Ask Paul: Wide deck vs. narrow deck engines

I recently received a question from a friend of a friend in Texas regarding the difference between a “Narrow Deck” and a “Wide Deck” Lycoming engine. This has been confusing to many, so let’s see if I can shed some light on the subject.

The difference between a Narrow Deck (ND) and a Wide Deck (WD) configured engine is easily determined by checking the specific engine serial number. The WD serial numbers end with the suffix “A,” such as L-0000-36A on an O-360 series WD compared to L-0000-36 on a ND.

There are a few exceptions to this, such as the O-320-H series and O-360-E series, neither of which have the suffix “A” but still contain WD cylinders. The design of these cylinders is considerably different from conventional engines, so there shouldn’t be any confusion once you’ve looked at them. The rocker arm configuration alone would be enough to alert you that they are not interchangeable with a standard cylinder.

One of the most noticeable differences is that WD cylinders are retained to the crankcase using a standard hex nut, while a ND cylinder utilizes a barrel type, internal spline nut with a “banana plate” that is installed over the cylinder flange for additional integrity.

The best way to tell the difference is to set a ND cylinder and a WD cylinder side-by-side on a work bench. The difference you’ll spot right off the bat is the thickness of the flange. This is considered the “deck” and the WD is thicker.

My guess is that this design came about when Lycoming began to increase horsepower and compression ratios of its engines. Prior to the WD cylinder, most engine compression ratios were 7.30:1, with none higher than 8.70:1. This, coupled with the fact that most TBO times were no more than 1,200 hours, worked out just fine. The WD cylinder was one of the things that allowed Lycoming to increase the engine TBO times over a period of time.

In 1963, when Piper introduced the PA-30 Twin Comanche, it used two IO-320-B1A engines, which used the new WD configuration. There was never a PA-30 delivered with anything but a WD engine. That set the stage and the WD cylinder was phased in on other Lycoming engine models from that point.

In the eyes of the FAA there is no difference between a ND and a WD engine in any of the Type Certificate Data Sheets. Therefore, it’s conceivable that on a Piper Navajo PA-31 you could have a TIO-540-A2C WD engine on the left and a TIO-540-A2C ND engine on the right.

Can a WD engine replace a ND engine? The answer is yes. You will probably have to trim some of the engine baffling to fit over the slightly larger WD cylinders, but this can be accomplished while still maintaining the proper fit for the cooling requirements of those baffles. What you can’t do as easily is install a ND engine in place of a WD engine unless you’ve got a “baffle stretcher,” which I’ve never even heard of, let alone seen, but you get my point.

Since I mentioned the TIO-540 series engines, I’ll alert you to one small thing when swapping a ND engine for a WD engine on this series. You must be aware that the prop governor gear drive ratio is different between these two configurations. This doesn’t cause any problems as long as you advise the facility that is overhauling the prop governor so it can make the necessary changes internally in the governor.

This may apply to other six-cylinder engine models, but none come to mind right now, so I’d suggest if you are confronted with this situation, contact either the prop governor overhaul facility or Lycoming Product Support before the work begins.