Engine failure analysis

[Robert Shafer]

At a recent Vintage race at Sebring I had an enginr failure with significatant power loss. After returning to the paddock I found I had noexhaust from the right side , 4-5-6. Checking further I found broken intake rockers on 4&5, and broken exhaust on 6. The camshaft nut was also of the cam, the sprocket was loose and the sprocket pin,intact, was laying in the bottom of the chain housing. Luckaly a leakdown test shows I still have decent numbers, 4-5% cold on all cylinders. Their is a good chance the engine was over reved on a down shift 4-5 hot laps prior to the failure. My question is if the over rev caused the failure why did it run OK for 5 additional laps?. Thanks for your help Bob Shafer

[Grady Clay]

Bob,

Most race engines with very high compression allow the valves to hit the piston squarely and, in many cases, don’t bend the valves.

In your case I suspect you had very stiff valve springs so the over-rev didn’t float the valves so they didn’t hit the pistons then. However the sudden acceleration can unscrew the cam nut. I suspect it took the five laps until the nut became sufficiently loose for the pin to fall out and the cam timing to change leading to the valves hitting the pistons. The good news is you have cast rockers that broke.

If the engine were mine, I would do a maintenance rebuild and replace all the valves on the 4-5-6 bank. I think rod inspection and replacing rod nuts & bolts critical. I would also carefully inspect the cam & related parts, replace the chain and replace the three bolts in the jackshaft. Sprockets are also susptct.


The cost of maintenance rebuilds is insignificant compared to a blow-up. On my 8300+ engines I do a maintenance rebuild every 25 at-speed hours (I add up lap times, in & out laps don’t count). All I do is touch-up the valves, replace the top ring, new bearings, rod bolts & nuts and a gasket set. I carefully inspect the 2.0 rods (polished and shot peened) with magnaflux, X-ray and die penetrate. I have never found a problem with the three identical engines. I also never had a failure.

Best,
Grady

[RWebb]

8300+...

that means rpm -- not the number of motors that Grady has blown up....

[Robert Shafer]

Grady
Really appreciate your input, had thought I might get away with only replacing the rocker arms but after your comments have decided I better do a rebuild. Thanks again Bob Shafer