Ask Paul: Why are holes larger than dowel pins?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I recently saw your article about Narrow Deck vs. Wide Deck and believe you are just the person to ask this question. A fellow AMT has been field overhauling his Lycoming O-360A1A engine. It is a “first run engine” out of an early Mooney. It is a Narrow Deck engine. (Yes, I know it is way past the 12 year time frame).

Here is the issue: The engine has small “stepped” dowel pins in the case half main bearing saddles (where the main bearings are installed). The removed bearings (original from factory) have small holes to match the small dowel pins. The new bearings (except for the forward {long} bearing) have much larger holes for the dowel pins. [Read more…]

Why mineral oil for break-in?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I’ve seen this in Lycoming literature — “Keep in mind that all turbocharged Lycoming engines must be broken-in on ashless dispersant oil only” — but why is it true? My engine overhaul shop, Central Cylinder in Omaha, Neb., insists, in writing, that for its warranty to be valid, I must break my turbo’d IO-360 in on mineral oil.

PAUL MILLNER via e-mail

A: This requirement has caused some confusion for Paul and I bet he’s not alone. [Read more…]

Is 172 with H2AD engine a good buy?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: Hi, I am a Mexican looking for my first plane. I found a 1980 Cessna 172 with an H2AD engine that has just 100 hours since overhaul. The serial number ends in 76T; I had read about 76A. Is it OK? Is it the same as the 76A?


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A puzzling compression problem

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: My engine, which now has 300 hours on it, is an O-290-D2B which had a new-limits field overhaul. It has settled-in as a nice smooth-running, economical and reliable powerplant with plenty of power. It gets about 7-8 hours per qt. (Aeroshell 100) at 6.5 gph. All EGTs and CHTs are normal and it always has a clean oil filter element (it is cut and checked at each oil change, 50 hours or six months), and has a healthy oil analysis report (no anomalies or unusual artifacts). However, I’ve noticed the compression going down at each annual on all cylinders and a recent pre-annual check-up has them in the low 60s (over 80) with a static compression test. It also takes a bit of “prop-rocking” to get it to settle in. I can hear some blow-by in the dip-stick tube. It seems uncanny that all cylinders would have this problem, so I’m a bit puzzled.

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Is it possible to convert my engine?

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: I have a Lycoming O-360-A1F6D that was originally installed in a Cessna Cardinal. This engine has since been used in an Van’s RV-6 and I will be installing it into a Van’s RV-8. It does not have a magneto with it as the previous owner used electronic ignition. I have not settled on an ignition source but do not want to go back to the Bendix D4LN-2021 dual mag. I recently noted in a General Aviation News article that Teledyne (TCM), now the owner of the Bendix mags, has stopped support for the dual mag. Is it possible to remove the “D” on this engine configuration by changing accessory housing and gears? Maybe it’s possible but not economical?

DAVE PARADIS, Hubbard, Ore.

A: I really wish you had asked me a different question because I’m afraid my answer to this one is not going to make you a happy camper. [Read more…]

Ask Paul: Are these spark plugs approved for my plane?

Q: Could you please help my pretty blonde wife and myself with our spark plug problem? We are desperate! Our mechanic is going to put the new fine wire plugs we just bought into our 1976 Cessna Cardinal RG, but we have conflicting information on the proper plug.

We bought some SR83P plugs from a dealer at Sun ‘n Fun, who assured us that the plugs were the same as the Champion fine wire plugs that fit our Lycoming IO-360-A1B6 engine, and that they cross-referenced just fine.


A: After doing some research of Lycoming Service Instruction 1042Y dated Sept. 1, 2009, which covers all “Approved Spark Plugs” for all Lycoming engines, I was afraid you may have a problem because there is no SR83P listed. I had a strange feeling about this, so I did some further checking with an old friend from the industry, Frank Gurko, who spent nearly a lifetime with Champion Spark Plug Co., then started his own company,, after retiring. He confirmed the SR83P spark plugs you bought are approved for your IO-360-A1B6 in your Cessna Cardinal RG and will work fine to replace your present REM38S plugs.

It would appear that there was an oversight by Lycoming during its last revision to SI 1042Y from earlier versions. As a matter of fact, the SR83P was shown as approved for your engine in SI 1042X, which I believe was dated in 2002. I discussed this with Lycoming and they will address this oversight when SI 1042 is next revised. So Jimmy, it looks like you’re good to go with the SR83P spark plugs for your engine.

I really appreciate you sending in your question because it reminded me of something I’ve been meaning to address. I’d like to make certain all readers understand that the outcome for Jimmy could have been much different had these spark plugs been the incorrect type for his engine. If incorrect plugs had been installed, there is the possibility that several nasty things could have happened and none you’d like very much. [Read more…]

Ask Paul: Can this be correct?

Q: My Tri-Pacer has an O-320 with a narrow deck (A) engine. It has SL32000N-A1 Millennium cylinders, which are are installed with no plates and splined nuts. Can this be correct? I have no wide deck cylinders to compare them to.

BLAIR MOHR, via e-mail

A: Let me see if I understand what you’re telling me: You have an old Lycoming O-320 Narrow Deck engine that has Millennium cylinders installed using the spline type cylinder hold-down nuts? Yes, it could be possible that these cylinders would be installed without cylinder base hold-down plates, which was done on the early low compression engines.

The spline type nuts have been long gone but, as you mentioned, apparently still in use. Should you wish at some point to convert to the more recent Cylinder Base Allen Head Nuts, this may be accomplished by complying with Lycoming Service Bulletin 213A, dated way back in 1957, but I’m not certain I’d go through the work. I think what you’ve got should work just fine.

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:

Is new technology worth the investment?

Q: I am about to change my Alfa Romeo boxer engine to a Lycoming O-360A1F1 on my experimental Glastar aircraft. I read your answer recently to a question if LASAR is really useful (Will my engine benefit from new technology? Jan. 12 issue). Its actual price is $2,500. Do you think that it is really worth this high sum?

My mechanic has doubts, saying he is not confident in electronics — that if the electronics don’t work, the engine will stop. He also says that standard magnetos usually always work.

G. FRANK, Italy

A: I appreciate the fact that you read my article on the LASAR system. One thing I overlooked when explaining the system is that any interruption of the three data sensors — manifold pressure, cylinder head temperature, or rpm — and the system automatically reverts to the fixed timing on the engine just like the LASAR system was not there. Therefore, the engine would not stop as a result of the LASAR system experiencing an electronic failure.

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Ask Paul: Why do my serial numbers differ?

A: I have a narrow deck configuration in my 1964 PA 30-270. The first entry in the aircraft logbook, made on Jan. 3, 1964, shows radio installation and LE serial #S or L 606-55 and the RE L 640-55. However the engine log books show LE L 938-55 and RE L 909-55 and the first oil change at 55 hours. Both are IO-320-BIA. I have not yet confirmed the numbers on the installed units — too many screws — and I don’t know the exact place of the info plate.Also, the N number was changed from N7373Y to N4009Y in 1966, but I have no idea why. Please help! I am baffled.

KARL ROESCH, Arlene, Montana

A: Karl, the difference in your logbooks is interesting and confusing, to say the least. The only thing I can think of is that the original engines were replaced at some point with the higher serial number engines. I am surprised, however, that this isn’t reflected in the aircraft log. [Read more…]

Ask Paul: The seal of approval

questionable seal

Q: We’re building an RV-6A using an O-360 narrow deck engine. There are six studs that hold this engine together. The center studs are wider at the base than the outer four studs. The case has an indentation where the seals fit. All six are counterbored. The size of the center studs at the base is slightly larger but the seal fits over the stud and rests into the counterbore OK. Should my gasket set have contained a slightly larger seal for these center studs or does this look OK?


questionable sealA: Thanks for your inquiry regarding the use of “O” ring seals on your O-360 narrow deck engine. After looking at the photos, it appears your crankcase is in compliance with Lycoming Service Instruction 1123D, which covers the “Installation of Dowels and Rubber O-Ring Seals at the Crankcase Thru-Stud Locations.”

While it was difficult to actually determine from your photos, the information you provided would lead me to believe your crankcase does incorporate the counterbores. I’d suggest you review a copy of the Service Instruction in order to actually confirm it. You should find this SI in your Avantext library. Also, any FAA approved repair station should have a copy of this publication if it maintains Lycoming engines. I think you’ll find after reviewing this publication the seal fit you spoke of is correct and no larger seals are required.

The RV6A is a very nice aircraft and I know you’ll enjoy owning and flying it.

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: