View Full Version : Another "How would you make it" question.

11-05-2003, 01:14 AM
Reading Peter S' post about the diving pump crankshaft reminds me of a bit of machine work which has baffled me for many years. I once came across an old Bugatti engine, possibly a Type 23, and probably made between about 1921 and 1926. The cam followers on this were banana shaped, i.e. circular in section and bent into an arc of a circle. I can see how they would have been made, but HOW would they have machined the guides in the OHC cylinder head to fit the banana shaped followers in 1921 (and how could it be done now)? IIRC, on the racing versions of these engines the cylinder heads were not detahable, but were cast in one piece with the block.


[This message has been edited by franco (edited 11-05-2003).]

Doc Nickel
11-05-2003, 04:31 AM
Presumably a single-point boring bar, with the boring head on a sort of parallelogram linkage. As the bar advances, the head is moved both forward and tilts. The arc traversed would be something like that of a car's unequal-length A-arm suspension.

Perhaps simpler still- the same boring head on a long pivot arm whose radius matches that of the cam follower.

Or, the hole was bored considerably oversize, and filled back to size and shape with babbit.

The real question is why? What did that gain, other than maybe clearance for some other part or component?


Peter S
11-05-2003, 08:27 PM
I dug out my book "Bugatti, Le pur-sang des automobiles" by HG Conway, it gives some details on these engines.

The first of the 8 valve 4 cylinder engines appeared in 1910, the cylinder and head are one-piece, they had vertical valves actuated by a single overhead cam. The curved tappets were required to allow the one cam to operate the vertical valves.

Here is a description of the engine taken from a journal of the day.

"A novel feature of the valve actuation is the employment of curved tappets. These are of oblong section about 1/2 in. by 5/16 in., and are so curved as to bring them up to a suitable position under the cam shaft for actuation. The manufacturing difficulties involved in this construction have been very simply overcome by arranging the tappets in pairs, each in a bronze casing into which white metal is run with the tappets in position, so that the necessary accurately fitting curved slot is obtained without machining. As the valve tappet guide casings carrying the white metal are cast in pairs and bolted to the main camshaft housing, they are handy for the white metalling process. The tappets themselves are probably machined in the lathe in the form of complete rings and split off into sectors of the required length."

How did you work on the valves of a fixed-head engine? Here is the continued description of the engine.

"The camshaft and valve tappets are arranged in a seperate housing or casing, covering the entire cylinder block. The inlet valve is in a detachable seating, but the exhaust valves seats direct in the cylinder head. It will be noticed that it has been made slightly smaller than the inlet in order that it may be assembled or removed through the inlet valve port.

By 1920 the engine was running 4 valves per cylinder, and the diagrams show the same idea, 4 curved, rectangular tappets allowing the SOHC to operate the vertical valves. Each set of 4 tappets is contained in its own little housing which is bolted into the main cam box.

These were the engines that began Bugatti's great run of racing sucess.

Later engines like the Type 35 straight eight still had fixed heads and vertical valves, with two inlets and one exhaust, but used horizontal levers or rockers, pivoted at one end.

It wasn't until around 1930 that Bugatti went to inclined valves with twin cams, this time just two valves per cylinder, still with fixed head.

This is a whole interesting story in itself, because Bugatti aquired two of the Miller front-drive cars which had gone to Monza to race in 1929. The twin cam Bugatti engines probably copied the Miller design. The two Millers sat in storage at the Bugatti factory until being re-discovered and purchased by Griffith Borgeson in 1959. One of these cars now resides at the Smithsonian.

Doc Nickel
11-05-2003, 11:33 PM
"White metal" meaning babbit, I assume?

I don't know about the tappets only being 1/2" long and 5/16" in diameter though... That would only give the cam a usable "lift" of about a quarter-inch at the absolute most, probably less.


11-06-2003, 01:14 AM
Back in them thar days, .250 lift was HUGE! These were the "racing" engines of the past. Most engines had valve lifts of .160 to .200. That could be why they went to four valves.

11-06-2003, 02:04 AM
I believe the 1/2 x 5/16 refers to the cross section, not the length.

11-06-2003, 03:29 AM
Peter S

Thanks for that - it has finally cleared up the mystery for me after about 50 years! A reasonably simple solution. I think the engine I saw was a 2 valve per cylinder one, so was probably even older than I guessed. I never saw the camshaft cover off, but had the (wrong) impression that the tappets were of circular cross section. I did read that valve clearances were adjusted by adding or subtracting shims to a thimble on the end of the tappet.

I also read that one reason for using that design of tappet was to minimise side thrust on the valve stems.

I still wonder if there is any way to bore a curved circular hole though!


[This message has been edited by franco (edited 11-06-2003).]

11-06-2003, 04:23 AM
<font face="Verdana, Arial" size="2">Originally posted by franco:

I still wonder if there is any way to bore a curved circular hole though!


This might help answer your question.


[This message has been edited by Mech (edited 11-06-2003).]

Doc Nickel
11-06-2003, 06:22 AM
Nice to know I can think of something somebody else thought of a century ago. http://bbs.homeshopmachinist.net//biggrin.gif


11-06-2003, 07:44 AM
The machine and the engine mentioned but not shown are examples of the work being done at this time, with what is to us primitive tooling.
Add to this the fact that many of these guys did not have 12 years of schooling. I did not say a High School education because so many of today's graduates cannot read and write, let alone add and subtract.
I always have to laugh when someone says he could not have done this or that without CAD or CAM, or cannot live without a DRO.

Al Messer
11-06-2003, 07:49 AM
J.C., I heartily agree. All they have to do is look at the first 150 years of the machine age and check out the accomplishments.

11-06-2003, 09:19 AM

Thank you for that link - now, having seen the drawing, I understand how it can be done. I could see what Doc was getting at, but couldn't visualise how they would have driven the boring bar. I've been surprised several times recently at the complexity of some of the machinery that was becoming available in the 1860s.

Regards, franco.

11-06-2003, 08:45 PM
I am fortunate to have several old books on machining and from what I have gathered from reading them more has been forgotton about machining then we now know!

Peter S
11-11-2003, 03:21 AM
Here are some images from the Bugatti book mentioned above, shows 8 valve and 16 valve motors with their unusual curved tappets.



11-11-2003, 02:43 PM
Thanks for that. I was having some difficulty envisioning those parts. What an odd solution! I would have the cam bear directly on one row of valves and use rocker arms for the other row.

I once overhauled an AC Bristol sports car. It used a single overhead cam bearing on the valves with cups on top of the valve stems using shims to control the clearance. What a PITA that was. Torque everything down, measure the clearances, remove, re-shim, repeat until acceptable. I point out that removing the cam to obtain access to the cups and shims meant breaking the timing chain.

[This message has been edited by Evan (edited 11-11-2003).]

Pete Burne
11-11-2003, 05:01 PM

You have to be into British sport cars to appreciate the "bucket and shim" design. Jaguar used it in their dual overhead cam XK engine from the '50s thru the '70s. It also helps if you can tolerate an oil spot on your garage floor.

From an XKE owner.


11-11-2003, 05:51 PM
I am so proud of my wife (sometimes- the dern fool bought another Jaguar). Any way I blew her engine. V12 Jaguar with shims. She tore it down to crankshaft and rebuilt/replaced everything her self. As final step she set up the valve train with a dial indicator. Measured the travel, selected from her assortment of shims (she had many because I holed three pistons and drove a valve side ways through the head - thus one new (used) head was for spare shims. I took a bunch of shims to work and had them ground to her specs. The men at work had bets about her abilities and the ground valve shims. EVERY valve was near the desired valve lash.

Bucket shims are mean but no worse than the old flat head fords that had to have stem ground to adjust. That meant, remove heads, valve keepers, springs and grind stem-if i remember correctly.

11-11-2003, 06:10 PM
Why do Jaguars have two seats?

One for the driver and one for the mechanic.

Peter S
11-11-2003, 06:39 PM
Talking about Jaguars and buckets, the 6 cylinder engines had a bad problem with the stationary sleeve (in which the bucket ran) coming loose in the cylinder head and starting to go up and down with the bucket. When they came up far enough, the cam lobe would start grinding away on them, so you also end up with an engine full of metal particles.
The same engine I worked on (4.2 litre) would also blow head gaskets between cylinders, bores were a bit close together....

A great book which traces twin cam engines is "The Classic Twin-Cam Engine" by Griffith Borgeson (out of print).

The Grand Prix Peugeots starting around 1912 were using DOHC and 4 inclined valves per cylinder, roller main bearings and dry sump in 1913.

The classic cup-type cam follower was used by Ballot on their suberb 3 litre straight-eight race cars in 1919.

Apparently Fiat twin-cams in the 1960's came up with a method of changing shims without removing the cams, but I haven't seen how this works.

11-12-2003, 08:30 AM
Peter S,

Not far from where I live is an as-yet-unrestored Maudslay car of about 1910 vintage. The engine has a single gear driven overhead camshaft. The cylinder heads are not detachable. Each valve sits in its own cage, which can be unscrewed from the engine to work on the valve.
The camshaft bearings are hinged in such a way that the camshaft still in the bearings can be swung clear of the valves so they can be worked on WITHOUT disturbing the valve timing. Pity some of the modern designs which use thimbles and shims for the valve clearance adjustment haven't copied this camshaft arrangement!