918-S

Some of you may remember I killed my 1980 euro spec "S" motor. I brought the damaged head in for repair. Considering I had these heads done 200 miles ago I was concerned about how much more these heads could be cut. After some research I managed to educate the shop enough to have them measure the thickness. Turns out the heads were cut too much even for the 1.4 mm gaskets. So I purchased a set of euro "S" heads to replace mine. Turns out these heads were cut too much also. They are at 23.1 mm the spec is 23.6 minimum thickness with a 1.4mm gasket.

After all the measuring here's what I figured happened to my motor. The shop that repaired my head before rebuild didn't check the heads before starting work. It is likely the heads had a prior pass across the cutter before he started.

One head was 23.6 mm on one end and the other end was at was at 23.5 WTF! The spec is 24mm stock and 23.6 min. thickness.

The head that broke the valve was at 23.1mm across the board. I used a stock 1.1 mm gasket instead of 1.4mm (which was still was too thin) I think I had a collision causing the damage. (Both the machinist and me!)

Ok so here's where I'm at now.

I'm having my head repaired. After the work to the head and the I think


So here's my thought, I'm shipping the replacement heads back to a very reputable vendor. (thanks guy!)

I'm having my damaged head repaired. After the repair I will have the other head shaved to match. I will run the thick 1.4 mm gasket.. I will have the piston notches cut for clearance.

After all this my question is: Does anyone think I will need to change to the adjustable cam gear setup or do you think off set keys will be enough?

mark kibort

I dont think the valves would touch, even without cuts! well, it would be close. actualy, what i measured was about .3" at TDC before the valves hit the piston pocket. (max lift on the cam is near .5") the idea is that the valve chases the piston down, and its a steep ramp, but i dont think the shaving of the heads would cause an impact. The gurus should chime in here.

mk

918-S

Thanks for posting Mark. Now let me see if I understand. 24mm stock, I lost .9mm. The cams may have been a quarter to half tooth off. I'm wondering if this may have contributed. Normally cam margin wouldn't effect this but maybe the combo would. Still thinking out loud trying not to repete the problem.

mark kibort

where is this measurement made? if i understand what you did, was the machine shop took near .035" off the heads. well, if i measured .3" before the valves hit the pistons, the shaving only brings the valves 10% closer to the pistons. i was off 2 teeth on the cam pulleys at a race at Laguna, (didnt run too well after this slipped) we fixed it and the car has been great ever since, so i dont think a 1/2 of a tooth pitch off would add to any issue.
again, i would consult with the experts here, but i dont see how 1mm of head shaving would do anything at all. again, when the piston is at TDC, the valves can drop .3" or more (a lot more) if its an early euroS . this is the worst possible condition,, the problem is if you only have .3" clearance and the cam pushes the valve down .5", well , you have a bent valve! however, if you dont have max lift or even half max lift at TDC, the pistons will not touch the valves.
if you look at the cam lobe position and the valve timing, it usually starts to open at TDC, and closes at near BDC. somewhere in between, it is at max lift.
If we use simple math, thats 90degreesATDC for max lift, and if it was linear, 45degrees for 50% of the lift (or .25") so you can see that at 45degrees after top dead center, there still could be some clearance and thats the extreme case. if you are off a tooth on the belt, thats 7.5 degrees on the pulley, or 15 degrees on the crank. I dont think the cams are at 50% of max lift (or near .25" at 15degrees past TDC. ) and even if it is, the piston has moved down as well.

mk



mk

918-S

Thanks again Mark. You have a good handle on how these things work. As I still work to understand what happened I can't help but think Porsche put a notch in the piston because there was an issue with the potencial for a valve collision. It would seem eventually through the timing of the piston moving away from the valve on the down stroke, the valve speed is faster than the piston. If you close the space the events will change dramatically for the worst. In addition if the cam gear is phased off by a half tooth this could excellerate the disaster. Am I on track here or with a head cut to 23'' and using a 1.1'' gasket do I still have enough clearance...

mark kibort

I guess the valve is going down faster than the piston, but it is still going down. this is the reason for the cuts, as well as giving the capabiltiy for the engine cams to be moved if the belt is not connected. on the S4, you need to have the engine at 45 degrees BTDC.

sure its close, but like i said, if you have a near .3" of clearance for the Valve at TDC, the question is does the valve move the entire .3" plus the .2" faster than the pistons go down .5"as fast as the piston . the answer is no, but you are asking where would the collision occur. I think most have found that 3+ teeth off can do this for our stock engines. so, i dont think that 1mm of shaving off the head is going to be a factor in a piston/valve collision as long as it was timed to spec.

still waiting for the gurus to chime in here.

mk

Randy V

Why is it whenever I see one of these 'thinking out loud' discussions, the danglerb alarm goes off?

Probably 'cause this is him, yet again?

918-S

What does that mean? I must have missed out on something. Are his questions too technical?

Randy V

Do a search here for that user name if you are curious.

918-S

Can we stay on post here? BTW: I see your a moderator. Your comment about danglerb wasn't intended as an insult was it?

Randy V

If the shoe fits...

No direspect intended - just thinking out loud.

918-S

What's your problem Randy? If you have a personal problem with me get it out in the open as long as your thinking out loud and trashing my thread with your off topic insults.

Aren't intelegent conversations allowed or are only cup holder and wheel up grades allowed?

Richard S

Geez Randy, give it a rest. 918-S installs a 928 engine in a 914, needs a little help in figuring out the engine failure so as not to repeat, and comes here for help. I appreciate the job you do as moderator (Thank you, Randy. I know it can be a thankless job) but a bit more tact as a moderator would help.

Rich

FlyingDog

That depends, are you using Amsoil?

Randy V

No need to get yer panties all in a twist. I was just making an observation.

Feel free to make a big deal out of it if you are compelled to do so.