Hispano Suiza V8

[RReid]

Thank you, Roger.

I got three of the cylinders completed, then we had to go to our daughter's house for a few days. So no machining, but lots of downtime from grandfatherly duties to spend time on in CAD, refining some of the “rough draft” designs and adding in some of the extra details. Lots of progress there, but hardly done.


One bit I spent time on is the interesting valve adjusting arrangement that company co-founder and Chief Engineer Marc Birkigt employed. The tappet, or “valve adjusting head” has a threaded stem with screws into the top portion of the valve stem. The valve spring cup slides over the valve stem in such a way that it can move up and down but not rotate. Its top surface has serrations that made with matching serrations on the bottom side of the tappet. If the spring cup is pulled down against the spring, the serrations “unlock” and the tappet can be rotated to adjust the valve lash. Clever.

My version is not an exact copy, but I hope a reasonable and doable facsimile.

[Vixen]

Hello Ron,

I always thought the Birkigt “valve adjusting head” was a neat way to adjust valve clearance on a overhead cam engine. I also worried it would be too small and too delicate for use in our reduced scale models. The more traditional bucket and shim is not much better in small scale. So, what diameter are you planning for your the valve stem (inside the valve guide) and what thread are you proposing to run down the inside of that stem?

Cheers

Mike     

[RReid]

Hi Mike,

As drawn, the valve stem diameter is 0.10", with a 1-72 thread. That will leave a wall thickness outside the thread of about 0.013" (0.34mm). Small and delicate without a doubt. Call it an experiment, or a gamble, as you wish.

Answering the question makes me think, though. It would make sense to increase the upper valve stem diameter to 0.125" to take advantage of standard stainless steel rod stock, with only a(nother) small concession to fidelity. So "Thanks!", for triggering a further refinement.

[Vixen]

Hello Ron,

I'm not familiar with the UNF thread system, so I had to look it up. I find it is very close to M1.8 x 0.35.  Gee that's small, even in a 0.125" dia valve stem.  The small size of the serrations in the tappet heads and spring cups will be another challenge. Sounds like you need to make a sample or two to decide if the Birkigt “valve adjusting head” is actually feasible in model scale.

It would be nice if they could be made to work, as they would solve a lot of problems with any overhead camshaft model, which normally do not have the luxury of rocker arms and screw adjusters. I will keep a close eye on what you do, as I have a similar requirement on the Mercedes engines.

Cheers   

Mike

[RReid]

The small size of the serrations in the tappet heads and spring cups will be another challenge.

Too much of a challenge for me, which is why I punted on those. If you look closely at my rendering, you'll see that the spring cup has vertical bits sticking up. The cup will be a brass fabrication, and those bits will be strips of 0.032" (0.8mm) brass soldered in. Machining matching grooves into the tappets won't be too hard. There won't be as many as Mr. Birkigt's serrations, but I think enough.

The #1-72 is the smallest thread size I normally use, but I have used it often in my engines. In this case I may also do a fabrication, with a stainless steel machine screw or stud soldered into the stainless steel tappet head.
 

[Vixen]

The small size of the serrations in the tappet heads and spring cups will be another challenge.

Too much of a challenge for me, which is why I punted on those. If you look closely at my rendering, you'll see that the spring cup has vertical bits sticking up. The cup will be a brass fabrication, and those bits will be strips of 0.032" (0.8mm) brass soldered in. Machining matching grooves into the tappets won't be too hard. There won't be as many as Mr. Birkigt's serrations, but I think enough.

Yes, I saw those bars on the spring cup. As a minimum; you only need a single vertical bar sticking up from the spring cup to engage in any one of the grooves in the tappet head. A single vertical bar could be machined, easier and stronger than solder. The tappet head (with machined grooves) could be silver soldered to stock 1-72 threaded rod.       you're right, it could be do-able.

Mike

[RReid]

As a minimum; you only need a single vertical bar sticking up from the spring cup to engage in any one of the grooves in the tappet head. A single vertical bar could be machined, easier and stronger than solder.

That's a good point, and a better way.

[Charles Lamont]

How would you make the key that fits the slot in the top of the valve stem to ensure the cup cannot rotate relative to the valve? 

[Vixen]

Charles, Ron,

Part of one of the 0.032" (0.8mm) bars on the the spring cup could be made to extend inwards, forming the key.

Typical tappet clearance for a small model is only 3 or 4 thou". The pitch of a 1-72 thread (or metric equivalent) is 0.0139". The number of slots in the tappet head will determine the incremental adjustment. It looks like 12 slots will be required to achieve fine adjustment (1.2 thou" per slot). 12 slots looks to be the limit for a 1/4" tappet head.




The machining is probably do-able ??? But more of a concern, are the load reversals on the thread and the side loads created by the rotating cam every time the valve is opened. Thread wear and fretting will be an issue. At the very least, the valve internal thread, the tappet external thread and the spring cup key would need to be surface hardened (how?) to resist the wear and fretting. More practical in a full size engine than in a miniature model engine.

I think I will be looking for an alternative design solution for the Mercedes engines.

Mike

[Admiral_dk]

A very interesting Idea - but I honestly don't like the odds of it, not have a Major breakdown .... 

A shame that it isn't a simple idea to downsize BMW's 'Finger-Followers' - as they are adjustable with a simple tool (matching whatever head is on the 'screw').

Mike mentions Bucket and Shim - I believe that it was George that did similar, by Lapping the Bucket to the desired clearance (no shim).

Per           

[RReid]

Charles,
The render doesn't show it, but the spring cup is drawn with a key projecting below the top surface, much as Mike suggested above. As far as I can tell from the Hispano Suiza drawing, their design doesn't use a key, but just the lobe shaped center opening, which seems intended to engage the valve stem slot while allowing the tappet stem to pass through. I've included that feature in my drawing also, mostly as a place holder for the idea.

Mike,
I went through that same analysis before proceeding with the drawing, and came to the same conclusion. Six or eight slots is easy but with more limited adjustment, twelve is better for adjustment but getting crowded, though I believe still doable. My DOHC twin has acceptable valve lash with no adjustment available at all, just carefully made bucket tappets juggled around to give the best result. So I see any adjustability as an improvement. I drew in six slots as a starting point, the other options are there for when the time comes make the final choice. I imagine using my dividing head and a slitting saw.

Regarding the loading issues, I understand your concerns. I can't do much about the load reversals, other than have as many threads engaged as possible, but the spring cup was given a much deeper skirt than the Birkigt version, allowing it to always be partly engaged in the bore into the head with a relaxed running fit. This is intended to help mitigate the side loading. There is also the intended usage of the engine, which, based on my tendencies with the other engines means fairly infrequent, short runs, and no load. Given all that, I do think it will be interesting to give it a try. I can retrofit bucket tappets if it doesn't work out. Failure is an option I accept in this circumstance.

[Vixen]

Hello Ron,

I agree with you, once you have set the tappet clearance, by whatever means, they rarely need to be redone. The trick is to find the most appropriate 'whatever means'. On an engine with rocker arms, you have the adjusting screws.. easy. OHC engines with finger followers usually have some means of changing the position of the finger followers. Direct acting OHC, the cams usually employ shims in the bucket tappets to set the clearances. On your DOHC twin you selected the bucket tappets on assembly, to achieve the desired tappet clearance. I once owned a Coventry-Climax designed engine which required the correct shims to be selected from a very expensive selection box. If there had been a surface grinder available, I could have made my own.

Like you, I find something intriguing about the Birkigt screw threaded valve stem. I think it should be possible to come up with an alternative simplified design which combines the threaded valve stem with the simplicity of selected distance pieces. At the same time, eliminating the load reversal issues. Have a look at the rough sketch below:




The valve stem is tapped 1-72 as per you design. The tappet head, spring cup and threaded rod are combined to resemble that of the original engine and is screwed tight home into the valve stem, to eliminate the load reversals. A short section at the bottom of the 1-72 threaded rod has the threads turned off to form a short pin. The protruding pin can be shorted as required to provide the required tappet clearance. The resultant valve assembly will physically resemble the original, it does not suffer from load reversals, is infinitely easier to manufacture and adjust for the required tappet clearance. And will not require any fancy heat treatment.

Just a thought for you to mull over

Mike

[RReid]

Hello Mike,

That's definitely an idea worth considering, and I certainly will. Thank you.

[RReid]

This is a rendering of the simplified tappet arrangement as suggested above by Mike. For clarity I've sectioned the valve and the spring cup.

The tappet head, with threaded stem, is now integral with the spring cup. The gap showing between the top of the valve and the inner face of the spring cup may be reduced by shortening the length of the un-threaded portion at the bottom of the tappet stem, thus lowering the tappet head and increasing valve lash. I suppose tiny shims could be punched out and placed under the stem to move it the other way, should it be needed.

I think I'm starting to like this idea. As Mike says, it's stronger, still looks the part, and certainly would be much easier to make.


This morning I also found the US patent for the original invention, filed in June of 1915, awarded almost two years later in May of 1917. I haven't yet found a clear, definitive date for when the engine was introduced, but it was either 1914 or 1915. For anyone interested, I've attached a PDF of the patent below.

[Vixen]

Hello Ron,

That's looking nice. All the essential elements of the original Birkigt threaded valve stem design; together with a simpler and more elegant means of setting the overall length. It's interesting to speculate whether Birkigt would have evolved his own design in a similar direction, given time. Though perhaps he was too wed to the patent design to want to change.

Not sure about punching out tiny shims    I would be inclined to make a couple of extra tappets and adjust the length to fit with a light touch on the grind wheel.

Cheers   

Mike