Post rebuild thought....

Doug Zielke · 01-03-2003, 06:34 AM

In the spec book, Porsche cautions that the heads can only be flycut (once) to a maximum of 0.25mm.
Does this mean flycut heads are essentially scrap if they required a future resurfacing job, say..... another 100k miles down the road?

HawgRyder · HawgRyder's Garage

Doug...it probably has more to do with compression ratio.
But you can flycut several times if necessary by shimming the bottom of the barrels to lower the ratio.
This is contingent on having enough room for valve to piston clearance of course.
Bob

tiorio · 01-03-2003, 07:32 AM

Wouldn't shimming out the bottom of the jugs compensate for the change in piston/valve clearance anyway?

Jamie79SC · 01-03-2003, 09:10 AM

Doug,

Are you really concerned with the value of heads with 200,000 to 400,000 miles on them? You ARE frugal. ;-)

Can't wait to see you on Northwestern highways this spring! Where to for the maiden voyage?

Superman · 01-03-2003, 09:15 AM

My heads and cylinder tops were flycut a total of .25mm. You're supposed to use a .50 base gasket instead of a .25, or two .25's, but I broke another rule and used two .25's. If I had it to do over, I might just use one .25 gasket. This would have increased my compression ratio, and I am told there would be no valve interference. When you do this, you move the cam sprockets, relative to the crank sprocket, so you have to watch for cam chain problems. Do-able, though.

Doug Zielke · 01-03-2003, 11:23 AM

Quote:

Originally posted by Jamie79SC
Doug,

Are you really concerned with the value of heads with 200,000 to 400,000 miles on them? You ARE frugal. ;-)

LOL!
Now don't confuse me with that notorious cheapskate from 10-9-0

Just curious more than expecting to keep the motor running forever.

01-03-2003, 11:42 AM

I know with VW heads if you flycut too much then you can start losing cooling fins as well as compromising the structural integrity of the head.

I've said this over and over, but I'll say it again. I know it's blasphemous to compare Porsche parts to VW parts, but I still think that there's some universal truths that govern what you can and cannot do to either one of them.

Zeke · 01-03-2003, 11:49 AM

Quote:

Originally posted by robh
I know with VW heads if you flycut too much then you can start losing cooling fins as well as compromising the structural integrity of the head.

Yes, that's true, and when you true the tops of the cyls, it exacerbates the problem. Do you guys ever true the tops and bottoms of cyls to blueprint the motor, or just buy new ones?

Superman · 01-03-2003, 12:00 PM

Well, ummmm, they do need to be straight. That's why the cylinder tops were flycut with the heads. They need to be flat and true.

Oh, and I never confuse you with THAT guy, Doug. Although, you are pretty smart, like he is.

kwikt 911 · kwikt 911's Garage

Damn, I had my cases resurfaced and and didn't use the .040in. base gaskets supplied. Instead I used the stock base gaskets and still had clearance to spare. The shop told me I would just get a little more compression and cams and housing should still line up fine. I have been driving the car for the last 2 weeks and it runs great.

Kevin
72 911T/E [linebored,valvejob]

Superman · 01-03-2003, 03:34 PM

Yeah, that's what I'm saying. You can probably get away with stock base gaskets. Perhaps the engine will have higher compression and make more power. If you don't want all that power, then be sure to install the thicker gaskets.

emcon5 · emcon5's Garage

That raises an interesting question, how much will using the standard base gasket raise the compression? Assuming a 95mm bore, as we seem to be talking about 3L engines here.

I would think the tensioners should be able to handle the sprocket being .25mm closer.

Any thoughts?

Tom

TimT · 01-03-2003, 04:19 PM

One thing you have to visualize is this:

If you excessively flycut the heads, the cam center moves in a like amount.

That makes it a ***** to seal the cams in the chain housings

That can be corrected though, by machining the chain housings.

In order to keep everything dimensionally correct. You should add an additional gasket that equals the amount flycut from the head to the thickness of the base gasket. ( or you could increase the CR by leaving the additional gasket out)

The timing chains can live with minor differences side to side, especially when using new chains and ramps.

TimT · 01-03-2003, 04:19 PM

wow caint type biatch

Doug Zielke · 01-03-2003, 06:37 PM

Just for the record, my machinist cut the heads 0.25mm, so I used the 0.50mm base gaskets as per factory instructions.

Whatever the small increase in the CR may be (by using a thin gasket), I'd rather have my valves staying away from the piston.

HawgRyder · HawgRyder's Garage

Yup good idea Doug....keep the HP down (so I can keep up with you). LOL
Bob

speeder · 09:46 AM

I just had my heads cut by C.E. during valve job, .25mm I was told. Using shims at base of cylinders has to do with precision of height of cam towers:chain covers, compression increase from .25mm cut= significantly less than .10 of CR, , so performance increase would be nonexistent. Valve/piston clearance would also be a non-issue w/ a .25mm change, normal clearance is quite a bit more.