When A Valve & Piston Kiss.

Twin Cam Valve To Piston Clearance Check.

Building a standard twin cam is a reasonably straight forward process, but when the decision is made to step away from a “Standard engine build” there is a need to be aware of many things in order to avoid potential problems.

The aim of this twin cam build was to obtain a larger capacity engine utilising an L block, 1600 cross-flow stroker crank, 83.5mm pistons, larger valves, high lift cams, and a special rally pack of valve springs and retainers, purchased from Tony Ingram in the USA.

Once the pistons and rods were installed for the first and initial dry engine build, the first action was to ascertain the actual side clearance between the edge of the inlet valve head as it entered the cut-out pocket in each piston,
beginning with number one piston set at TDC, this is an important check, because with a high lift cam lobe, the valves are going to spend quite a bit of time “down” in the pockets during the critical stages of the 360 deg rotation of the crank shaft.

What We Found Surprised Us.

To carry out this test, the recessed valve pockets were painted with engineers blue lay out die, then the inlet and exhaust valves were assembled into the head with some very soft, hand compressible springs (Not the normal valve springs), the head was then torqued down with an old head gasket to act as a spacer.

With pistons set at TDC on number one and number four, each valve on one and four is then pressed down into the pocket twice, then the crank is rotated to bring number two and number three to TDC position, and the valve actuation process repeated again.

After removal of the cyl head, we found that the edge of each inlet valve was just kissing the piston on the outer edge of the inlet valve cut-out or recess, which was plain to see as the very thin coating of engineers blue had been scraped from the side of the pocket, this was simply far to close for comfort in any engine builders book, that’s something you definitely DON’T want to entertain inside of the engine.

The pistons in question were Aussie made by JP Nornda. As it turned out I kind of wished that I had not decided to use the JP pistons, while they were designed to fit the twin cam lotus engine, and nicely machined, they are simply not on the same manufacturing level as CP, JP, Mahle, and many other imported high performance pistons.

The overall piston weight was another concern, which was corrected by milling the internals with a ball end mill to remove excess weight, then of course weight matching and careful balancing had to be carried out, the operation turned out to be a lot of work which we would have preferred not to do. But once bitten, twice shy, as they say, we would definitely NOT recommend JP Nornda pistons for a Lotus twin cam engine rebuild. This may seem to be a bit harsh, but lets just say that I was expecting a better product for the money spent.

The alcohol based lay out blue is an excellent clearance indicator, because when it becomes dry after application, it will scrape off and leave bare metal even when very light contact is made between two moving parts such as a valve and the edge of the valve cut out in the piston. Bearing blue could also be used, but may not indicate as well as engineers lay out die.

Luckily, with all of the exhaust valves, there was no sign of any valve to piston contact, there was plenty of built in clearance. (Edge of pocket as well as pocket depth).

At first we used plasticine as an indicator, but that was a bit useless, so we resorted to the engineers layout blue, and its quite surprising what the blue dye showed up. If an engine build was carried out as it was, it would have lasted about five minutes after first start up!

The pockets only required a smallest amount of metal to be removed, and to cure the problem requires that each of the four pistons be mounted on a custom made holding fixture which is set up on a rotary table, which is then clamped and aligned on the table of a vertical milling machine so that more metal can be machined out of the OD of the pockets to provide that little bit of extra clearance for the edge of the valve, because, as any engine builder will tell you, pistons have a tendency to rock a little (Even new engines) in the bore between the change of stroke direction, and we certainly did not want to risk that.

As my friend Rohan said to me, you have to check for proper clearance in critical places such as this, if you don’t check, you could be asking for big trouble down the road. Imagine if the problem wasn’t detected, or it was simply over looked, the engine build was completed. I’m sure there would have been some serious issues pretty soon after start up of the new engine.

The process of of fabricating and engineering a special holding jig to mount onto the vertex rotary table took several hours, but it enabled very accurate re-machining work to be carried out on the valve recess pockets in the vertical milling machine, and by doing the work in house meant that we had total control over the machining work.

We also recorded a video of the jig and fixture and the machining work we carried out, you can watch the video below to learn more.

The writer has had more than thirty years of Lotus experience, which pretty much covers every category of work that one may encounter during a rebuild or restoration, but he is by no means an expert, the thing with the Lotus elan is that you simply never stop learning about them. Listen carefully to the experience of others, there is always a new trick or hint to learn here and there.