Over rev at DE...
 

'74 911 2.7 MFI

I think I have a problem. It looks like I over revved on a downshift at the track. After a valve adjustment and a compression check, it appears only one valve is bent. Is this even possible, to bend only one valve? Motor ran great, no problems, and I can't afford at this time to rebuild. Is it feasible to go in and replace only one valve? Anything that really needs to be done? I know the list goes on and on for the "while your there..." but I'm on a tight budget.

Any suggestions or input greatly appreciated.

HI the big end bolts require replacing, if not expect a bigger bill and a dead engine, pull the top end off and the cylinders, with a bit of jigary pokary you can get the rods off the crank without striping the case, see the I MISHIFT AT THE GLEN thread.

regards mike

I'd second the recommendation to go with better rod bolts unless they've already been upgraded - as there's a good possibility one or more of the stock ones got a little hammered and could result in later catastrophe. No way to really know, though. ARP - $220 or so a set for the 3.6 - rock solid bolts that will take over-rev much better. Very easy to replace with the case intact believe it or not - I just did mine. Just need a little bendy-magnet tool and steady hands.

Easy enough to just bend one valve - probably had the weakest spring on it or something, or the peak rpm only had time to bend one of them - whichever was open at the peak, etc.

You may find more issues once you take off the heads, p/c's.

When you have the engine apart you should be able to see which pistons hit which valves. If you have good leak down on the other cylinders then you probably have only the one bent valve. Yes you can just replace that one IMO. It would be smart to replace the rod bolts as has already been pointed out. I'd use stock on that engine to save money. IMO only engines with the smaller rod bolts (9mm I think) need to have ARP or equivalent.

-Andy

ONce the heads are off it makes ense IMHO to r and r all the valves..

and get them checked for run out.

Even a light kiss will easily put them off..and in many cases they can be saved by a recut on a good machine..

Do not even think about skimping on new rod bolts btw...

Kind regards
David

HI do NOT skimp on a bent valve either, and if it is bent, it's bent, getting it re-cut will NOT straiten a bent valve, put a new one in otherwise order a head, cylinder and piston, con rod and poss crankcases (depending on the RPM the head of the valve drops off at).

regards mike

That is why its called the MONEY$ shift. I have been down the same road a couple of years ago. Unfortunately, there is NO "cheap" solution. The cams, and heads need to come off, etc.
Once you're in there, you will be in a better position to decide what to replace ie. new valve springs, rod bolts, rings, valve guides etc.

Michel

Yes, it is possible to only bend one valve. If you don't want to be down for the season, then you can rebuild just that head relatively easily, however, some of the other comments (about rod bolts, etc.) should be heeded for a long term solution...

-Wayne

I've heard of cases of a slightly bent valve, where the spring pressure could basically close it by rebending it just a tad... The valve head eventually breaks off from the repeated stress.

Replacing one valve vs. all of them is a risky proposition and would probably save you some $700-$800. Park the car and skip the beer for a few months, and save that extra money up.

Additionally I would ABSOLUTELY replace the rod-bolts after an overrev big enough to bend a valve. If they lose any clamping force from stretch, insert new motor here.

FWIW, I'd say that the whole point of removing all the valves is to check for tiny amounts of bend.: and have the opportunity to freshen the seats..

The really bent one is easy.

However, there may well be others which look fine, no wobble to see.

But the Serdi valve cutter will reveal tiny runout..this is actually almost always present in an end of season rebuild race motor if you listen to the sound of the machine when recutting..the first second only you can hear it..or you can pick it up with the dial guage.

Note: I'm taking no more than one or at most two thou here..many rebuilders would miss this and not regard it as important enough to mention...it will be taken out almost instantly by the stone..

IMHO such a valve will be as good as new for practical purposes after cutting...

Kind regards
David