Checking 993 hydraulic lifter lash
 

Trying to check the hydraulic lifter lash on a 993 motor. Does anyone know the proper procedure?

The Bentley manual is rather brief, and it appears that there is supposed to be a minimum of 0.2mm lash on the intake, and 0.6mm on the exhaust side, with everything installed.

When I try this with a brand new INA lifter, I have interference, not lash.

I did my own heads, and at most removed .2mm from the valve seats. New & old valves measured identical in length. Cams were reground, probably took 0.1 to .2 mm off to remove some light pitting. Rockers were resurfaced lightly. So I should be close to even between the valve seat and the cam / rocker combo.

Which leads me to believe that I am doing something wrong, or some important note may be omitted from the Bentley?

When in doubt, take things apart ...

A bunch of the old lifters and a new one.

http://forums.pelicanparts.com/uploa...1598628167.jpg

It would think that the small machined lip (arrow) on the plunger is supposed to be inside the lifter bore. Once it goes past the lifter bore, the teflon seal will get pushed off the lifter body. This means that the new lifters are at full extension as delivered - during operation they need to be shorter than this. So the Bentley manual is rather wrong with their description!

I took an old lifter apart & removed the spring & reassembled it. Using this to check lash, I have plenty. In fact, I have too much - there is a maximum permissible, .073 (not sure about the 3...) and .089 for intake & exhaust. My engine is exceeding those values, especially on the exhaust side. Replacement lifters of different lengths are, aside from insanely priced, only available shorter ...

Looking at the still pumped up (go Hans & Franz!) lifters that came from the engine, they are overextended. Probably why no seals were on any of them! Chances are, the engine has been running like this for some time. Not sure what would cause it, though.

All of which leaves the question of what to do about it. Machining the rocker shafts seems like the most easily accomplished remedy. Sinking the valves into the head would also work for this specific issue, but would cause others. Buying new cams & rockers might help, but I don't know how much was removed - might not amount to much. Should have measured the base circle before I sent them out ...

This may help.

Unfortunately Porsche never gives the installed height of the valve from tip to spring pocket when the valve is on the seat. That would help immensely.

What they suggest you do is as follows.

Drain "lifters" on an intake and Exhaust rocker arm as use this as a set up rocker arm. (no oil in lifter)
Make sure all rocker arms are the same as the "setup piece. No refacing on some and not on others. Make sure cam lobes are the same, for same reason.
Assemble the assembly with cam.
On the backside of each cam lobe, measure the travel (play) with an indictor. Porsche give a spec here. Min of 0.2mm, max of 1.85mm on the Intake side and min of 0.6mm, max 2.25mm on the exhaust side.

They sell shorter versions of the lifters to compensate for no lash. You could tip the valves in a pinch, but I'm sure Porsche have suggested this way as in most cases the heads would be assembled and fitted to an engine with decks attached. Changing the lifter is easier at this stage.

I'm sorry have no idea the part number but these are easy to find. From memory there are two sizes, 0.5mm and 1.0mm , I think. Best to check, I could be wrong.

Makes sense, and essentially what I did (unfortunately with the engine buttoned up already). And this method shows I have too much lash, not too little ... I don't think they sell longer lifters (nor am I willing to part with $2400 for 12 lifters, even if they did).

Draining the lifters alone doesn't work that well, as they seal air tight, and the air exerts considerable force when at maximum compression; as long as the spring is in there, it will always "recharge" the air in the lifter. Taking the spring out and then bleeding the air out while compressing it worked well.

I checked all my refaced rockers, comparing to a new one, and they are all within .005" of lash to the new one (same or tighter, actually ....). No discernible difference between intake and exhaust rockers - they look different, but dimensions appear identical. Which leaves the reground cam, or ... the engine tolerances just stacked up that way. Still seems odd. I would think if anything things would end up tighter after the 2nd top end rebuild.

In your cylinder rebuilding, were the valve stems tipped? If you refaced the rocker faces and removed 0.005", that will be close to 0.007" at the valve.

Adding all of the repair work together, you are probably were you are at, unfortunately. The proper way to do this work is to work backwards. Cams and rockers first, tip the vales as required (all the same) and cut the seats to suit what lifter sizes are available. Not really a good way if you are considering air flow across the seat, but you dnt have a lot of choices other than longer valves.

Another way would be to change the retainer and capture a lash shim in the retainer, but this is more work than sinking the valves into the seats. Cheaper than valves, and you hold the correct geometry and air flow.

Pick your poison.

I think I will try machining the rocker shaft flats.
0.015" is the most I should need to remove to be in spec.
They are hard, but at slow speed spending 2 or 3 carbide endmills ought to get the job done.

Interestingly, the bolt side of the shaft flats is not very parallel to the shaft axis. Only the cam tower side is, which is what I want to cut. Will have to spend some time fixturing...

Good luck. You are a braver man than I am. I would never solve one problem by creating another.

Understand and solve the first problem before you go cutting rocker shafts. We have built many of these engines over our history, and never run into anything like this.

Either the cam and or rocker faces were cut too much, or the valves were tipped too much, all without regard to the end result. Did the cam decks or the cyl heads on the spring side get surfaced?

Will it work. Probably but the valve geometry will all be wrong and the performance will likely suffer. Also, you have to be very careful about the contact stress levels going through the roof on the rocker faces. The radius is critical to where it contacts the lobe otherwise the levels can go through the roof and you suffer from accelerated wear. Spring force and cam lift will accentuate the problem even more.

Be careful. Certainly not how I would solve the problem.

Just to provide some background:
Motor is junkyard sourced, Vin had been removed, so unknown provenance.
It has had a top end refresh, looked like good quality work. Exhaust guides were worn, intakes still perfect. Bottom end, my guess, has 200K on it.

I replaced exhaust guides & valves with new. New valves measured the same length as the used ones. I reused intake valves - I did order a set of new ones, but the used ones were in great shape. Same length as new. So valve stems are not shortened. I recut all valve seats - that should decrease lash.

Heads were not stamped or marked, but may or may not have been milled. Seems any head milling would decrease lash, not increase it?

Rockers measured the same or less lash as a new one I sourced. So they weren't cut very much, or the new parts are way out of spec - not unheard of with aftermarket Porsche parts (Glyco main & rod bearings, anyone?). Either way, buying a new set would solve nothing.

That leaves the cam. I would guess .010" to remove the pitting, assuming the operator didn't want to waste time sneaking up on it. That's about .014" at the valves, and it gets all but 3 valves within upper end of spec, which may have been the target for the original design - after all, virtually anything outside of new valve seats would decrease the lash. Can't think of anything that would increase it - cams & rockers don't wear, they pit. Probably should have bought new cams, but I didn't.

Assuming the cam is the main culprit, machining the rocker shaft faces should actually restore correct geometry somewhat by my figuring. It gets the rocker shaft closer to the angle it's supposed to be sitting at, rather than leaving it relatively more angled - the rocker face sits on the cam, cut the cam, the rocker changes angle. Cut 1/2 the distance off the shaft, the angle moves back to where it started. Or is my logic bass-ackwards? It is known to occur ...

There is of course one last possibility. Maybe Porsche just started slapping together parts in the mid 90's, and if it started, they shipped it, screw the specs. Porsche did go to the Toyota Production System (aka TPS) with the 993, and I bet lunch that Toyota does not carefully measure and hand select parts for the theoretically perfect engine. Porsche was near bankrupt at the time. I know it's heresy here, but I did work in the OEM auto industry for 32 years - it's certainly plausible, and I dare say even likely. All my castings are late 1994, so they would have been the lead parts blazing the way for TPS. Maybe the TPS reports didn't have the correct cover sheet?

Don't grind on anything if the lash is excessive.

Obviously we don't know what happened in a previous life, but if the lash is excessive you can fix this with 8mm "lash caps." Depending on the manufacturer of the lash caps, you can get them from 0.020" to 0.080" thick. Probably even a wider range if you look around.

John Dougherty was nice enough to measure the base circle of an un-ground 993 cam for me - 1.260" - 1.265".
My reground cams measured 1.263" intake, and 1.258" exhaust side. There is about a 2:1 ratio of lash-to-shim inserted under the rocker pad, so the regrind accounts for .007" of lash over a purchased new cam.

993 105 141 51 should be Size 1- 0.5
993 105 141 61 should be Size 2- 1.0
Have you thought about going with a Mechanical rocker kit, kinda pricey but takes all the variables out albeit with more maintenance in periodic valve adjustment.

Richard

Mystery solved. The issue is the replacement exhaust valves. While the same length (-.002"), the valve stem height when installed is .027" less than the old valve (on the one head I checked). The valve margin is about the same, so the sealing surface must be cut to a slightly larger diameter. Thus the valve seats closer to the stem, and thus I get extra valve lash.
http://forums.pelicanparts.com/uploa...1598998103.JPG
http://forums.pelicanparts.com/uploa...1598998103.JPG
(New valves were from si-valves; old ones might be original with a regrind or just OEM, depending on what was done at the last top end refresh)

Richard, the engine is in a mid-engine kit car, access to the lower valve covers is very difficult without pulling the motor - I am highly motivated to keep hydraulic lifters!

So, using tools at had (something for a different thread) I refaced the old valves. Turned out pretty good, and they seem so seal just fine in the one head I've got apart.
http://forums.pelicanparts.com/uploa...1598998103.JPG

As an experiment, I also machined a rocker shaft down .010". Took a bit to get a repeatable setup. I checked the rocker on the cam, and the goo pattern on the face suggests this is also a viable method.
http://forums.pelicanparts.com/uploa...1598998103.JPG
http://forums.pelicanparts.com/uploa...1598998103.JPG

Any thoughts on which is preferred? Old valves with somewhat worn stems refaced using home shop equipment, or machined rocker shafts and new valves that protrude slightly more into the combustion chamber?

So others can avoid my frustration:
https://forums.pelicanparts.com/911-engine-rebuilding-forum/1072393-valve-stem-height-vs-lifter-lash-993-3-6-a.html#post11016820