While shopping the local machine shops, it seems I keep coming across the same scenario:

Machinist that are willing to do the 4mm overbore, and even some that are experienced but none that have a deck plate for the job. The common response when I ask about this is 1) The factory didn't require them but 2) yes it would be better to do the bore job with them.

So, what does it take to have a deck plate made up. I'm not talking about the Sterling kind that is being discussed elsewhere to close the deck, I mean the kind you bolt on in place of the head to pre-stress the block while boring.

Should I even consider going forward with the bore job without one? I've seen what a difference it makes on cast iron closed deck domestic blocks, which is pretty major. I would think that the open deck aluminum block would benefit even more.

 

Hey ATB....

For the materials...I hear the Boeing Surplus store has all sorts of aluminum, titanium etc...in various thickness'. Maybe a good place to start your search.. and then take it with an old head gasket as a template to the machine shop of your choice?? Just a SWAG....

Later,
Tom
midlman@rennlist.net
89GT

 

Adam,

Yes, it would be highly beneficial to have a deck plate in place for boring.

No, I personally would never have a hole bored without one. Especially with the style of block we have. (Damn Porsche saving money on the casting.)

Stock size, I can point you in the direction of my former business associate.
104mm you'll need to get your own made, unless somebody else has had them made.

Greg

 

Not knowing any better myself, Id probably do somehting like Tom suggests. A large chunk of metal cut in the pattern from a head gasket. Im sure SOMEBODY on here knows the answer, source or how to make one.

Perhaps make one then rent it out to others that are doing the same thing...recover some of the costs?

 

Adam,

It might make better financial sense to use a shop that has already done 928 block boring to 104 instead of machining a new deck plate?

Wasn't there someone saying that the 928 block should be preheated before boring to hold the tolerance?

OR get a group of future stroker motor builders to invest in the making of the deck plate.

Maybe a few 104mm 944 deck plates are floating around

 

Why not take an old head and bore throught it? They are cheap and have all the correct holes and reliefs.

 

Now I know what to do with one of the two useless 86 heads I have

Adam,
Want one? Free. Just return it when done with four 104+mm holes in it

 

How about installing 'real' deck plate first and then take bores to 104mm? At least towers would stay put better than without it.

Erkka

1992 928 GTS 5-speed rebuild engine will have deck plate for sure
Oak Green Metallic 22L
Classic Gray MX

 

John Veninger wrote:

That's what I'm finding to be so strange. Locally here (Seattle area), it appears that it's the norm not to use them. I called Powerhaus to ask if they use them and the tech's response was "how much boost you planning on running?" When I said none, but maybe 5psi , he said not to worry about it.


Different shops will go to different lenghths when boring a block in order to simulate the same internal stress the block is under while running. This includes bolting on components that stress the block (deck plate, water pump, motor mounts,etc.) and heating the block to operating temps while boring. I guess it just depends on how extreme you want to be. The machine shop I'll probably be using related that part of what takes so long in properly machining the alusil block is that the heat generated by the cutting stone during the boring process causes the cylinder to expand and move away from the cutting stone resulting in false tolerances. A couple passes are made, and then the block is allowed to cool before the stone is run again since the tolerances are cold tolerances.



Greg wrote:

No doubt about it Greg, but the big question is what am I leaving on the table without it. Probably not a performance issue, just a longevity issue. And maybe just a minor one at that(?) I'm assuming that Porsche didn't use a deck plate when originally honing the factory blocks, and look at the kind of mileage we're able to get out of our motors.



Is there a reason a stock size plate couldn't just be bored oversize? This would be a quick and easy fix.

Thanks for the input guys.

 

Mark A. wrote:


John V. wrote:

I'm going to talk to the machinist right now about your proposition John. If he says it's a workable solution, we've got a deal.

 

Apparently there's some science to this whole deck plate deal.

Machinist said that boring through a head would take a lot of work, and that he wouldn't be interested. Better off getting one made (he suggested BHJ, anyone heard of them?) or just proceeding without.

Bummer.

 

Lag wrote:


I would think so, but here's the question. What affect does the thickness of the deck plate have on it's functionality? Is there a point where a maximum rigidity is reached? Why not 3"? or just 1"?

 

Theoretically you would want a deck plate which has the same ridigity that a cylinder head would have . In that way IF , BIG IF the cylinders distort when the head is fully torqued into place that same distortion would occur during boring and honing . Whether or not that occurs with an open deck (unsupported cylinders) is the subject of some dispute . Porsche in the WSM does NOT specify that a deck plate be used l . Illustration 13-25 shows it being done without a deck plate .

 

Gbyron is right, they are beneficical.

Deck plates make a lot of sense for longevity issues when you are pushing beyond the factory specific power output and changing the bore signifciantly. This one is off a 968. I have another to do a pair but one is on loan to a machine shop. I also have one for a 100 mm. bore, but not a pair for the early cars. Using a casting that has been bored out is time consuming and not worth considering and much less likely to generate the stress required to simulate the true clamp of a head stud. Here is one of the deck plates. If you want to rent a pair, I might be able to help you out. No promises, but I will see what I can do.

Take a look. It is the item on the right. They are expensive and you will probably only use it once.

Thanks and good luck!

 

Jim B. wrote:

Jim's response is basically the same I got from a Porsche trained mechanic (not machinist) that I spoke to. Admittedly, I don't quite follow the science, but he stated that on the open deck design a torqued deck plate will just distort the cylinders in a different way then without one. Porsche spec'd the bores to be honed without one, so there is no need for them.

fst951 wrote:

I'd be very interested. Please let me know regarding availability. Even if only one is available, I can bolt a head on the opposite side.

You can PM me or email me privately at adamb777@att.net

Thanks fst951