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Dropped a valve...

noquarternoquarterMember
Frowner
Yes, on the last test run before heading for Spa trip next weekend, cylinder 4 intake valve broke. It shouldn't, no high rpm, and fully warmed. As pictures show, broken rocker, 2 valves, piston with deep groove. Looking at the valve stem on the faulty valve, it looks to me like a mismatch between retainer and locks. I'll bring the head to a local machinist Monday, I'll want his view as well. When playing with a retainer and locks with no valve stem inside, the locks touch each other at the bottom, but there's 0.6mm gap at the top. Looks mismatched to me. The head needs a new seat and a valve guide, hopefully that's all.

The engine is a MME Racing 408 dynoed at 593HP. I called MME Friday, they were interested in helping solving the issue, but of course Mark wanted to see the pictures. I sent pictures, but I didn't hear back from them Friday, I'll call them Monday. The engine is 4 years old, but only has 4000 miles on it.

I'll keep you posted.

Sad pictures below
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noquarternoquarterMember
Visited a few machine shops this morning. Different views, some think that the valve locks are OK, and that the broken valve is from rpm++. I shift at 6000, occasionally 6500.

Will call MME shortly...

The local parts guys I normally use can handle stock engines, but not this. When I explained the guy what happened and handed him the exhaust valve to see if he could find a replacement, he asked "Is this the intake or the exhaust valve?" applause
Last edited by noquarter
noquarternoquarterMember
I've read that post and that was my initial thought that maybe this was the same. The machinists I visited today however said that it's been overrevved, he could feel at the back of the other valves that they had bounced.

I don't know, I want to get it running now, and winter time I'll take it apart and refresh it. With an agressive cam and 1.73 rockers, it'll never be a high rpm screamer IMO.
joulesjoules"Racing is Life, anything before or after is just waiting" - Steve McQueen
I previously had a long discussion with my engine rebuilder on rev limits, he bases it on max piston speed, so strokers naturally do not rev as high, you are relying on the higher torque (which is what you want anyway).

For a heavy duty engine with forged crank & pistons 4,000 feet per minute piston speed is the recommended max; for a 408 that equates to 6,000 rpm rev limit. This somewhat subject to valvetrain components being equally capable, but a good rule of thumb IMO.

Julian
George PGeorge PAdmin.
You are correct Ron.

The rocker arm failed (the roller broke off) first, not the valve. This allowed the end of rocker to straddle the valve tip and push down on the spring retainer. Pushing down on the retainer allowed the keepers to loosen, fall out of the groove and fall off the valve stem. Once the keepers let go of the valve, the valve dropped. The piston hit the valve & broke the head off.

Look at the bottom of the rocker & the top of the spring retainer, it is evident they were in contact with each other.

The question is, why did the rocker arm break?

-G
noquarternoquarterMember
Stainless roller rockers may be in the plans for this winter. Trade a little power for longevity might be a good idea.

Is the consensus for a 408 stroker max 6000 rpm? Earlier performance engines I've had lost a little power at 5500 so it was natural to switch gear. But this one feels strong all the way, so it's tempting to overrev. Maybe a rev limiter for my ICE ignition...

Thanks for all the suggestions, I'll try to deal with them all to avoid it.
lastpushbuttonlastpushbuttonMember
I hate it for ya mikael.. My 408 has ss steel rockers. It pulls hard all the way to 6200, Then I put in a 6800 chip. I have not hit it yet. I was thinking about a 7000 limit... but NOW, maybe 6800 is enough. Coil bind could put a lot of stress on the rocker. That is why the behive springs allow more compession of the spring as the center area of the spring is larger diameter than the top and bottom areas.

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