Thanks TJ. So you pulled all the cylinders to find the one with the overweight wrist pin?

I had an R-985 which right out of high $$$ overhaul had a shake. It was balanced by a well known balance service as part of the overhaul. Overhauler said it’s my ailerons or other problem I finally flew it without the cowling and baffles so I could see the valve covers shake. Tried hard, soft engine mounts, machined washers so each mount had the exact same amount of crush. Removed 3 blade Hartzell and tried two different Ham standards 2 blades with 6101 and 6167 blades different diameters. Problem never went away. I sent the engine back to change blower ratios 12:1 to 10:1. Sent the 3 blade Hartzell to prop shop. It was factory new when installed at first. Checked blade angles and static balance, Tracking was perfect. Flew without the spinner.

Vibration would follow prop RPM and not airspeed.

I flew the airplane to Hartzell in ohio. Engineers put velocimeter in cockpit and on enigine. Tried to balance and no luck. I flew it multiple flights to get vibration survey. They thought it was not propeller but in the engine. Spike at 1/2 crank RPM.

I flew it to another midwest engine shop, pulled one cylinder to check counterweights, All appeared correct, We tried prop balance but no luck. .02 ips solution is worse than no weights at all. .78 without weights according to balancer. Something like 67 grams needed. I’d have to look back in notes.

Had 5 different balancers try to balance it. No luck. Balancer just goes in a circle with weights. I tried aligned with and 90 degrees to the master rod

Had 3 different sets of mags. tried a 50 and two different 70 amp Jasco alternators, Tried different alternator drive ratios, Had 4 different carburetors, 3 prop governors, removed vacuum pump

I pulled all 9 cylinders and bought another set of pistons rings and wrist pins. I mixed and matched parts to get within a gram of the complete assembly weights and matched the components as best as I could before matching the totals. The originals were 11 gram spread to each other. The total weight of the entire assembly is unknown because the small ends of the rods are factored in and it is impossible to do this for an assembled R-985
It is slightly overbalanced normally. I have the formula but it’s in my notes. The engine has 3 dampers on the crank counterweights.


No good change. Engine shop refused to believe it. So 8 years and 500 hours later I bought a Dynavibe with spectral capability. I did ground runs and flew the airplane. Same problem as before it would wander around and never come to a solution or if it did come to a solution I could get to .02ips and it would shake so much worse I would have to take the weights off. But the balancer said it was great.

I tried velocimeter on the back of the case instead of the front. One time it seemed better but I could never get back to that even with the weights in the same spot.

The EGT and CHT are almost perfectly matched. Rich of peak or lean of peak it is perfect. Slides right in to LOP with no fuss if I want to and if too lean just slows down with no vibration. The takeoff and altitude power is fine and compressions are all 79/80 or perfect.

I have a theory. I notice a spike way on my spectral reports down at something like 1/8 crank speed. Dynavibe said it’s just ground resonance the balancer is picking up but.... The radial has a 4 lobe cam intake and 4 lobe for exhaust and if It has one bad exhaust or intake lobe. Or possibly it’s mis profiled. That lobe would hit on a different cylinder every 4th time at 1/2 crank speed. Since it’s a different cylinder each time it would not show up as a EGT or CHT indication because they all get it.

Is there possibly a problem with the cam ramping or other machining done for a T303 engine that is not known by the overhauler or cam grinder. Or change TCM has done to the cams that is wrong for the RPM range and manifold pressures the T303 operates at. Is there a difference between the normal and counter rotating engine cam machining that has crept into the specs unnoticed. There can’t be many of these engines overhauled each year and with the low initial production run some knowledge may have been lost.

http://www.nar-associates.com/technical ... screen.pdf

This is for a bottom induction Continental. The top induction T303 engine should be better tuned. We will find out when the 6 point CHT EGT is installed.

I don’t know which cylinders share a cam lobe for intake or exhaust. Wear there or even reground rockers could make a difference.

Of course it could be a piston or pin mismatch like originally thought due to the recent cylinder work. Very hard to pin this down unless something was done and the vibration started when it was smooth previously.

All the exhaust valves have their own lobe. Intake valves share.

Thanks Paul, It’s been a while since I had one apart.

Kirk "My yoke shake isn’t in pitch and my ailerons don’t flutter. I only see a bit of vibration in the left wingtip and static wicks”.

I have the exact same problem. The aileron static wick actually wore the rivet holes out on the bottom of the aileron and got loose so I had to move it to another location. It is just riveted through the skin with cherry rivets from the factory but there is vibration there.

Have you tried balancing the prop after the reindex to see if it is any better?

I haven't had a chance to re balance the prop yet. Flying home on Sunday I tried 24" and 2100 RPM (ala Mike Busch style) and while only doing 159 knots TAS at 22 GPH, the vibration actually seemed better. I think I need to fly in another 303 that the owner reports as "smooth" so I can better gauge how bad my vibration really is vs the rest of the fleet.