Kissed pistons

les_garten · 03-26-2005, 08:23 AM

Hello,
I rebuilt my 1985 930. During the process I changed the cams to SC cams and kept the same Stock Pistons and Cylinders and re-ringed them. During the break in process I had a sudden stoppage of the car with poor running afterwards.
I pulled down the motor and found four kissed pistons. There is a strange pattern to the imprints on the pistons. The right and left side of the motor have mirrored damage patterns.
Pistons 2 and 5 have exactly the same impact mark as you can see on # 2 in the picture. Pistons 3 and 6 have identical marks and the are shallower than the marks on 2 and five, but as I said, the marks on 3 and 6 are identical are also.
Pistons 1 and 4 are untouched. The only valves affected are these 4 exhaust valves. To be perfectly honest I don't fully understand the valve timing on Porsche's and feel that I screwed up the valve timing.

My questioin is: Has anyone seen this pattern before.

jpnovak · 03-26-2005, 08:49 AM

Did you measure valve clearance?

les_garten · 03-26-2005, 09:02 AM

We used the stock 3.3 pistons and at the time felt it was not necessary.

Tinker · 03-26-2005, 09:03 AM

SO you are saying it was idling and then just stopped? Or did you miss a shift and over-rev it?

What did you time the cams at? Did you reset the valve spring heights for the SC cams?

Tinker

les_garten · 03-26-2005, 09:08 AM

The spring heights were set to 34mm. The cams were timed at 1.4 I believe. It's been a while since we did this. There were no missed shifts, just driving down the road at 70. I'm mainly interested if this was a cam timing issue because I'm not certain we timed them correctly. I'm also interested in the pattern of damage and if anyone has seen this? Like why not all 6 pistons?

Tinker · 03-26-2005, 09:18 AM

From my memory the SC/Carrera cams have been timed between 0.9 to 1.7mm depending on the year.

If you did in fact time them at 1.4mm at TDc overlap, you should have been fine. Somethings screwie.

Unusual to see a mirror pattern on both sides which would lead me to think the cam timing was off.

Tinker

WERK I · WERK I's Garage

les_garten,
I rebuilt the 930S engine in my car 2 years ago. It already had a SC grind in the engine, but the original pistons were retained. There should not be any valve clearance issues with that grind or spring height setting. The only thing that comes to mind is improper cam timing procedure with that type of symptom. Is it possible that you timed the cams on the wrong side of the cam lobes? (down ramp vs. up ramp)
Is this a Porsche SC camshafts or aftermarket "SC" camshafts? If so what type? What are their specs for timing?

les_garten · 03-26-2005, 09:34 AM

I'm obviously going to do something different this next time. I'm doing 3.4L 7.5:1 and 964 Cams. I believe the spring heights are 34/33.5 for Intake/Exhaust. I'll also figure out how to time these Cams or get someone to do it for me. I'll also have to check Piston to valve clearance this time.

les_garten · 03-26-2005, 09:39 AM

I purchased the regrinds from Stephen at Imagine Auto. The 964 cams are reground on my original Porsche cams from EBS racing. The spec from Imagine was 1.4 to 1.7. It seems I may have had the cams retarded, correct?

WERK I · WERK I's Garage

Quote:

Originally posted by les_garten
I purchased the regrinds from Stephen at Imagine Auto. The 964 cams are reground on my original Porsche cams from EBS racing. The spec from Imagine was 1.4 to 1.7. It seems I may have had the cams retarded, correct?

Retarded seems logical. One other thing....were the valves adjusted properly?(e.g. valve lash) Pardon my bluntness, but is it possible you either not not adjust the lash for 2,3,5 and 6 exhaust valves or maybe did not tighten the lock nuts on the lash adjusters?

les_garten · 03-26-2005, 10:34 AM

The lash was checked for sure. Also there is a distinct pattern here that I believe would not result from incorrect lash.

WERK I · WERK I's Garage

The thing is, cylinders 1 and 4 have the same valve timing as the others. So 1&4 should have the same symptoms as the others, kissed pistons.
When you said you "had a sudden stoppage" of the car was that because of vehicle impact or the engine suddenly stopped?
Did this occur when the engine was up to operating temperature for the first time?

john walker's workshop · 03-26-2005, 10:58 AM

i see that a lot on turbos. even ones that still ran well and didn't bend any valves. generally from an over-rev.

WERK I · WERK I's Garage

Quote:

Originally posted by john walker's workshop
i see that a lot on turbos. even ones that still ran well and didn't bend any valves. generally from an over-rev.

Yep,me too. My engine had the tell-tale signs of an over rev on the piston tops. The pistons had been machined to remove the protrusions resulting from the collision done by someone on a previous rebuild.
Even though a valve interface check isn't really necessary with a 964 or SC grind on these flattop pistons, you should always check your deck height on all cylinders prior to mounting the top end. You might have been on the shallow end of the tolerance.

les_garten · 03-26-2005, 02:25 PM

The stoppage was after about 700 miles of break in. I was driving on the interstate at about 70 mph.

strokher racing · 03-27-2005, 12:46 AM

I have a set of intake and exhaust valves and set of 3.4 cylinders if you decide you are in the market

Eric Hood

les_garten · 03-27-2005, 02:32 AM

Thanx for the offer, I just got all the parts together. What are the chances that I did any rod bearing damage? These marks are not huge. I am just worried about dinging the bearing or bushing. Can the rods be pulled and the bearings and bushings changed without splitting the case?

Les

WERK I · WERK I's Garage

If you had 700 miles on the engine, I don't think the incident occurred because of an incorrect cam timing procedure. If you were going only 70MPH the engine was probably just over 3K RPM. Normally not an engine speed where the valves start floating. Unless you accidently downshifted into first......

What work was done to the heads prior to engine assembly?
Did the rocker arms for the affected cylinders break because of the impact? Can you show us more evidence with pics?

"To be perfectly honest I don't fully understand the valve timing on Porsche's and feel that I screwed up the valve timing."

Is this your first 911 engine build? There may be something basic you may have overlooked that lead to this event. Or there may have been something done to the engine on an earlier rebuild that masked the problem. Do you have any history on the car and engine from the previous owners?

les_garten · 03-28-2005, 05:20 AM

There is no other damage to the engine. This is my first 911 build. I've owned the car for 14 years. The engine was stock and nobody had been in the engine before.

Rob 930 · 03-28-2005, 11:07 AM

If you'd driven 700 miles with no apparent problem (and there was no over-rev), then suddenly this, I think you've got more going on than incorrect valve timing. Did it run well prior to the stoppage? If so, that would further support that valve timing was not the problem. It sounds like you had some sort of sudden failure that caused cam timing to be lost. I'd be looking at cam chains, sprockets, chain ramps, tensioners, intermediate shaft gears and so forth. Sounds to me like something let go in the cam actuation train.