mrmoots

Its a very long and confusing story, but in essence I am rebuilding my 964 engine and using new cylinders and pistons. I had the early style heads, so have had these machined to accept the later style cylinders. In having this done, they have been slightly over machined and face slightly off square with the cam carrier surface.

I have decided to build up the engine dry and check all tolerances as I go. That way any problems can then be thrashed out before I rebuild properly.

All heads have been machined so the cam carrier face is not square with the cylinder face. All heads taper by approximately 0.2mm.

The lowest head measurement I have is 83.99mm. On that head I have a deck height of 1.06mm to 0.90mm (as the face is not square). Measured with crushed solder.

I have measured the compression ratio and its 10.59:1. I am using new Mahle cylinders and pistons, from what I can determine it’s a misnomer that these will give a true 11.3:1 ratio.

Today I have finished the valve to piston clearance check using the most over machined head. I have done the valve clearances twice and reset the cam timing twice and on each occasion recorded the same valve to piston clearances. The minimum clearance I have is Intake 2.5mm. Exhaust in excess of 4.5mm.

Waynes book says clearances should be minimum 1.5mm intake and 2.0mm exhaust. Providing these figures apply to the 964 as well, then I am OK. Anyone know?

It also looks like the cam shaft to timing chain housing seal on the 964 will accommodate more removal of material off of the heads than on earlier engines. Again can anyone confirm this?

From what I have determined so far, I cannot see why I cannot have the heads refaced square then use them. This will give me a head height of 83.99mm, which is 0.29mm over machined from what Porsche recommend. Can anyone think of any other issues I may create in doing this?

Thanks

Geoffrey

The cylinder head needs to be square top against the bottom, and all of the heads need to have the same height. Failure to do so can create issues like the cam binding in the cam tower, improper cylinder to head sealing, and oil leaking. .2mm is .008" and that is a large difference.

The Mahle cylinders are generally in the 10.5-10.8:1 compression range from the ones I've measured.

You should not have piston to valve clearance issues with a stock camshaft. I've run them as low as 1.3mm at 8000 rpm in my own engine, but would probably not run that tight in a customer engine.

As you machine more material off the head you may have issues with the cam chain box 0-ring seal for the camshaft drive end as well as the power steering end.

I've used heads that have been machined about as much as yours and they were Ok, however this was a race engine that only lasts 3 years anyway. When it was dissassembled, the heads were leaking at the cylinder due to the loss of strength on the side of the head between the studs where the first fin was paper thin.

It sounds like you had a non-Porsche machine shop machine the cylinders without the proper tool. It is not that hard a process to do, but your tool must be square and it sounds like it wasn't.

mrmoots

Geoffrey. Thanks for your input. How thin do you think it is safe to take the 1st fin? Currently they are about 1.2mm. In theory this should not change much if I have the heads faced off square, but I would have thought some will come off to give a decent finish.

It looks like most of the cylinder surface is contacting with thicker portions of the head, but it certainly makes them very thing around the left and right edges. Do you think it is worth getting the lower fins welded up to make them solid before refacing? I have heard of race 3.8 having this done.

You have worried me a little now as I thought after the checks I had done I were home and dry.

The machine shop do have a jig and apparently have done hundreds of heads before.

I am just really glad I checked mine before bolting it all together.

Thanks

Geoffrey

can you post a picture of the thin and thick sides?

mrmoots

Two photos of the most over machined head. What do you recon now?

Geoffrey

They certainly are thin and I think you will have longevity issues, espeically if you track this car. It looks like it tapers exhaust to inlet (or vice versa), am I seeing this correctly?

mrmoots

This is the thick side.



The taper is an optical elusion. The machined surface is flat, but the cast fin is tapered slightly.

That is not really what I wanted to hear. Do you think welding the lower fin up is possible, or will the heat cause more problems?

NineMeister

With these heads you really only have two options:
1. Weld up the lower fins and have them all remachined flat, irrespective of the final overall thickness.
2. Throw them and start again with another set.

Obviously with option 1 you will then have the problem that the heads have been over machined. The solution to the chain housing misalignment with the camshaft is to machine the same dimension from the bottom face of the timing chain housing (the faces that bolts onto the crankcase). So the big question is then how much can you machine off the head before you run into problems?

On a custom road-race engine we built some years ago we actually deliberately milled 0.5mm (0.020") from a set of RS heads in order to increase the compression ratio and run a tighter squish clearance. The engine ran fine for some years and we even ran it with higher lift cams than stock, so I would be comfortable to machine the same amount from your heads if you are using the standard cam but not without welding up those lower fins first. If you cannot get anyone local to do this work we could do it for you.

If you do ship the heads, make sure that you pack them in pairs, face to face with a piece of stout cardboard between the faces and then tape them firmly together; then put all three pairs next to each other with more cardboard dividers and tape them all together as a block - this way none of them can bang into each other in the box. Make sure you also use a suitably large box with plenty of styrene foam filler, otherwise yours would join the ever growing pile of transport damaged scrap heads that have previously arrived at our shop.....

dutchcrunch

should not the machine shop give your money back and maybe some compensation for their mistake?? seems like not much to ask for considering how much the cost to replace/repair

darth

mrmoots

I'm in the same boat (i.e. and worse) having discovered my heads were overmachined almost .75mm by a PO. I drove it 8 years/115,000km's in that state unknowingly. A slight weep developed a couple years ago on #3 cylinder/head which was remedied by a head stud retorque and I discovered after dissassembly that the head gasket had broke. I'm not sure how welding the fin will help as the sealing surface is further inboard on the cylinder barrel. To compensate for the over machining of the head a copper shim was installed at the base of the cylinder which interfered with the seal at the base of the cylinder. A workaround for this is to have the case spigot machined to accept an 993RS seal. I'm thinking of machining the base of the chain housing to remedy the seal misalignment. Although not as elegant as Colin's or Jeffery's suggestions (i.e. who I have the upmost respect) it has worked for the 8 years I drove the car and who knows how long before I owned it.

Hope this helps
Bill
90C2

mrmoots

Colin - I will give you a call this morning and discuss this further. Thanks very much for your input.

dutchcrunch - I actually have not paid for the machining yet. But I have paid to have the heads rebuilt with new guides etc, so I am loathed to just chuck them and start again if at all possible.

Bill - We have PM`ed each other on Pelican. Thanks for the info you sent before, it was good to have it in the end. I will let you know what the outcome is.

Thanks all.

mrmoots

A little update on what has happened.

I had a very helpful conversation with Colin from ninemeister over the telephone, which has resulted in me dropping off a second set of heads to him to work his magic on.

I am very lucky to have had a spare set of raw heads to full back on.

In the meantime, I spent Sunday playing with my model engineering lath. I have made myself a wrist pin C clip installation tool, which utilises an old wrist pin as the punch and a alloy "bullet" to slide the "O" rings onto the through bolts. Not supper high tech billet alloy wonders, but functional none the less.


Thanks all for your help and comments.

Will keep you posted.

NineMeister

When you come up to collect the heads, remind me to show you the gizmo I designed and made to put the pins and circlips into the back cylinders of the 996 & Boxster engines.....