930 deck height mitutoyo vs solder

nocarrier · 03-25-2012, 10:16 AM

Hey everyone. So I am checking the deck height on a 930 engine.

I've tried two methods...

The First using a mitutoyo depth gauge with the cylinders "snugged up the the case and the piston at TDC.

Parallel with the piston pin i get a consistent reading of .040. I'm sure this will measurement will get a little smaller by probably a thou one the heads are on and torqued. See pics attached....

nocarrier · 03-25-2012, 10:21 AM

The seccond method I used was the infamous "solder" method.

For the life of me, I cannon get a consistent reading with this method. It seems as soon as the solder is about the be crushed. It gets squeezed out by the chamfer of the cylinder heads.

I had one measure at .0470 and the other at .0460.

I couldn't repeat these numbers because of the solder being pushed away by the chamfer.

nocarrier · 03-25-2012, 10:25 AM

Am I good to go with the measurement from the first method?

Or is there a problem why I cannon get a good reading using the solder method? Are my heads over machined? Is the diameter too large causing me problems as there is no "flat" surface above the piston?

Thanks for you help and thanks for looking

docrodg · 03-25-2012, 11:37 AM

The depth mic method is the one to use considering you have flat-top pistons. The solder method is meant for domed pistons where there is no surface that you can "register" a reading from and is not as accurate as a mic. The heads are probably fine, all heads are shaped in a dome more or less except exotic racing heads with a squish area and other engines that are that way. For a better measurement just clean the mating surfaces and torque the cylinder down before you measure so you get proper "crush" figured in.

nocarrier · 03-25-2012, 12:14 PM

Thanks for the advice.....

What is a proper way of clamping the cylinders to the case without damaging the surface of the cylinders?

The washers aren't exactly square with the sealing surface of the cylinders.

I can imagine a proper made "clamping plate" that has been surface ground to an exact height would be a perfect tool for this.

docrodg · 03-25-2012, 12:30 PM

They do not need to be clamped at full torque and you can usually just install the head, torque, then remove the head and snug the nuts down on the cylinder with a spacer under the nut. The torque will cause the crush to occur but the copper will not rebound too much before you can snug the nut back up. Not sure how much base gaskets actually crush, but some of the copper head gaskets for US V-8 cars will crush quite a bit... like 3 thousandths or more. The clamping plate you describe is very much the perfect tool, I have one for V-8 Chevy's that is used for machining and measuring, but probably overkill in this case. Unless you plan on stressing this engine a lot and running a tight clearance you probably have more than adequate clearance and can suffer the error of not having a crushed gasket.

nocarrier · 03-25-2012, 03:20 PM

In the first set of pictures where the depth mic is reading .040 I was taking the picture after installing the cylinder head and torquing it down to crush the base gasket.

I removed the head, set it to TDC and that is where I got the .040.

I just finished checking the other bank of cylinders and I got the same reading.

AlfonsoR · 03-25-2012, 07:53 PM

You can buy some 1" square tubing and drill holes for the studs so that it's oriented in a diagonal. This way you can get the cylinder a bit more square. Tighten just a bit, maybe 5 ft-lbs, then use your depth mic. I believe this might give you better repeatability. You also might want to put some cardboard (like from a case of cola cans) under the tubing to protect the cylinder.

KTL · 03-26-2012, 08:04 AM

Definitely protect the cylinder if you're going to torque the cylinder to the specified amount for complete assembly. I use some copper water pipe for my cylinder hold downs and I thought i'd be fine if the copper squished a bit. I was wrong......

I put a slight ridge in the cylinder and I had to file it down and fill it with some JB weld.

Eagledriver · 03-26-2012, 10:54 AM

I use PVC pipe for my spacers. Don't have to worry about damaging the cylinders that way. You are measuring two different things with your measurements. The first method measures the deck height. For it to be accurate you have to have a sharp enough probe to catch the very edge of the piston. The solder method measures piston to head clearance (not deck height).

If you do the compresion ratio calculations you'll find large differences with small deck height changes. For my engine .031" was 9.86 and .039" was 9.64.

-Andy

jwasbury · 03-26-2012, 05:59 PM

Quote:

Originally Posted by Eagledriver

If you do the compresion ratio calculations you'll find large differences with small deck height changes. For my engine .031" was 9.86 and .039" was 9.64.

This effect is much less on lower static compression ratio engines like the 930 lump in question.

I measured deck height using my vernier calipers, but I like your mitutoyo depth gauge better.

nocarrier · 03-27-2012, 03:52 PM

Thanks for looking and chiming in everyone.

Being that this is my first Porsche engine build, I'm constantly learning more are more about these motors.

I read in "the book" that the solder method was an accurate way to check the deck height on these engines. However, after placing a cylinder head on an empty cylinder, It is very clear how the solder method really doesn't work in this case.

When looking through the cylinder to the head, it became apparent that the solder method will only measure the piston to cylinder head or in my case; the piston to the chamfer on the cylinder head.

I'll try and take a picture of what I'm trying to explain.