O-320 H2AD 160HP ENGINE...Sorry.

My first thought  - the vacuum pump vanes. They like to make strange noises.

I've been on the phone with Lycoming and Penn Yan tech departments and they feel the cause may be the Rockers or Guides.  At least, that's a starting point and can be further reviewed without too much time and effort.  I'll let everyone know what we find out.  Thanks.

Bryan,

That was one of the ideas that we had early on but it is "not faint" and it seems to move with the cylinders within the engine top itself. 

In this case, it doesn't appear to be a steady noise in the vac pump area.  They're going to take a valve cover or two off and look at the rockers.

07100R.V. Wrote:
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> I've recently performed a pre-buy on a '79 172N
> and when we did the compression check we found awe
> strange noise???  It sounds like an old rocking
> chair when the prop is rolled through the
> cylinders.  The noise appears to travel through
> all the cylinders as the prop is rotated.  It is
> not a belt...but something in the engine that
> seems tight.  The engine has recently had some TOH
> work and cleaning done to the cylinders but no
> notes in the logs on rings.  Any ideas?

Hello Adem;

Has this been a problem since the engine was last overhauled?  if so, you may have a camshaft to crankshaft timing problem.(Please see my post from 6/25/2006). According to the POH and the Type Certificate Data Sheet for the 172N, the Static RPM should be between 2280 and 2400, with maximum cruise RPM of 2700 (Red Line). The Type Certificate Data Sheet is available on the FAA web site. The symptoms you described are what I experienced, which turned out to be the camshaft to crankshaft timing problem. The camshaft to crankshaft timing problem can be determined using the following procedure from the Sacramento Sky Ranch web site:

Crankshaft to camshaft timing off. Note: It is easy to assemble the Lycoming O-320H2AD with the internal engine timing 1 tooth off.

This condition may be checked by first disconnecting starter. Remove top spark plugs and rocker box cover on #2 cylinder. Turn engine to TDC on compression stroke #1 cylinder. Engine timing is checked by first observing the number two cylinder valve rocker arms. Both valves should be closed or nearly closed. Move the propeller slightly in one direction. Rocker arm motion should be seen as one valve starts to open. STOP. Now rotate the engine back to the original position with the #1 at TDC.. Now move the propeller slightly in the direction opposite from the first movement. Rocker arm motion should again be seen as the other valve starts to open. If the two valves started to open as described with only a small amount of engine movement in each direction, the engine timing is correct. If movement in either direction exceeds twenty degrees of engine rotation before motion of the rocker arm occurs, the crankshaft to camshaft timing is not correct and the engine was assembled incorrectly.

Check this before you attempt any repairs, as I spent a lot of money on a propeller overhaul, carburetor overhaul, magneto timing check, compression check, new tachometer, and other "fixes" that didn't solve the problem.

Hope this information is helpful.

Jon

The "H2AD" engine is just as dependable as anything that Lycoming makes """Providing"""" that the Serial Number is suffix "T". Without the suffix this design is sure to make for tiny pieces of metal in the filter or screen. The original design used a modified version of the Ford 302cid lifter which did not last. The "T" mod as they call it, increases the lifer diameter about two fold, which also requires a new or refurbed case. It is getting harder and harder to find an H2AD without the "T" mod. As far as the magneto goes, it is a bit more expensive to overhaul, but if properly maintained it is a pretty dependable piece. One thing about this magneto that is very often overlooked, is when overhauling this mag, it is prudent that the condensers be removed from the cap/harness and sent to be checked for condition. Most of the dual mag overhauls that I have seen were simply a bad condenser eating points.

Tom Wrote:
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> The "H2AD" engine is just as dependable as
> anything that Lycoming makes """Providing"""" that
> the Serial Number is suffix "T". Without the
> suffix this design is sure to make for tiny pieces
> of metal in the filter or screen. The original
> design used a modified version of the Ford 302cid
> lifter which did not last. The "T" mod as they
> call it, increases the lifer diameter about two
> fold, which also requires a new or refurbed case.
> It is getting harder and harder to find an H2AD
> without the "T" mod. As far as the magneto goes,
> it is a bit more expensive to overhaul, but if
> properly maintained it is a pretty dependable
> piece. One thing about this magneto that is very
> often overlooked, is when overhauling this mag, it
> is prudent that the condensers be removed from the
> cap/harness and sent to be checked for condition.
> Most of the dual mag overhauls that I have seen
> were simply a bad condenser eating points.