Pistons Hit the Chambers, Help Please!
 

Here’s the situation:
I’m building up an engine for a track car.
• Porsche MFI on a 1977 2.7L
• JE domed pistons (11.0:1 designed static CR)
• Cams are DR40’s.
• Crank fired/twin plugged
• Planning to run race fuel or a blend of race/pump fuel.

During deck height check I ran into a problem. The piston domes are touching the chambers.

Initially I’m using some ‘purple” assembly lube to check for interference and soft putty (plumber’s putty) to check for clearance on the valve pockets and piston dome. I’ll be using the acid core solder method once I have the preliminary clearances achieved.

As stated JE domed pistons are touching the cylinder heads just at TDC. The engine will rotate completely but the interference is definitely there. On the attached photos you can see the area on the piston dome (recreated for effect) where the transfer of assembly lube is occurring. Also on the attached photos you can see the actual witness marks on the chamber where the interference is occurring.

Static condition of the test:
• Base gasket total thickness: .025”/.64mm
• Deck Height clearance: .029”/.74mm
• Piston to Chamber clearance: .000”/.00mm
• 2 Head Nuts torqued on diagonal: 15 ft. /lbs.

So now I need to get the piston dome away from the chamber. I’d like to have .050” clearance. That means I would need to change the base gasket stack.

Anticipated condition:
• Base gasket total thickness: .078”/2.0mm
• Deck Height clearance: .082”/2.10mm
• Piston to Chamber clearance: .053”/1.3mm

I’m concerned about detonation of the end gases with a Deck Height clearance of .082”/2.10mm. Could some of you experienced engine builders provide me with some advice please?

Thanks,
Gerry
http://forums.pelicanparts.com/uploa...1348335552.jpg
http://forums.pelicanparts.com/uploa...1348335610.jpg
http://forums.pelicanparts.com/uploa...1348335783.jpg

Assuming that the heads have not been excessively cut, I would get your deck height to .040" and measure the inference. It sounds like your compression height is wrong and thats a piston problem.

Steve,
Thanks for the reply. I don't know the history on the engine. Bought the car as "basket" case. I haven't had the heads cuts. EMS in Englewood rebuilt and twin plugged them for me. They didn't mention anything. How would I check them to see if they have been cut? I'm not set-up with the buret equipment to do a proper CC check on them.

Thanks,
Gerry

Gerry,

You need to measure the depth of the sealing surface and compare that to an untouched head that you KNOW hasn't been previously machined.

It most likely that your pistons were made wrong and the compression height is too high. Thats why the domes are touching the heads. You may have sufficient material to machine the domes a bit, but I cannot tell you that for certain from here.

I do think you will need to buy-borrow the equipment to make accurate CR measurements in order to know where you are with this and find resolution.

As Steve says, measure the compression height. Lucky you, you have a lip on the edge of the piston to measure from. Insert the pin from the outside of the piston so it sticks out. Take a dial caliper and measure from the bottom of the pin to the lip on the piston top. Now subtract half the pin diameter. What is that measurement?

What is your cylinder height?

Pistons
 

It looks to me that the heads are still close to full thickness based on what I see beyond the fresh machined seal surface, I seems to me that the dome radius is wrong for that combustion chamber for them to hit the head there, call Jerry Roche at JE with the job # on the piston and have him review the build sheet.
Mike Bruns

Great info and suggestions guys. It really helps. I'm going to do some measuring this afternoon and see what I find.



Thanks,
Gerry

More Measurements
 

Well Gents I’ve talked to Jerry Roche (JE Pistons) and done a lot more measuring. Here’s what I’ve come up with:

• The Compression height is 1.3445”.
• Jerry (JE Pistons) recommends increasing the base gasket thickness or having the piston domes machined and referred me to an outfit name Rebco. He believes the domes are thick enough to accommodate removing material at the radius of the dome to achieve proper clearance.

I checked deck height and dome to cylinder head clearances using the acid core solder method with three different base gasket thicknesses; 1mm/.039”, 1.5mm/.057”, and 2mm/.075”. Below listed are the results:

1mm/.039” Base Gasket
• Deck Height: .055” (used one piece of solder, I believe the piston rocked)
• Dome to Chamber: .000” to .010” (used 4 pieces of solder: 10, 2, 4, and 8 o’clock on the dome radius)

1.5mm/.057” Base Gasket
• Deck Height: .060” (used two pieces of solder on opposing sides)
• Dome to Chamber: .020” to .025” (used 4 pieces of solder)

2mm/.075” Base Gasket
• Deck Height: .075” ( used 4 pieces of solder on opposing sides)
• Dome to Chamber: .030” to .035” (used 4 pieces of solder)

So it appears regardless of the base gasket thickness, the clearance from dome to combustion chamber is not going to be right unless I have the pistons machined.

Comments and suggestions welcomed.

Thanks,
Gerry

What's compression height?
 

Gerry,

Could you help me understand what's compression height means? I'm not familiar with this term and really wanted to learn about it. I've recently done an engine rebuild for a racing motor but have not used this parameter (compression height). Thanks.

Tony

Tony,
As I understand it:
It is the dimension from the center of the wrist pin to the top of the piston at the point where the deck height is measured. On a flat top piston it's pretty easy. On domed pistons it can be harder. It would be the area around the top edge of the piston just adjacent to cylinder wall and would not include any of the dome.

Maybe some of the experts could chime in and give a better (or more correct) description.

Regards,
Gerry

Gerry,

FYI a larger volume at the base of the dome as apposed to the top of the dome is not a good setup at all. Not sure what you have going on but it is not good. I went through some steps to get just the opposite thing to happen, close clearance at the base more at the top.

You will need to send your head(s) out to get the pistons re machined to fit the heads. If you have another head you might want to see if it has the same results.

Stay tuned,

PFM