One of the defining features of pre-war Bugattis is the monobloc construction of the engine’s top end. In an earlier article, Keeping the Fire Inside the Motor, I explained the monobloc concept, along with the more commonly found design of a separate head casting and gasket. Alfa Romeo, Maserati and others produced engines with both designs, depending on the type and purpose of the engine.

Bugatti never varied, sticking with the monobloc from the first car, the Type 10, to the last Type 57s that trickled out of the factory after the war.

The Evolution of Bugatti’s Monobloc Design

The earliest Bugatti engines were four cylinders with a single block casting integrating two valves per cylinder. An overhead cam, driven by a tower shaft and bevel gears, operated Bugatti’s unusual banana tappets – curved bars with rounded ends running in babbitt blocks.

The next generation design moved to four valves per cylinder while retaining an evolution of the banana tappets.

When the Type 30 straight eight appeared, the banana tappets were replaced with much more effective finger followers, pivoted off shafts running parallel to the cam. Valve count dropped to three per cylinder, two intakes and one exhaust. This system carried through all the Type 35 variants until Bugatti decided to go to twin cams, reportedly after having had a good look at Harry Miller’s Indy-winning engines (which themselves were patterned after Peugeot’s Type L3 from 1913, winner of that year’s Indy 500).

The four cylinder blocks, both 4-valve and 3-valve, used valve guides that threaded into the block. With the twin-cam Type 57, Bugatti decided to not thread the guides, instead engineering a light press fit. This may have been an attempt to improve heat transfer from the valve into the cooling jacket, as there is more contact area on the pressed-in guide versus the threaded one.

Because the 90° included angle between the intake and exhaust guides causes the valve centerlines to intersect the cylinder bore, there is not enough room to install the valves through the guides. The valve must be inserted up through the larger guide bore, then the guide is slipped over the valve stem and driven into the block.

Getting this interference fit right is part of the art of building the Type 57 engine. Too loose and the engine will smoke as oil seeps past the guide’s outer diameter. Too tight and the valve job and assembly process becomes excruciating. The guides go in and out multiple times in the setup process, typically removed with a forked lever that fits through the port and pushes the guide out the top. But each time a guide goes in and out, the press fit is worn a little looser… so you must start with it on the tight side if it is to remain correct when finished.

The guide bores in the iron typically stay the same size, while guides with oversize shanks are turned down to fit. The difference between too tight and too loose is a very small amount of metal, perhaps .0005″, so it’s a fine margin to work within. Of course, it is possible to glue the guides in on final assembly if there is a concern about oil seepage past a loose fit.

Spark Plug Cups and Casting Architecture

Another major change from the single-cam engines is the centrally mounted spark plug, located between the valves rather than through the side of the block. This hemispherical combustion chamber design is one of the major improvements from the previous generation of Bugattis, and it is why so many winning cars of the era used the same design.

To manufacture this, Bugatti designed a spark plug cup that is pressed into the open cooling water jacket at the top of the combustion chamber. I’m not sure why they couldn’t have done this as Alfa and Maserati did, for the 8C 2900 and 8-CTF respectively, in one solid casting around the spark plug. It makes for another potential water leak and it is a time consuming job to fit new cups and seals.

I also find it curious that Bugatti decided to go to one long casting for all eight cylinders, while the previous designs used two individual four-cylinder blocks. Perhaps to reduce machining costs? Again, Alfa and Maserati went with twin four-cylinder blocks for their straight eights of the period.

Common Problems and Cracks

A regular issue with any pre-war Bugatti is due to the oil drain-back tube design: a copper tube inserted into the casting with a tapered steel wedge driven in at the top and bottom to expand the copper and seal it against the iron. These are frequent sources of leaks, putting coolant into the engine oil. Care must be taken when fitting new tubes and tapers, as too much force can crack the block at the taper.

Cracks between the spark plug and the valve seats are occasionally a problem. The potential for this varies greatly based on the casting. Some old blocks used a 5/8″ spark plug thread, which leaves very little room between the thread and the valve seat. The 14 mm plugs give a wider band of iron that is less prone to cracking. There may be a few special blocks out there that were made to use a 12 mm spark plug to allow even larger valves.

The block in the pictures that follow had cracked in multiple cylinders through the exhaust seat and was welded, an extremely tricky job to work at the bottom of that long cylinder! Re-cutting the seats with a hand-held Neway cutter, carefully whittling away the weld bead, was a slow exercise of precision and patience.

A Block Failure On the Dyno and More Seat Work

The engine illustrated next was making good progress in dyno testing with the older block casting that the project came with. The owner wanted to chase power and we had two piston seizures in that search for more horses. It had a larger volume blower from a Type 50 and our initial pistons were in the high sevens compression ratio range, working under about 7 PSI of boost. It ran on race fuel and we also tested alcohol jetting, on which it produced close to 300 horsepower.

The latest piston revision lowered the compression ratio a bit and had coated skirts, with another increase in piston-to-wall clearance. Nearing the end of the dyno session, a chunk of iron blew out of one of the chambers and opened a hole into the cooling jacket, bringing everything to a swift halt. That block was quite unfortunately now scrap metal.

A new block from Crosthwaite & Gardiner arrived after some time, and all of the parts were test fitted to the new casting. The as-delivered valve seats were slightly smaller in diameter than the ones in the old block, and fit poorly on the valves we were running, high up on the valve face. The seat needed to be opened up a hair to place the contact closer to the center of the valve face.

The Neway cutter set of three comes into play here. A special pilot with a tip that grabs the cutter body from below is inserted through the guide. The 45° cutter is sent up the bore with a long needle nose tool and fit onto the pilot, then turned against the seat to increase the diameter of the existing face. Then the 60° cutter takes a small corner off the inside to narrow the now-wider seat and move the contact band down the valve face. The shallow 30° cutter then clips the outside of the seat face to move the other edge of the seat up the face.

This is a somewhat tricky operation, as you only want to remove enough seat material to achieve the goal. If you go too far, there is no going back without fitting a seat insert, a much bigger operation.

This new casting came with single-angle seats, just the 45° face. Adding the two small cuts on either side of the 45 helps airflow past the seat, the classic “three angle valve job”. In addition, the as-delivered block has the main port entry to the seat bored in a straight operation, and the intersection with the as-cast port has a sharp edge that can disrupt airflow. Breaking this edge with a carbide burr and a little smoothing with a sanding roll improves flow a little more.

The Monobloc Didn’t Die with Bugatti

Although Bugatti as a company did not survive long past World War II, the fate of the monobloc concept was not sealed with the last Type 57. Harry Miller’s initial design that inspired Ettore evolved through the years with versions made by Offenhauser continuing to win at Indianapolis well into the 1970s with turbo boost. Johnny Rutherford took the engine’s last win at the Brickyard in 1976.

In total, monobloc Millers and Offys won the Indy 500 thirty-nine times between 1922 and 1976. In fact, most of the winning engines up to that 1976 finale were monobloc engines, from the 1913 Peugeot to the Maseratis that won in ’39 and ’40. It is probably harder to list the winning engines that included a head gasket in their parts inventory.

If you happen to be in the market for a new Bugatti cylinder block, Crosthwaite and Gardiner currently lists a fully machined unit for £8,725 ($11,803).