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Damaged Piston

 
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CMXXXI  



Joined: 05 Nov 2002
Posts: 1939
Location: Vicksburg, MS

PostPosted: Tue Nov 11, 2003 7:02 am    Post subject: Damaged Piston Reply with quote

While waiting for the replacement cylinder head to arrive I did some wondering and worrying about this damaged piston I discovered. JL924T has been so kind as to sell me a used replacement piston that is in his used parts stash, and I've decided to replace that piston.

As you can see in these photos, it's pretty chewed up because of a broken valve. What I found disconcerting is that ridge that walks along the dish wall. I initially thought that it was a crack, but it is a raised ridge. Between the big gouges, that bothersome ridge and the deformation caused by a piece of the valve stem (semi-circular dent at piston edge), I think the smart, safe thing to do is to go ahead and replace it.
..(Click for larger images)
Since the engine is out, and I've decided to replace it, it didn't make sense to me to NOT go ahead and rebuild the engine. New Deves rings, new main & rod bearings, new seals and new gaskets are all on order. It'll be another couple of weeks before I'll have all the parts in-hand, so it looks like getting the car running again is a New Year's target instead of Thanksgiving...

I'll keep the Board posted on my progress.
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'79 Eurospec 931
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Joes924Racer  



Joined: 03 Nov 2002
Posts: 11966
Location: Oregon, Denver Colorado native!

PostPosted: Tue Nov 11, 2003 8:27 am    Post subject: Reply with quote

good idea.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.
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wdb  



Joined: 02 Nov 2002
Posts: 2024

PostPosted: Tue Nov 11, 2003 9:59 am    Post subject: Reply with quote

are those pistons from an N/A or a turbo engine . I thought the 931 pistons had valve reliefs in them
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Lizard  



Joined: 03 Nov 2002
Posts: 9379
Location: Abbotsford BC. Canada

PostPosted: Tue Nov 11, 2003 10:22 am    Post subject: Reply with quote

these are from a 79 931 w/ 7.5:1 cr, the 8.0:1 and 8.5:1 iirc have the valve reliefs
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924 turbo  



Joined: 02 Nov 2002
Posts: 1566
Location: Simi Valley, CA, USA

PostPosted: Tue Nov 11, 2003 5:35 pm    Post subject: Reply with quote

I've never seen an 8.5:1 931 piston, but the 8.0:1 pistons don't have valve reliefs either.
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Jon Furst
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-nick  



Joined: 16 Nov 2002
Posts: 2700
Location: Cambridge, MA

PostPosted: Wed Nov 12, 2003 3:02 am    Post subject: Reply with quote

Hey CMXXXI-
You might want to check into getting everything balanced at the machine shop- if you haven't already. Especially if you're mixing pistons sets. I don't know if sets were weight matched at the fatctory at all?

Looking good though!

-nick
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CMXXXI  



Joined: 05 Nov 2002
Posts: 1939
Location: Vicksburg, MS

PostPosted: Thu Nov 13, 2003 5:16 am    Post subject: Reply with quote

They are indeed stock turbo pistons from a '79 Euro-spec engine (M31.01) with a CR of 7.5:1

I was under the impression that none of the turbo pistons had the valve relief cuts. I mean, it's been "agreed upon" that all stock 931 engines are interference engines. With relief cuts, wouldn't that make it non-interference? As an aside, JL924T sent photos of his new setup, new pistons that are flat-topped and shorter (to maintain a similar/equal CR as original), in essence making his a non-interference engine. Sounds pretty neat, but I wonder if the flat-top design will rob any performance.

Balancing... yes, I've given thought to that. I see in the manuals there are tolerances for weights of individual pieces. Wouldn't it really only matter that the piston/rod assembly be balanced out, not the piston by itself and the rod by itself? None the less, I have a digital scale accurate to 2 grams, and a single beam balance accurate to within 1/10 gram. I had planned on doing what I could to get things pretty equal. Is there a "special" place on the rod and the piston where material can be ground away to achieve the balance?
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-nick  



Joined: 16 Nov 2002
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Location: Cambridge, MA

PostPosted: Thu Nov 13, 2003 6:32 am    Post subject: Reply with quote

My machine shop just removed material from the underside of the piston. I can't remember where on the rods the material was taken off- I seem to recall not much material being taken off them at all.

I'm not too sure what kind of balancing method/formula the machine shop uses. I do know it was a fair expense to have everything balanced- flywheel/crank/etc. FWIW they did grind a fair bit of material off my crank counterweights to get it balanced. I assume that is what makes up the cost and it may be the most significant source of imbalance (just going by the amount of material removed).

I was under the same impression that valve reliefs don't make any difference on an interference engine?

Flat top 931 pistons eh? Are these JE sets? I've never heard of them before. The dish on my JE's is flat, unlike the hemispherical OEM dish. Is this what you mean? Mine are definately still interference though- I took of my timing belt after lining everything up and turned the crank by hand just to see, and sure enough they touched (don't worry, I didn't apply much pressure!).

PS- did you check the piston skirt taper on yours? That was the bad source of wear on mine which required the replace/rebore.

-nick
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CMXXXI  



Joined: 05 Nov 2002
Posts: 1939
Location: Vicksburg, MS

PostPosted: Thu Nov 13, 2003 8:47 am    Post subject: Reply with quote

The only calipers I have large enough for this project is a set of dial verneer calipers, good only to 0.001" accuracy, and the throat depth isn't enough to take mid-piston measurements. I'm afraid that I'm just going to have to measure where I can to see if the replacement is "close" based on the accuracy of the tool I have. As for checking tolerances on the other three, I'm going to just have faith that everything is OK. I'll clean things up with solvents, but I'm not heading to the machine shop unless I find something that is really noticeable. (fingers crossed) The crank will just have to stay the way it is, as I don't have the money or real interest to "tweak" things to that degree.

Here's one of the photos that JL924T sent me of his flat-topped pistons. As you can see, the top is flat from cylinder wall to cylinder wall, no dish at all. I'm presuming that the#1 & #4 cylinders are at TDC, and you can see that the piston crown is short of the top of the cylinder. Perhaps he'll be reading this post sometime soon and can comment on what he's done and how.
(click photo for larger image, caution, it's a large file)
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Lizard  



Joined: 03 Nov 2002
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PostPosted: Thu Nov 13, 2003 8:53 am    Post subject: Reply with quote

looks like #1 is at tdc but not #4
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