YYC930

We took it apart after a knock developed.

The 3.2 Carrera case was machined for the Mahle 3.5 P+C set....and the piston to head clearance was plastiguaged 2-3 different times to ensure clearance.

What's up with that eh ?

We don't want to do double gaskets and lose compression, so we were thinking we'd machine the pistons for more clearance as there does not appear to be enough meat on the heads before you cut into the valve seats.

Any ideas from the engine builders here who've done this before ?

Also I found out the tech plastiguaged the calico coated Jerry Woods bearings.....a definite no no ....but doubt that would have caused our problem.

TIA for any answers........and I am going to cross post this in the 911 forum.

Geoffrey

What RPM do you run the engine to? What connecting rods are in the engine? How much do the pistons weigh? What was the piston to head clearance? What was the deck height clearance?

The issue with plastiguage and the JWE bearings is that the bearings can only be crushed once, not twice. This is why you use a bore guage and measure each ID of the connecting rod and then an outside mic to determine each crankshaft journal. From there you order the bearings for the clearance you need/want to run. You don't need plastiguage.

YYC930

7,500 RPM....Carrillo rods......piston weight 537.5 grams......and the other measurements, my tech knows I don't have that handy.......

Geoffrey

Well, you'd need the other measurements to try to determine what has occured.

YYC930

I know.........I though you were asking for them. Gave the tech **** for not reading the JWE instructions as well.

Geoffrey

I guess you need to figure them out so it won't happen again.

DWalker

The use of shims wont drop compression enough to worry about, so get your piston to head and piston to valve clearance set right.
Helpful information would be:
Piston type and part#
Rod length
cylinder head type and part#
chamber volume
valve size
crank part# and stroke

Geoffrey

Not only will shimming the cylinders not drop the compression, it will ruin the deck height clearance which is integral in the squish and ability to run proper timing. Too much deck height clearance and you'll have a host of other issues to deal with (if peak performance is your goal).

911pcars

I wasn't aware they make Plastigage in the thicknesses necessary to measure head-to-piston clearance. Can you confirm with your builder?

Measuring bearing clearance is usually the function Plastigage (0.0015" - 0.006").

Deck height is in the order of many times that clearance (~0.025"+ or so) and is usually measured with clay or soft solder.

Sherwood

altonj

Jerry recommends against plastigauge for all bearing, not just coated ones.... IIRC. Has nothing to do with the "special bearings".......

Were the heads chamfered to accomodate the larger bore and deck height checked? Shimming the cylinders does reduce compression, just not that much.... But too much deck height can yeild pre ignition issues. You can see the differnces in deck height with this simple calculator.

http://www.csgnetwork.com/compcalc.html

You can raise deck height slighly, to gain some clearance as I believe this motor is twin plugged??


You can check the piston to head clearance easily with either solder or clay.

cheers