June 2013
Useful tips for understanding your airplane’s turbocharger.
We get calls every week from someone across the country regarding bearing play. During pre-buy, annual inspections and routine maintenance, the common practice is to reach into the compressor housing inlet, grab the turbine wheel shaft and give it a wiggle. You might be surprised to know that you can expect to find some play when you do this.
A little explanation is in order. As you can see by the photos (directional info here), the main bearings in these turbochargers look more like a bushing.
There are two items to consider when understanding proper bearing clearance: (1) the clearance between the outside diameter (OD) of the bearing and the inside diameter (ID) of the bearing housing; and (2) the clearance between the ID of the bearing and the OD of the turbine wheel shaft.
If you measured the bearing housing ID (.9832) and the main bearing OD (-.9780), the difference—that is, the clearance—of these turbo main bearing components would come to .0052. Similarly, a measurement of the main bearing ID (.6272) and the turbine wheel OD (-.6251) shows a clearance of .0021.
To find out the total clearance in main bearing components, add .0052 and .0021 and you’ll find your total clearance is .0073. So you see, it’s not surprising to find some obvious play in the turbo shaft.
But how much is too much?
The turbocharger overhaul manual has a special section in it devoted to this subject. The procedure to check this clearance is to remove the turbo; the task also requires use of an indicator set (P/N 6450 (English) or 6451 (metric) from Central Tools).
Remove the oil drain fitting and place the dial indicator plunger down through the oil drain port of the bearing housing. Rest it on the shaft of the turbine wheel. Work the shaft back and forth toward the dial indicator, then away from it, to obtain the total play.
Most small AiResearch and Kelly Aerospace turbochargers have a minimum of .003 and a maximum of .006 to .008 of total clearance. See the table (directional info needed) for specific information about your model.
Let’s say we took our measurement inside the bearing housing and came up with .007. If you multiply that times four inches out to the end of the shaft, it’s easy to see how you could have .020 or so of play out at the end.
A quick (and unofficial) check would be to take up the play in the shaft with your thumb. As you move the shaft to the side of its play (where it normally would stop), rotate the compressor wheel. If you don’t feel the wheel scrape against the compressor housing, your bearing clearance is okay. This technique is not meant to replace the Approved Data, but it can be a quick and very general method of assessing the play.
Once you know the factors involved in calculating bearing play, the procedure for accurate measurement, and the published numbers, you too will be able to discern how much is too much.
Gary Main started working at the family FBO after school and on weekends. He obtained his pilot license in 1976 and became an A&P in 1979. Since then he has had extensive aircraft maintenance experience, including turbocharger and systems troubleshooting repair and overhaul. Main is instrument single and multi-engine rated. Send questions or comments to editor@www.piperflyer.com.
Resources
Main Turbo Systems, Inc.
mainturbo.com
Central Tools Inc.
centraltools.com
