valve - piston clearance on stock 3.2 engine
 

Does anyone know the clearance between the valve and piston on a stock 3.2 engine?

I had some cylinder head work done on my stock 3.2, ended up needing new valve seats, and I no longer have adequate clearance. I didn't think the 3.2s ran all that tight, so I'm wondering if I loused up somewhere, or if the valve seats are too far into the combustion chamber.

Did you change cams or something also? I didn't think the 3.2 cams had enough profile to be an issue?

V to P
 

If its not too late what is the height from the area on the head where the valve spring seat/shims go to the tip (above the keeper grooves) of both valves, the factory has a dimension for this to determine if the seats are too deep and it will determine if they need to be deeper as well

Mike Bruns

Well...did you screw up something...NOT you, but the machine shop probably did.

They might have shaved the heads too much...is the shop a well know Porsche machine shop??

Did you have any work done to the cylinders?

I'm just fishing for an explanation from one of the Forum experienced guys...

As far as I can tell you didn't open the case or take out the rods.

Let us know how you do with this.

Regards,

I thought the motronic cams would have lots of clearance as well.

Jugs are stock, heads did not get machined except for new valve seats, and are stock. Not sure I can measure the valve spring seat distance - everything is bolted up, and I lack a valve spring compressor. I'll have a look-see tomorrow.

Exhausts are at 2.2mmm, intake at 1.25mm. I was curious how close to stock this is.

Some closer inspection suggests a combination of factors. I did not remove all of the carbon from the pistons as well as I should have, and there are enough spots (verified w/ borescope) on the pistons to possibly make a 0.25 - 0.5 mm difference. My bad. Chances are the seats are 0.25 - 0.5 mm too shallow also, but I don't know this, I just suspect it.

I was debating pulling it all apart again and scraping the carbon off the pistons so I can verify that that's the issue, or retarding the cams by 1 hole (6 - 7 mils, or 0.15mm, valve lift at TDC). They'd both still be within spec, though at the bottom end as opposed to right on the money, and the intake clearance is exactly 1.5mm if I do that. The chains have 14K miles on them, so I won't get the "break in retard" that a new set of chains would provide.

I'm kind of tired of messing around with it, so I'm leaning towards option "B".

V to P
 

I would like to know how you checked the clear. I start at 7 deg BTDC on overlap to test the exhaust valve, starting with close to zero lash, 1 turn on the adjuster is 1mm, and 10 deg ATDC on overlap for the intake. I like a min. of 1mm on the intake and 2mm plus on the exhaust.

Mike Bruns

If you know how many shims are under your valve spring seat cups, someone (maybe even me) can measure the height of a cup and tell you nominal shim thickness if you can't find that. That would allow you to measure above the top of the cup to get the distance to the keeper or retainer or whatever is needed on that score.

But it looks like you fall within Mike's comfort zone no matter what. I don't recall ever seeing a factory spec for clearance in a factory publication. But perhaps where those do appear (as in our host's book), those came from Porache somewhere?

Sounds like you used the "screw adjuster down" method of determining clearances? And at various crank angles to catch where the closest approach spot was?

Yes, screw adjuster method. Closest spot for the intakes was about 10 deg ATDC. I found the close spot really easy - start with 1.5 turns in and turn the engine over carefully. Where it binds is where the close spot is!

Well, I retimed the cam and got the required clearance on the intake side. Alas, the exhaust side went below 2mm, so that plan is out.

I also took 1/2 the top end apart, scraped the remaining carbon off the pistons, and reassembled it. Still same answer. The #4 intake valve was slightly higher off the seats than the other 2 on that bank.

Who would design such a finnicky engine? Worse than my kids & food. Lousy Germans ...

I think I will check all the cylinders, and if none of the intakes are worse than 1.25, I will run it. After all, I fully expect a 0.25mm variation over the different cylinders, and if #4 happened to be 1.5, per the book I am good leaving it alone.

V to P
 

You were fine before you went through all that, I have taken apart race engines that have had .5 mm on the intake clear. and not touched, I would not have wanted to try that but I always check engines as they are disassembled to see what they have for clearances, if you had seats replaced and the shop doesn't have the factory spec book then they may well be more proud than they should the good thing is you have room and reduced the chamber vol. to bump your compression up a tick

Mike Bruns

I checked all the cylinders, the intakes ranged from 1.2 to 1.6mm, the exhausts from 2.0 to 2.4. So there is indeed some tolerance going on, probably based on how much grinding the valve seat needed to fit the new guides.

Thanks Mike, I feel somewhat better forging ahead now. I will still be nervous my first few track days ... and probably it will always sit at the back of my mind how the carbon buildup on the pistons is slowly thickening ... but hey, 0.25mm is 0.010", and it isn't a lot of extra carbon.

I've done some thinking about all of this, and come to the conclusion that the 1.5mm intake valve clearance "spec" Bruce Anderson & Wayne have in their books is on the high side, as some posts, backed by experience, in this thread already suggest.

The intake valve closes a bit after BDC during the compression stroke. It then has the power & exhaust stroke during which it stays closed, so one complete revolution of the crankshaft. Then, during the intake stroke, it opens a bit faster than the piston recedes, making the closest spot about 10 deg ATDC.

But since the intake valve is opening, and has been closed for 1 crank revolution, it is impossible for the valve to be floating - if it was, it would have been killed during the compression stroke, and no amount of valve clearance would have saved it.

The exhaust valve is closing when it comes close to the advancing piston, so if it floated due to overrevving, it could indeed contact the piston. So, the >2mm clearance seems appropriate there.

Is it possible you have the cams swapped, left vs. right?