009 to .011 for exhaust only?

PostPost by: 1owner69Elan » Thu Mar 05, 2020 7:08 pm

When I used exhaust settings at .006-.008 I constantly burned exhaust valves on my 69 Federal S4 with Strombergs. There was no updated information in the workshop manual that I had purchased at that time, at the factory, with delivery of my car, and I assumed was appropriate for my car.
Then, I received updated info from the fellow that had actually created the Federal Stromberg smog system that the clearances should indeed be larger. I switched to 010 and thereafter avoided the valve burning failures.
Now with a Weber head I also use 0.010.
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PostPost by: Esprit2 » Thu Mar 05, 2020 7:45 pm

Rob,
The valves live in a very hot environment, especially the exhaust valves, and their only chance to cool down occurs while they are closed and in contact with the water-cooled head. Smaller valve clearances shorten the cooling time, and can result in burned valves. The exhaust valves run much hotter, require more cooling, hence their larger valve clearances for longer cooling.

The valves hammer their way up into the valve seats over time. It's called "Valve Recession". As the valves go up into the seats, the ends of the stems get closer to the cam lobes, and the valve clearances close-up... ie, get smaller.

The smaller value in each clearance range is the Wear Limit... time to re-shim the valves. While shimming to just over the minimum is technically "within spec", it's a bad idea unless you really want to be back in there right away, re-shimming. Shim the valves to the top of the range in order to have the most time on the road before the task needs to be repeated.

0.005" to 0.007" is correct for the intake valves only... you're correct

Exhaust valve clearance was spec'd at:
0.006" to 0.008" up to engine number 9951.
0.009" to 0.011" after that engine number.

Those are the exhaust clearance specs given in the manual. Unfortunately, I'm not aware of the specifics of WHY the exhaust spec changed. Cams did change at some point(s), but I can't confirm off the top of my head that a cam change took place at engine # 9951. So it would be wise for you to just check your engine number and use the exhaust clearance that is called out for it.

While larger clearances provide better valve cooling, excessive clearance can lead to cam/ tappet/ valve stem damage. The cam lobes have "opening ramps"... ie, areas of transition between the constant-diameter base circle and the actual lobe. The "opening ramps" provide a transition area that more gradually makes contact, then accelerates the tappet & valve stem before the actual lobe makes contact and gets serious.

If excessively large clearance is used, the cam may rotate beyond the "opening ramp" (skip right over it) before first contact is made. And then it's more of a collision than gradual first contact & acceleration. In that case, too-large clearances can result in excessive noise/ clatter, and the parts beating one another mercilessly.

Too little valve clearances risks over heating/ burning the valves... the exhaust valves in particular.

Too much valve clearance risks excessive wear as the parts hammer on one another harder than is necessary.

B-Cam . . . . C-Cam . . . D-Cam
Standard . . SE Cam . . Sprint
264° . . . . . . 272° . . . . . 270° . . . . . . Duration
44° . . . . . . . 52°. . . . . . . 50° . . . . . . . Overlap
0.375". . . . . 0.340" . . . . 0.350" . . . . Lift
.005-.007. . .005-.007. . .005-.007 . . Intake Clearance.
.006-.008 . . . . . . . . . . . . .009-.011 . . Exhaust Clearance.

So, when did engine #9951 and the exhaust clearance change occur relative to B-C-D cams?
I don't know off the top of my head.
The obvious change in lift occurred between the B & C cams.

Failing a full understanding for now, FOLLOW THE SPEC FOR YOUR ENGINE NUMBER.

Regards,
Tim Engel

PS...
The Lotus 907 also used C & D cams with the same profiles, the same chilled cast iron tappets, and the same shims. For them, Lotus specified:
0.005" - 0.007" Intake
0.010" - 0.012" Exhaust.

Just stirring the pot...
Last edited by Esprit2 on Thu Mar 05, 2020 8:07 pm, edited 6 times in total.
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PostPost by: elansprint » Thu Mar 05, 2020 7:47 pm

Not to mention spinning the main bearings on the BMW V8 engine. i keep hearing about all this German reliability everybody tells me Jags are crap but i have had a 12 year old sovereign for the last 10 years with only a new alternator & dpf reair flexi still on original battery. TVR dont even go there bought new in 96 rsd otter switch only problem in 24 years yet all my mates BMW & Merc nothing but niggling problems even when new Porsche 997 IMS bearing failure so i take a lot of this with a large pinch of salt. So lotus & TVR etc warm them up before opening the taps and drive smoothly look after them and they will look after you
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PostPost by: mbell » Thu Mar 05, 2020 10:04 pm

I daily drive an older (18 years old) BMW that I have had 9 years and I describe it as a reliably unreliable car. In that there a lot of issues with them but their all common, well documented, easily to predict when they happen and normally not too hard to fix. So along as your ok with fixing things and that things that are lifetime parts on a lot of cars are service parts on a BMW the older ones are fine.

I have no interest in more modern BMW thou, they seem to still have the same issues but are much more difficult to fix due to packaging and electronics. They only make sense if you going to lease them for a few years and change them out.

I do enjoy driving the thing thou, well unless I've been out in the Lotus that is.
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PostPost by: 2cams70 » Fri Mar 06, 2020 2:46 am

draenog wrote:I guess you're using "rooted" in the Australian sense, i.e. "f***ed"


Yes - and would you believe the owner of the BMW who described it as "rooted" was a gynaecologist :roll: (I kid you not, it's true) Let's hope he doesn't describe his patients in the same way!! :lol:
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PostPost by: rgh0 » Fri Mar 06, 2020 4:19 am

Why Lotus changed the exhaust clearance specification has never been explained. It may have been a change in valve material, a change in the cam clearance take up ramps or they realised the original clearance was just to small and risked burning exhaust valves.

At least in my experience the exhaust clearance closes up a little due to the exhaust valve expanding a bit more than the head does as it runs a lot hotter. The inlet valve clearance does not appear to change much between hot and cold, So my bet is that Lotus just decided the clearance on the earlier engines was just a little to tight and to avoid ay liability for failed earlier engines just increased the clearance after a certain engine without explanation

Given all the changes during rebuilds that have occurred with most engines over the years and almost all would have new valves fitted, I would use the later 9 to 11 thou clearance setting.

My S4 came with standard B cams originally and its engine number is well after the change point so I dont think it was related to cam type itself or Lotus would have been more specific than just an engine number. I have always set exhaust valves at 10 thou

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PostPost by: Esprit2 » Fri Mar 06, 2020 6:59 am

rgh0 wrote:Why Lotus changed the exhaust clearance specification has never been explained. It may have been a change in valve material, a change in the cam clearance take up ramps or they realised the original clearance was just to small and risked burning exhaust valves.
I've always felt is was probablly the latter... realizing the original clearances was too small. However, I had a shred of doubt since the new spec was from a change-point forward when it would have been so easy to just change the exhaust spec across the board with a TSB.

Regards,
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PostPost by: rgh0 » Fri Mar 06, 2020 7:48 am

Esprit2 wrote:
rgh0 wrote:Why Lotus changed the exhaust clearance specification has never been explained. It may have been a change in valve material, a change in the cam clearance take up ramps or they realised the original clearance was just to small and risked burning exhaust valves.
I've always felt is was probablly the latter... realizing the original clearances was too small. However, I had a shred of doubt since the new spec was from a change-point forward when it would have been so easy to just change the exhaust spec across the board with a TSB.

Regards,
Tim Engel


Yes - but knowing Lotus I am sure a TSB would be seen as an admission of error and an opening for compensation claims from people with burnt exhaust valves from before the TSB issue. Colin was not very keen on paying out hard earned money i think.

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PostPost by: 2cams70 » Fri Mar 06, 2020 8:28 am

Wildcard but could it have been linked to change from Die cast head to sand cast? Maybe the material spec changed a bit and the thermal expansion properties of the head changed?
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PostPost by: oldelanman » Fri Mar 06, 2020 9:25 am

Miles Wilkins makes this comment in his book....

For all engines, inlet clearances should be 0.005-0.007 in, and exhaust clearances 0.009-0.011 in. If you are rebuilding an engine with the original exhaust valves still fitted (this applies to engines before number 9952), use an exhaust valve clearance of 0.006-0.008 in.

....seems to suggest that the exhaust valve material was changed.
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PostPost by: rgh0 » Fri Mar 06, 2020 10:04 am

2cams70 wrote:Wildcard but could it have been linked to change from Die cast head to sand cast? Maybe the material spec changed a bit and the thermal expansion properties of the head changed?


I thought this may be a possibility but the earliest type 2 sand cast heads start around engine number 6075 well before the exhaust valve clearance change at engine 9952

A bit of head history here

viewtopic.php?f=39&t=31879&start=


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PostPost by: rgh0 » Fri Mar 06, 2020 10:08 am

oldelanman wrote:Miles Wilkins makes this comment in his book....

For all engines, inlet clearances should be 0.005-0.007 in, and exhaust clearances 0.009-0.011 in. If you are rebuilding an engine with the original exhaust valves still fitted (this applies to engines before number 9952), use an exhaust valve clearance of 0.006-0.008 in.

....seems to suggest that the exhaust valve material was changed.



Certainly a possibility but I would have expected a Lotus technical service bulletin or other information around using the later clearances if rebuilding a head with new later valves apart from the reference in the the Wilkins book which is the only one i have seen and it has no details or attribution unfortunately.

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PostPost by: oldelanman » Fri Mar 06, 2020 11:40 am

rgh0 wrote:
oldelanman wrote:Miles Wilkins makes this comment in his book....

For all engines, inlet clearances should be 0.005-0.007 in, and exhaust clearances 0.009-0.011 in. If you are rebuilding an engine with the original exhaust valves still fitted (this applies to engines before number 9952), use an exhaust valve clearance of 0.006-0.008 in.

....seems to suggest that the exhaust valve material was changed.



Certainly a possibility but I would have expected a Lotus technical service bulletin or other information around using the later clearances if rebuilding a head with new later valves apart from the reference in the the Wilkins book which is the only one i have seen and it has no details or attribution unfortunately.

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The Parts List shows an exhaust valve change at engine no. 9952 and there's also a brief reference to it in the Robinshaw and Ross book.

Parts list - engine.png and
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PostPost by: rgh0 » Fri Mar 06, 2020 7:33 pm

Hi Roger
I missed that in the parts manual, it certainly looks like a exhaust valve material change as nothing else changed dimensionally that I am aware off unless a change was to make the valve slightly shorter to accommodate the increased clearance with the standard nominal shim thickness in the design. Does anyone know if the original fully dimensioned drawings of the twincam head still exist somewhere?

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PostPost by: UAB807F » Sat Mar 07, 2020 7:08 am

oldelanman wrote:Miles Wilkins makes this comment in his book....

For all engines, inlet clearances should be 0.005-0.007 in, and exhaust clearances 0.009-0.011 in. If you are rebuilding an engine with the original exhaust valves still fitted (this applies to engines before number 9952), use an exhaust valve clearance of 0.006-0.008 in.

....seems to suggest that the exhaust valve material was changed.


Brian Buckland mentioned the valve material change in his book and links the different clearances to changes in thermal expansion. Mine's the first run of the book but I would expect it to be in the latest reprint as well.

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