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'78 3.0 rebuild, interesting mess

Have a 78 3.0 that was supposedly rebuilt by a reputable builder less than 20K miles ago. After checking leak down I knew I had a problem with the engine. Pulled it and found 4 of the 6 with broken rings, pistons caked with carbon etc.

After cleaning the 1/8" thick carbon off the pistons I find a nice half moon dent in each of the pistons with the same radius as the valves, the obvious suspect with this evidence. Here's a pic.

My thoughts are that something bad happened, maybe tensioner went and valve timing skipped, which caused all the pistons to bump a valve and leave a mark. If it was a current issue there would have been evidence on the carbon build on piston face. Guy rebuilt the engine using same pistons. Comments on my theory appreciated.

I would like to use the Euro pistons that I have and the current 143/144 Cams which I think are S cams. Second question is - I thought I read "S" cams are higher lift which maybe caused this valve issue initially with Euro pistons? Or is the answer obvious, go with a 964 cam and it fixes everything?
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Old 04-04-2018, 07:32 AM
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Obviously the exhaust valves hit the pistons at some point. Do you have the build sheet from the last "rebuild"? Probably new valves at that time?

Was there any leakage out the exhaust when you did the leakdown?

"S" cams will not work with stock SC pistons, as there is not enough clearance in the piston. "S" cams will not work with CIS either.

Broken rings is another matter. What cylinders do you have and what shape are they in?

FWIW, IMO, I would not believe "rebuilt by so and so" without seeing a build sheet. Heck, I still have the build sheet from 1985 on my old motor. YMMV.
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Old 04-04-2018, 09:28 AM
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Agreed with Gordon. I've seen low mileage Carrera 3.2 pistons & cylinders I bought used that were covered in carbon and revealed smiles like that on all six, but not as prominent. I believe mine were a mild mechanical overrev that was ignored and the car had to be driven a fairly long time after that for the carbon to build up and cover the doinks in the piston tops. Couldn't have been driven too long because the P&Cs were said to have only 15K miles on them before the car was totaled and based on the condition of them (ignoring the undisclosed smiles under the carbon) I believe the mileage was as claimed.

Since your valve contact is much greater than I found on mine, I would think your issue was much more severe. One could suspect the engine was torn down and fixed (bent valves and new guides?) and then back on the road again. I don't think it's a tensioner failure because both would have to fail to get valve contact on all six pistons. Remember, the valve timing on each bank of three cylinders is independent of the other due to there being two sprockets, two chains, two tensioners.

How do the piston skirts look? Heavy contact like that cocks the piston and then the skirt scrapes the cylinder wall.

The broken rings could be careless installation? It's easy to crack a ring if you're not gentle with guiding the piston into the cylinder with the ring compressor around it. Especially with a split ring compressor. Which actual rings are broken would also be a telltale of the problem. Improper ignition timing will also break rings.

It's a common misconception that the S cams have more lift and that's why they won't work with certain pistons. The S cams actually have just about the same amount of lift as the so-called "mild" SC/Carrera cams. The issue with the S cams is the duration which dictates how long the valves are open and the camshaft timing spec at TDC overlap point. There's also the narrower lobe separation angle that has the S cams experiencing a longer period of time where the intake and exhaust valves are both open. Take a look at the cam specs courtesy of camgrinder/Dougherty Racing



When the valves are open the same distance/lift for a longer amount time (larger number of degrees of camshaft rotation) then there is greater opportunity for the valves to be in contact with the piston. Hence the reason the S pistons have valve reliefs in them and the SC & Carrera pistons do not. Granted, the SC and Carrera pistons are an assymmetric wedgy horseshoe sort of shape vs. the S pistons being symmetric domes. But the duration and the timing spec is the issue here, not the lift.



It's not a direct comparison when looking at early S pistons vs. the later SC pistons because the S heads have a smaller combustion chamber diameter with a taller roof. So that allows for a different piston dome.

You should check what the cams are because that's obviously an important thing to match with your pistons and you need the timing spec. That numbering convention of the cams sounds like a Webcamshafts number but I think it's an older one because they no longer list it in their current online catalog. A call to them could dig it up for you.

Camshaft technical information will make your brain hurt

http://www.elgincams.com/campaper.html
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Old 04-04-2018, 11:05 AM
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Wow, Thanks for the great info. A few bits of information:
- the cams are 911.143.00 and 911 144.00, based on the end stamp. Who know if thats accurate
- The piston above is the worst, all have small smiles from a valve. All were caked with carbon
- I had the heads reworked by a very reputable shop. Only one issue with valves, one of the valves was not ground all around the seat. Weird. I don't know which one it was.
- Shop inspected all the cylinders, said they are perfect. They said Use 'em. They are Mahle
- I only remember cyl #1 as having good rings, 4 of the other 5 were bad.
- Pistons are definitely Euro 95mm, all in good shape and no skirt damage
- Broken rings could have been from bad install, timing, airbox issues, etc.or all of it.
- According to Waynes book the cam p/n I have are from a 74-77 CIS or S. My engine s/n is from '78 as a side note.

I'm certain the engine was torn down and rebuilt not many miles ago and to be honest they could have used the Euro pistons, with the smiles, at the rebuild. That damage may not have happened on that car or engine. Who knows.

So since this is what I have and I got it cheap, cheap, I want to at least do it right. From what I'm reading I need to check these cams and make sure it matches the numbers or pull them and regrind to 964 for great performance.

Just need to make sure these parts work together to save me another tear down.

Thanks for you comments.
ken
Old 04-04-2018, 06:29 PM
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Do you know who did the rebuild?
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Old 04-05-2018, 05:16 AM
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My mistake. I misunderstood your description of the cams and made an assumption based on how Webcamshafts names their cams. The later 911S that had CIS has a much milder profile in terms of duration and lift. It's actually the first cam listed on that Dougherty image I posted above. So now that we've confirmed your cams, those combined with your Euro pistons shouldn't be the cause of your valve contact

Check with Dougherty about the regrind potential. He may require a different profile to use as donors to grind the 964 profile. This is because your CIS S cams have less lobe separation (110 degrees) compared to the typical candidates like the SC/Carrera profile (113) and they also have quite a bit less lift.

Here's another tidbit of info I recently found from one of our forum members here. Nice info on how he does heads at his shop. http://www.exotechpower.com/36401.html

Well you should be OK with the pistons and cylinders if they check out ok visually and measure OK. Make sure to measure the cylinders with a 0.0001" resolution bore gauge to check for ovality and calculate your clearance.

T
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Old 04-05-2018, 02:30 PM
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Mark, yes I know who did the build and I reached out to them before I dug into it. They wouldn't respond. They are on Rennlist and this site.

Kevin - I will check P&C again but those were also checked by my engine shop. I will talk to Dougherty, extexh and Webb.

Thanks for your great information.
Old 04-07-2018, 05:32 AM
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update.
all pistons are cleaned and look great and confirmed to be Mahle 3.0 Euro with 9.8 CR. I think the right choice is to go to 964 Cam. From what I have read the 9.8 CR is stretching things a bit with low quality gas and could have caused the detonation, carbon buildup and the ring destruction.

Should eliminate any deck height issue and be a great little engine when done. Anything I'm missing.
Old 04-23-2018, 07:44 PM
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If you saw my other post re: the Web 964 cam, I'm moving forward finally. Anyone have any comments on doing a 3.0 with 95mm 9.8 CR Euro pistons and 964 cam?
Old 11-12-2018, 06:24 PM
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I think that would be a fine combination. However if you are worried about detonation and you want to lower your comp, I have some 8.5 3.0 pistons that I would be more than happy to swap with you.

However I am sure your combination should work perfectly.
Old 11-13-2018, 12:05 PM
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Quote:
Originally Posted by jellonailer View Post
TTT
If you saw my other post re: the Web 964 cam, I'm moving forward finally. Anyone have any comments on doing a 3.0 with 95mm 9.8 CR Euro pistons and 964 cam?
Sounds like a great combination. Just make sure your timing is not too advanced.

It would be a very good idea to actually measure and calculate your actual compression ratio. Slight changes in deck height and valve depth can make difference. You wouldn't want to end up above 9.8 IMO.

I am running JEs at 9.8-1 with no issues. (yet)

An exhaust backdate will really wake up your motor, if you are still running stock SC stuff.

Yea, I also have a perfect set of 8.5 pistons in a box....
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Last edited by Trackrash; 11-13-2018 at 02:15 PM..
Old 11-13-2018, 02:08 PM
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Thanks Gordon for the input. I was actually going to post a question regarding deck height and CR. I thought I read somewhere that I can double up the metal cylinder gaskets to get slightly less compression, did I mis-remember reading that?

To add to your comments I have SSI's and Dansk exhaust, and they are Mahle pistons.

As a last wild card I may go PMO's for the hell of it.
Old 11-13-2018, 03:58 PM
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Yes, you can double up the base gaskets, but they make thicker ones which would be a better solution. Measure with one gasket first to get a baseline.

Bruce Anderson's Performance Handbook has a section on measuring and calculating the compression ratio. Kind of fiddly doing all the measurements, but IMO, necessary for a build like yours.
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Old 11-13-2018, 07:47 PM
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If you are using Mahle 9.8 pistons, you don’t need to worry about thicker gaskets. They won’t give you 9.8 anyway.

-Andy
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Old 11-15-2018, 06:10 PM
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