Benson bearings

[DanR]

In the Benson engine manuel it gives information on converting the mac drone engine to AX bearings the manuel only describes replacing the rod bearings if I am not mistaken, is there any bearings on the crankshaft that need replacing? What was wrong with the original bearings? and how would one tell if he has AX bearings or the old style?

[Alan Coats]

Dan,

R&D Aeronautical Engineering carries parts for Macs. Their catalog has a section on parts that should no longer be used, and why. Their catalog shows an e-mail address of harold.smith.30gfe.net.

Alan

[DanR]

Thanks Rick,

I will have to find a catalog some where.

Also would anyone on this forum, running a McCulloch just happen to remember what jet size they run (Holley 1920) ? Thanks

[mikezcnc]

DanR,

Just for the record, the Holley 1920 has to have a main jet replaced when used in a gyro application, as per Rick and sonnyj.

Any Holley performance shop stocks them although they won't be of much help when you ask them specific questions.

The part number is 122-67 BUT, notice that it means that hte part has a 0.068" diameter!!!! What it means is that Holley part numbers do not reflect the diam,eter although they are tracking each other. You need a 0.068" jet which corresponds to a p/n 122-67. You will also need a gasket. I got mine froma carburetor rebuilder shop for free.

While I am at it, the air filter that fis the 1920 carb is from a local parts store: made by Dynachrome, 4: OEM Style. Fits perfectly but you need to replace a provided clamp with a regular automotive hose clamp: Chinese are great masters of copying and the provided clamp looks very similar to a stainless hose clamp BUT functionally it is USELESS. Mike

[scottessex]

Dan, originally the Mac engine was designed for a military target drone aircraft. I was made to run flat out until it got shot out of the sky. (not made for long term use)
BUT certain things can be done, and upgraded to make a better reliable engine. That is what's "wrong" with the other bearings.

[C. Beaty]

There were 3 styles of connecting rod big end bearings used on a Mac.

The earlier ones came with bronze caged bearings which had 3 needles per compartment.

Later Macs had a full complement of uncaged needles.

The AX style bearings were custom made for Bensen by Torrington and used steel cages with one needle per compartment.

The bronze caged bearings had a propensity for breaking up and destroying the engine.

The full compliment bearings weren’t too bad.

The AX bearings were best but probably are no longer available.

[Rick Whittridge]

I use the full complement uncaged bearing in all my motors with great success!

[gyromike]

I also use the uncaged bearings in mine.
No problems so far.

[C. Beaty]

The idea behind caged bearings is that the rollers can be guided near their ends, reducing roller-to-roller friction and can be kept running true rather than skewing.

I have also used full compliment bearings without problems.

[DanR]

Ok fellers thanks, I had the cylinders off at the time I posted my question on the bearings, while i had the cylinders off I thought it would been nice to know if I had the right bearings. If I understand yall right you can do away with the cages and and run just the needle bearings by themselves? I will try to find AX bearings as I have several mac's and want another for backup. Been dusting, polishing and rechecking everything on the gyro after its sitting in back of hanger all winter, I have a sack full of different jets found a .068 and installed it and defeated the power jet, its ready to ground run and (gulp) fly!

[C. Beaty]

Another point sometimes overlooked is that if the cylinders have been rotated, the wrist pins also must be flipped.

Bensen used exhaust ports up but muffler installations require exhaust port down orientation.

The wrist pin is hollow and only one end is plugged.

If you look through your exhaust ports and can see the unplugged end of the wrist pins, they must be flipped. Hot exhaust gas enters the wrist pin and makes short work of the small end needle bearings.

[Timchick]

I'm planning on rotating my cylinders to do an exhaust system. Where can I find new cylinger gaskets?

[gyromike]

I'm planning on rotating my cylinders to do an exhaust system. Where can I find new cylinger gaskets?

R&D Aeronautical Engineering.
1-903-527-2006 Phone.

[Rick Whittridge]

Dan, Try the loose silver tip bearings , instead of the AX . they work fine!The best rod to use is the copper plated A rod that is centered on the crank journal. The E rod is allowed to float at the crank journal & is centered on the crank by the piston by thrust washers on both sides of the rod at the wrist pin.This combination does not work well causing wrist pin bearing failure due to the close fit at the wrist pin & lack of oil to the wrist pin bearing. I have modified the piston & thrust washers to correct this problem with good results! I can do this mode for anybody that is interested .