Piston to Valve clearance with Solex cams

neilca · 02:13 AM

My race engine is a hog pog of undesiarable parts I had laying about. This is the first engine in a new chassis and it is my belief that "the first engine in a new chassis will die a horrible death". Ususally, at the hand of some subsystem that failed. I tell you this so you will understand better why these parts are put together. BTW the engine has survived the first year!

I have 2.0L heads with 42 mm intakes and 38 mm exhausts. "S" forged pistons, these had been knurled to bring the diameter back to close to stock. I had to deck the block 0.060" in order to clean up the spigots from years of corrosion. I had been running CIS cams (143/144 PN) and the thing was dog slow. The dyno said 110 Hp at the rear wheels.

I decided to replace the CIS cams with some Solex cams I had laying around. Put them in last night and timed them per Wayne's book and everything looked good until I checked piston to valve clearance on the exhaust valve.

The method I use to check for clearance is, adjust the rocker to zero lash, now adjust the valve to be 0.100" compressed. Rotate the assembly, if you do not hit the valve then you know you have at least 0.100" clearance between the valve and piston. Mine went "clank". At the proper timing the engine will rotate with 0 lash but it will not rotate with any compression.

I have two theories at this point,
1. The 1969 heads I am using have larger valves than the 1966 pistons were designed for and I am not going into the valve reliefs as expected.
2. The 0.060 I took off the deck is coming back to haunt me and I will need to machine the pistons to get more relief.

What do you think?

304065 · 07-30-2010, 05:21 AM

Yep, if you skim 1.5mm off the cylinder spigots you have pretty much eliminated the piston to valve clearance provided for in the original engine. You might pick some clearance up if the valves are sunk deeper in the seats if there have been a couple valve jobs. Here is what the Factory said about it:

Quote:

Valve recess in pistons of engines up to Eng. Nr.
900 727 is somewhat shallow. Should it become
necessary to match a new cylinder head to
pistons of the above mentioned engine series, the
smallest clearance between the valve recess on the
piston and the valve head must be determined.
The required minimum clearance is 0.8 mm
(.031") (see Section En 21).
The easiest way to check this clearance is with
the aid of Plastilin (putty

You need new pistons anyway, why not spring for a set of JE's that have the right diameter and deep enough valve reliefs? I wouldn't think the knurling would hold up for more than a couple hours.

Walt Fricke · 07-30-2010, 08:46 PM

I think you are screwed up on both points:

2) Er, why not add a second copper gasket (which is under the cylinder at the spigot)? Or stack several? Or buy ones which are thicker?

You can buy back what you lost in deck(piston to head) clearance that way.

Lots easier and cheaper than having (old and basically worn out) piston valve pockets machined, no?

1) But, if I read Bruce Anderson's chart correctly, while you have the highly desireable '69S heads with their big valves, you also have pistons made for 39mm intakes and 35mm exhausts. 42 into 39 won't go, etc. Amazing that the CIS cam had lift low enough that this didn't show up then.

So you need '69S pistons, or J&E's version of same. Or spend some significant money to have your '66 pistons (now worn out) valve pockets cut.

Here's an idea: have your cylinders plated and rebored undersized? Naw, bet that would cost more than J&Es.

Steve@Rennsport · 07-30-2010, 10:23 PM

Walt,

As you probably know, the 2.0 heads are more prone to detonation issues and the last thing a fellow wants to wind up with is excessive deck height caused by too many base gaskets.

Practically speaking, anything more than a 2 gasket stack is much more prone to leaking and IMHO, the right way to fix the problem is by using the correct pistons that provide sufficient clearance in the valve pockets.

Its not the cheapest, but its the best solution.

neilca · 07-31-2010, 06:43 AM

Thanks for the input guys.

I am going to pull the pistons out and machine them to fit, I have this capability in house so the cost is my time only. I do not want to put any money into this engine because I am in the process of building a 2.4 to replace it and that is taking up a lot of funding.

I find it unusual tha Porsche would allow only 0.031" clearance piston to valve or is this refering to the side clearance on the diameter?

Walt Fricke · 07-31-2010, 01:24 PM

Neil

Hmm - how many of us shade tree types on this Forum just happen to have an in-house valve pocket cutting capacity? Good on you.

Steve: I was mixing apples with oranges here. Neil didn't ask about piston to head clearance, but it seemed with the decking he did he would have reduced that rather a lot, so he might want to gain some of that back. However, since the motor ran at whatever clearance that gave him, and his only change is to push the "too big" valves deeper with his replacement cam, thus creating a clearance issue, I should have shut up about spacers and deck height.

Now that you mentioned the early motors with the single plug and high dome pistons being sensitive to too much clearance over at the periphery (as opposed to what I tend to think of with my later motors - too little clearance), I remember reading about it. If I'd taken a multiple choice test on this issue I might have guessed right?

Speaking of clearance (since I think the well equipped Neil has gotten the advice he wanted), I am pondering what I should set my SS 2.8 (J&E 95s, 3.2 heads with 52mm intakes) deck height/piston clearance at. I searched the archives, and found advice all over the place.

Grady Clay said he ran a race motor at 0.021", but only because it was a 40 hour (or something short, maybe even shorter) motor. I assume that by replacing connecting rod bearings at those intervals (maybe checking wrist pin clearance and main bearing wear also?) he felt he could prevent a tolerance build-up from normal wear which would lead to contact. Bruce Anderson has a similar figure (0.533mm) somewhere in his book.

Anderson gives 0.035 (0.9mm) as the factory figure.

Someone suggested 0.025" (0.635mm) as a minimum.

Another source said 1.5mm (0.06") was a conservative figure.

Someone else listed 0.03 (0.762mm) through 0.04" (1.0mm).

Yet another said 0.8 (0.031") through 1.00mm (0.039") was the spec, and that 0.7mm (0.028") was "tight."

What should a Club Racer, shooting for the highest compression he can get, pick? What if he's shooting for a 100 hour motor?

Regards

Walt

Steve@Rennsport · 07-31-2010, 02:46 PM

Quote:

Originally Posted by Walt Fricke

Steve: I was mixing apples with oranges here. Neil didn't ask about piston to head clearance, but it seemed with the decking he did he would have reduced that rather a lot, so he might want to gain some of that back. However, since the motor ran at whatever clearance that gave him, and his only change is to push the "too big" valves deeper with his replacement cam, thus creating a clearance issue, I should have shut up about spacers and deck height.

Now that you mentioned the early motors with the single plug and high dome pistons being sensitive to too much clearance over at the periphery (as opposed to what I tend to think of with my later motors - too little clearance), I remember reading about it. If I'd taken a multiple choice test on this issue I might have guessed right?

Speaking of clearance (since I think the well equipped Neil has gotten the advice he wanted), I am pondering what I should set my SS 2.8 (J&E 95s, 3.2 heads with 52mm intakes) deck height/piston clearance at. I searched the archives, and found advice all over the place.

Grady Clay said he ran a race motor at 0.021", but only because it was a 40 hour (or something short, maybe even shorter) motor. I assume that by replacing connecting rod bearings at those intervals (maybe checking wrist pin clearance and main bearing wear also?) he felt he could prevent a tolerance build-up from normal wear which would lead to contact. Bruce Anderson has a similar figure (0.533mm) somewhere in his book.

Anderson gives 0.035 (0.9mm) as the factory figure.

Someone suggested 0.025" (0.635mm) as a minimum.

Another source said 1.5mm (0.06") was a conservative figure.

Someone else listed 0.03 (0.762mm) through 0.04" (1.0mm).

Yet another said 0.8 (0.031") through 1.00mm (0.039") was the spec, and that 0.7mm (0.028") was "tight."

What should a Club Racer, shooting for the highest compression he can get, pick? What if he's shooting for a 100 hour motor?

Regards

Walt

Hi Walt,

There are a lot of variables that affect optimal deck height and too much is just as bad (in a different way) than too little. One of the big ones is combustion chamber shape and piston dome design so this is why its hard to make generalizations about this,.....

IMHO, anything more than .040" is a problem due to detonation-related issues.

Ideally, one designs pistons to allow proper valve clearance, targeted CR and allow proper deck height. Thats a tall order in many cases, thus the need to juggle deck height and valve clearances.

Street engines should shoot for .039", race engines should see anywhere around .032-.035", in the majority of cases. As Grady said, sometimes we do some creative machine work to reduce this, but it comes at a price in longevity & durability.

Hope this helps,

neilca · 08-01-2010, 04:23 AM

Where do you measure the deck height on a hemi piston?

BTW another issue I found from cutting the deck 0.060" was the rods were hitting the bottom of the jugs. I had assembled the engine and was in the process of timing the cams when I found the engine would not turn 360. I thought at the time the pistons were hitting the head. I used clay to confirm I had plenty of clearance it was by accident I found the rods hitting the jugs. I cut 0.100" off the bottom of the jugs to get clearance.

I guess I cannot use the moniker of shade tree mechanic. I have 32 years of working on a variety of SCCA and IMSA race cars. I don't do it for a living, just a hobby. In all those years I never worked on a Porsche, so the information I get from this site is invaluable.

neilca · 08-01-2010, 01:36 PM

I pulled the engine apart and found the pistons are too thin to cut in reliefs. I am going to have to go back and shim the base of the jugs. Hats off to Walt.

neilca · 08-07-2010, 05:17 AM

Results are in, it took a 0.040" base shim to get 0.085" of clearance to the exhaust valve. I tried a couple of configurations and found the relationship of the base gasket to valve/piston clearance to be non linear. Here are some of my results.

Base gasket of 0.010" gave 0.000 clearance
Base gasket of 0.041" gave 0.085 clearance
Base gasket of 0.066" gave 0.126 clearance

The good news is I now have clearance and I only had to use one base gasket
(a 1.0 mm)

I then checked my comression ratio, which I had not done before. This is my setup for measuring the volume. This is a 50 ml burret and i had to fill it twice to get the volume filled. I would have to check my notes but I remember the volume to be about 85 cc. I then subtracted the step volume. ending up with about 70cc combustion chamber volume.

Measuring the piston crown is a bit more challenging due to the hemi piston. I placed a dial indicator on top of the piston at top dead center then rotated the engine untill the crown cleared the bore (about 0.650"). I then used the burret to measure this. Subtract the measured volume from the calculated volume (1/2 bore squared X 0.650")

After all the number crunching it came up to be 9.7:1 measured static compression ratio.

Other considerations, I had to use two gaskets on the chain boxes to move the boxes out in order to get a good seal on the cams.

I use a mechanical chain tensioner, moving the jugs out 0.030" made a significant difference in chain tension. this also had to be adjusted. Hydraulic tensioners probably would not have been effected.