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Date:         Sat, 14 Dec 2002 16:18:01 -0600
Reply-To:     John Rodgers <j_rodgers@CHARTER.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Rodgers <j_rodgers@CHARTER.NET>
Subject:      Top Overhauls
Content-Type: text/plain; charset=us-ascii; format=flowed

In my aviation days, engines given a "normal" life span, were rated for so many hours of operation and when that limit was reached the engine was shipped to a certified remanufacturing facility torn down completely and rebuilt back to original factory specs, in which case the engine would come back with a "yellow tag, indicating factory new specs, therefore one could expect factory new performance and lifetime. There were other types of rebuilding with less than factory spec that would still extend life, but that is not for this discussion.

Along the way from "zero" operating hours to life limit, there would be a time to do a "Top Overhaul" to extend to the full lifetime of the engine. Depending on the FAA rules involved in the operation where the engine was being used, one could do this top overhaul at the specified numbers of hours of operating time, or as indicated by oil consumption rate and compression tests. The top overhaul consisted essentially of rebuilding the top end,ie, the case was not opened. The cylinders, pistons, rod ends, rings, valves, guides, seats, rockers and shafts, all mike and components adjusted by replacement or repair to bring back to within wear limits. At the time of a top overhaul, a crankshaft runt test would be made with a dial indicator to determine the amount of wobble in the crankshaft. If that check was in spec, then the case was left intact. If not, then it was time to crack the case for overhaul, and that would include the case and all internal components.

But back to the matter of a top overhaul. On the WBX, is it an advantage to consider the half life of the engine, and do a top overhaul to get the full life that the crank and crank bearings would/could provide if the cylinders are kept up to snuff, or would it be best to just run it to the point the cylinders give up and then do a major rebuild including tearing down the case.

I am curious about this. I have often heard it said "Don't re-ring the thing or do a valve job, as the new compression will wear the mains and crank throw bearing out faster then you will have to rebuild it anyway. I'm not so sure about that, given its a common practice to top overhaul opposed cylinder aircraft engines.

I seek input of experiences from the list membership.

TIA.

John Rodgers 88 GL Driver


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