I Sherlock'd My Dead Big Block Chevy and Found Out Why It Broke!

Summary

The video details the process of diagnosing issues with a 632 cubic inch big block Chevy drag boat engine that failed during racing. After tearing down the engine and inspecting the parts, evidence of metal-to-metal contact between the exhaust valves and pistons is discovered, likely causing bearing failure. Measurements confirm lack of proper piston to valve clearance. The engine issues stem from a rushed rebuild prior to competition where clearances were not properly checked. The rebuild compromised power, reliability and ultimately caused catastrophic failure. While rebuilding again is possible, a different engine will likely go into the boat for the next race season.

Takeaways

• 😟 The 632 cubic inch big block Chevy engine broke at a drag boat race event last year, so Cleetus is finally tearing it down to diagnose the failure
• 😞 There were ongoing valve train issues with the heads that were never fully resolved, likely causing component failures
• 🤔 The crank, rods, and pistons were replaced when converting the engine for nitrous, but clearances seem to be incorrect
• 😟 Bearing damage indicates debris circulated through the engine, wearing down components prematurely
• 😟 Evidence suggests exhaust valves were contacting the pistons, damaging both
• 😔 All of the exhaust valves are bent from contact with pistons and guides are damaged
• ☹️ The lightweight NA parts were never meant for the rigors of nitrous and are now compromised
• 😕 Proper clearances are critical, the installed parts had insufficient piston to valve clearance
• 😐 The engine builder is talented but time pressures led to overlooked measurements
• 🙂 Despite engine issues, Cleetus still had fun racing with friends and learned a lot diagnosing failures

Q & A

What type of engine is being worked on in the video?

A 632 cubic inch big block Chevy engine.

What happened to the engine that caused it to break at the drag boat race?

The engine dropped a valve which caused it to lock up and break.

What mods were done to the engine to prepare it for nitrous?

It got a new crank, rods, and pistons.

What issue was discovered when disassembling the engine?

There was evidence of the exhaust valves contacting the pistons due to insufficient piston to valve clearance.

How did the lack of piston to valve clearance manifest itself?

There were circular wear marks on the pistons that matched marks on the exhaust valves.

What other issues were found with the engine?

The crankshaft bearings showed uneven wear indicating a possible issue with the crank flexing or taper in the journals.

What will need to be replaced to fix the engine?

The valves, pistons, and possibly the crank will need replacement. The cylinder heads may also need work.

Why did the engine builder miss the clearance issue originally?

He was in a hurry and didn't have time to check all the clearances before assembly.

What was done to measure the existing piston to valve clearance?

The engine was reassembled with lightweight checking springs and the clearances were measured with a dial indicator.

What engine will go in the boat next instead of rebuilding this one?

A proven reliable engine built by Sonny already sitting in the boat will likely be used next.