Dave928S

I'm rebuilding my heads at the moment and I'm considering if I do anything with the valve guides ... which weren't a problem when the car last ran. I still have to take comprehensive measurements, but guides are only a little sloppy.

The WSM and the Dimensions and Tolerances guide give various information in relation to (MY 82 Euro S M28.11) valve/guide dimensions and wear limits.

The WSM sets out the procedure for checking the wear using a dial gauge, with the valve stem end flush with the head top face, and give a limit of 0.80mm ... which seems a bit on the high side to me.

The dimensions and tolerances guide on pages 20 - 21 gives a stem to guide wear limit of 0.50mm for both intake and exhaust.

They list the valve guide bore (new) as 9mm with a tolerance of +0.015 ... the intake valves (new) as 8.97mm with a tolerance of -0.012 ... and the exhaust valves (new) as 8.95mm with a tolerance of -0.012. If you take the guide bore and valves at the extreme of the tolerances (when installed new) you get maximum and minimum clearance of 0.030mm - 0.057mm for the intakes, and 0.050mm - 0.077mm for the exhausts. Hope I got all the calculations right.

Does the 0.80mm rock/wobble test wear limit equate to the 0.50mm valve guide/valve stem wear limit? I'm thinking that the wobble test could also be an indicator of bellmouthing of the guide with maybe a lesser clearance midway, which raises the question of whether guides wear in a parallel fashion, or is there more wear at each end? So ... with these two measurements are we checking the same thing? If not ... what if it is excessive on one but OK on the other?

Bearing in mind that new maximum clearances are 0.057 - 0.077 .... a wear limit of 0.50mm seems a bit sloppy.

I'd appreciate the thoughts of everyone who's re-built these motors, as to what is excessive, what measurements are meaningful, and when guides should be replaced.

Bearing in mind the process of removal and replacement is a drama and risks stressing the head, I only want to replace if there's a good reason ..... I'm doing way too many WYAIT's at the moment. I'll post photos of the almost bare shell sometime soon.

Dennis Wilson

Dave,

On my 78 euro the valve guides were worn oval with the maximum wear being measured as in the picture above. If yours are under the wear limits they will probably be OK with new seals.

Dennis

Dave928S

Thanks Dennis ... it'll be good to hear what others have experienced also, so that I can understand the nature of the wear on these motors, and hopefully why Porsche set the limits they did. As I mentioned, the wear limit seems excessive to me, bearing in mind new clearances ... and what I've experienced on other alloy race, rally and road heads.

Dennis Wilson

Dave,

Considering the clearance is for oiling, it isn't as critical as long as the valve seats and seals are good. IIRC the earlier cars had a softer bronze alloy in the guides so they wore a lot quicker. Since the heads are already off, you might check with the local shops to see what they would charge to replace them WYAIT. It could be cheap insurance.

Dennis

terry gt

If there is any wear , For 6.00 each + labor , why would you even think about it .

Dave928S

As I said in my original post terry if I did all the WYAIT's while my car is virtually a bare body shell I'd be replacing absolutely everything .... because it's a 28 year old car and everything is worn to some degree.

I asked quite a few questions in my original post and I'm looking for some enlightenment as to why Porsche specified what they did. The shop doing my heads (who do race motors and work on other Porsches, Ferraris, Lambos etc etc) have said to me they regard the wear limit as excessive but they thought maybe Porsche had a good reason to set those measurements. Mine are within them ... so do I replace guides that Porsche regard as OK?? I will if they're likely to crap themselves shortly after I put the whole motor together again ... but if they'll last another 100.000km or more then I'll leave them alone. Gaskets, studs, bolts, broken parts, badly worn parts are a no brainer to replace while I'm there .... drilling, pressing, reaming valve guides is a process that I don't want to go through to end up with something that's not practically any better.

I'm looking for the sort of information that Dennis gave me (his were oval and to the wear limit), so that I can make a value judgement based on the experience of others, and not have blind reliance on WSM or specs that seem contradictory and vague. I'm following the head stud thread for the same reason.

Dave928S

Dennis
So the oval meant that you were getting side to side rock on your motor? Was the oval dimension less than the wear limit one way, and up to or more than the other way? What miles were on it? Did you have symptoms?

If I can see where mine are in the ranges of wear that others see, then I can make a value judgement as to where my guides are compared to average wear seen by others ... and therefore know whether I'm on the edge ... or have lots more useful life left.

Dennis Wilson

Dave,

The wear was side to side as the gauge pictured above is measuring. The out of round was at least .120". The symptoms were blowing smoke and oil out of the exhaust and carbon caked on the backside of the intake valves between the stems and seats. This is most likely the reason the car was parked for 10 years before I bought it. I had to do a complete fuel system rebuild to discover this.

Dennis

Dave928S

Thanks for the further info Dennis

Jeeez!! .. you weren't kidding when you said out of round. The valves were trying to get out of the head!! It stands to reason you were getting major symptoms ... I've had none.

I searched for threads on this issue but couldn't find much ... hopefully I'll hear from others to try and figure what happens and how quickly. I'm thinking that wear is likely to be very gradual up to the wear limit,and maybe rapidly escalates once it gets to the stage of 0.80mm slop, when it starts to slap back and forth in the guide with more energy ... and that's why Porsche decided on that limit ... it's the point beyond which wear and leakage gets rapidly out of control.

Dave

Dennis Wilson

Dave,

My head guy told me that Porsche went to a harder alloy on the guides in the early 80's so that may explain the different wear tolerances. The 928 uses a very short guide so the wear was more rapid.

It is amazing that all the valves were seating without one being burnt. Compression was good on all cylinders.

Dennis

GregBBRD

Dave:

The factory wear limit is very extreme. I think of a wear limit as something that is absolutely worn beyond use...and won't work, no matter what. Middle of the Sahara...928 apart...only way out, other than walking. Valve guide wobble=.8mm. Start walking.

I replace any guides that get close to .4mm "wobble", when I'm rebuilding....which is half of the factory wear limit.

Dave928S

Thanks Greg
That's the sort of info I wanted to hear and a great help. That limit seemed ridiculously excessive to me too.

toofast928

Just happen to stumble across this thread. I'm in the process of rebuilding the same 16V heads. The manual spec of 0.80 mm (or .031") doesn't make sense to me. Must be a misprint .08 mm makes more sense. Any engine I ever assembled specs called for .001" clearance and .003" is the max. I always replace the valve guides during a head rebuild. This head I found the valves stems to be worn .001" so all valves were replaced also.

So my recon head now has new valve guides with id of .346", and new valves with valve stem size .351"
Making the valve guide clearance .005". I can still feel some wobble in the valve. I don't mind some extra clearance since the engine is going to be SC, but still like to be around .002" clearance.

I'm going to have the valve stems "dry lubricated coated" to try and tighten up on the play. See how that works out.

Tony

AO

Relating to my rebuild and rather than start a new thread specifically on valve guides, I thought this was the best/most recent one to bring back to the top.

So what's the best source/make for replacement guides? SI, Porsche? Tony, how did the coating work out? Are there any other considerations one should look at when replacing the guides?

69gaugeman

I left mine alone at .5mm wobble. The engine runs fine. I just replaced the stem seals.

Now it needs rings and machining, but that is another issue.