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DanH 12-29-2008 11:05 PM

Baffle Mod
Lycoming cylinders have a classic baffle problem. I don't know if anyone else has taken this particular approach to solving it (lots of ways to skin a cat), but maybe the pictures will help the new guys understand the issue.

Here's the root of the problem. The intake side of the head has no fin depth in the area indicated by the pencil. but standard baffles for the left front and right rear cylinders place a plate directly against this area. There is plenty of air at "A", but there is no way for air to circulate down to area "B"


Here's the standard baffle (right rear) against the head. Flow to the lower fins and baffle wrap is blocked by the zero-depth fin area identified by the green tape. The standard "cure" is to place a washer or some other spacer between the head and the baffle, but that opens a gap along the entire length of the baffle particular the area outlined in a black rectangle above. Any air sneaking past the area in the rectangle is pure leakage.

It is quite easy to hammerform a small duct of sorts in the baffle plate. The "duct" is nothing more than a fancy dent knocked in the sheet.

The result is a passage to bypass the no-depth fin area (arrow). Now air can flow down the back of the head and pass in between the lower fins where you have a baffle wrap. The reminder of the baffle plate is not spaced away from the head and cylinder, reducing undesired leakage. Any air not passing between fins is pure drag; it didn't do any cooling work.

Not all builders have metal forming hammers, or may not want to try beating on an expensive baffle part as their first metal forming experience. I'll do the left front baffle using an alternate method and get it up here soon.

Have fun.

----->>>>POSTSCRIPT: See posts 24 and 25

carguy614 12-30-2008 05:29 AM

Baffle Mod
Great idea Dan. For sure, the clearance is an issue, as I lost 25 degrees just by adding a washer to the back of that baffle. This looks like a much more elegant solution. I'll give that a try on my next one.

Thanks for the tip!!


Captain Avgas 12-30-2008 03:44 PM


Originally Posted by DanH (Post 284884)
I'll do the left front baffle using an alternate method and get it up here soon.

A really interesting topic Dan. I'm looking forward to your front baffle solution. Very timely for me....guess what I'm working on.

RV7Guy 12-30-2008 04:59 PM

Great Idea
I used a couple of larger spacers. I really like the idea of hammering a dent to make it work. How great to get to use a hammer in a productive, non destructive manner:)

DanH 01-01-2009 09:00 PM

Here's the alternate method. Lots of ways to skin this cat.

Cut a hole in the baffle, carefully deburr it, then make a spacer (0.065" to 0.080" thick) and a cover plate:

When assembled, you have a duct to bypass the no-depth fin area:

There is a structural brace (a length of angle) above the inlet ramp and across the face of the cylinder. Space it forward a little so it doesn't block the duct.

Here's the other side:

The 90 degree flange on the cover plate is important.

Dimple and countersink as necessary, rivet flush on the cylinder side.

Wicked Stick 01-02-2009 10:01 AM

Great Post Dan. !

aerhed 01-02-2009 05:32 PM

A dent in the baffle, BRILLIANT!

Rick S. 01-02-2009 07:23 PM

Great you lay awake at night thinking of these great ideas/fixes? I love your posts!! Even when the flames are flying!!

Tom Martin 01-03-2009 05:55 AM

Another method, that I have used many times, is to rivet a piece of 1/8" flat bar to the outside of the area that Dan is modifying. The flat bar goes in the location where the mounting hole to the cylinder is. It is about a couple of inches long. This is a simple modification, solves the cooling problem, and strengthens the baffle at the attach point.

DanH 01-04-2009 06:39 AM

Thank you everyone for your kind comments.

It is an air-cooled engine worth more than my first house, so really good baffles are worth thought and careful fabrication.

BTW, I can't say if a 1/16" or an 1/8" or any other particular dimension is "correct" for this duct. Can't even say if the left front and right rear cylinders should have the same duct size; different pressures in their respective plenum areas. I am sure an install which doesn't allow any air to a whole section of the cylinder head finning is bad, and likewise, all leaks are bad.

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