To Flycut or not to Flycut, that is the question
 

Hey everyone just a quick question, I have Wayne's new book and am building a 2.7 to put into my 73. I have an option, is it better to flycut the heads or no. I know you get a little more power if you do, but it does weaken them doesn't it??

Thanks in advance from all you people with bigger, fuller, more knowlegeable heads than I. I'm trying to learn as much as I can since I've only had my 911T for 6mos now!

Mike

if your taking the heads seriously, you must be doing forged pistons.. then the cam act.. might be part of a nice package.....Ron

I machined heads and block for 9.5 compression with RS spec cams, new P&S; never heard anything about it being weaker? Runs great ......

Check out the thread,
http://forums.pelicanparts.com/911-engine-rebuilding-forum/96968-time-rebuild-2-7-a.html

Let us know what you end up doing?

As a follow on queston... For a "plain vanilla" top end (i.e. restore to stock specs), should you flycut? I am looking at my options and just want to freshen up a bit.

TIA

A little bit off topic

Hey Harry D

Congradulation on your new job. I hope everything work out for you. I'm gonna miss some of your colorful comment, now that you will be working full time again, that mean less time on this board.

Your (qualified) machinist will inspect the heads and answer that question for you.

If, for instance, he finds that the sealing surfaces are warped and/or erroded, a fly cut of, say, 0.25mm would likely be in order.

Engines that have been running around with a head broken stud or two, will have microscopic damage to the head sealing surface. Mine did.

this is why
 

I have access to a total of 8 heads, 4 are just fine. 2 have cracks in the valve bosses that are repairable OR I can use two that are flycutted.

I don't like the idea of a "fixed" part, especially one as important as this. So should I scout out 2 heads that aren't flycutted, or should I just flycut the other four and have six nice heads?

Thanks

Mike

flycutting is generally done to repair a pounded mating surface. once they are glass beaded, have a good look at the surface and take it from there. a lot of heads have already been flycut, and you can't tell unless you measure them. use a large micrometer to compare them between the cylinder contact surface and the outer surface where the cam tower mates. needless to say, they need to be all the same thickness. there are plenty of 2.7 heads out there, cheap, so i would find 6 uncracked, un-flycut heads rather than fix cracks, or have to deal with ones already flycut and needing it again.

Flycutting will raise compression, and higher compression means higher hp. If you are planning to rebuild your 2.4T to 2.7 RS spec's, compression will probably end up around 8.5:1. By flycutting, you can raise compression at least another 0.5-1.0 w/out creating any problems running on 93 premium street gas, and your engine will have more zip.
With all due respect to John Walker, I have had the pleasure to drive early 2.0S, 2.2S and 2.4S cars. Both the 2.0S and 2.2S engines engine have 9.8:1 compression vs the 2.4s and 2.7RS which have 8.5:1 compression. The 2.0S and 2.2S have a real snappyness to them at high RPM's, as oppossed to the 2.4S which pulls beautifully, but doesn't give you the same snap as the earlier year S's.
To me, part of the fun of driving an early 911S is trying to hold a tiger by the tail. Don't get me wrong, the 2.4S is great, it's just a different feeling.
I had my 2.4E rebuilt a few months ago with 2.4S heads ported and polished, as well as having the heads flycut. With the higher compression on the 2.4, it feels like a cross between the early 2.0/2.2S's and the 2.4S. I am very happy with the results, and every early 911 enthusiast who has driven my car is in love with my engine.
Obviously it means more money and more down time, but if you have someone who knows what they are doing, I would go with flycutting to get the extra juice out of the heads.

Good luck either way.

Thanks Ruf. I'll still be here. Just less frequent. Bad news is that my DIY Top end cna't be done. I'll need to pay my favorite wrench.

Sooooo, for my wimpy T, and what JW sez, this tells me that I don't need to heads cut unless there is some type of dammage.

Thanks to all.

Remember that when you change the distance from camshaft to crankcase (via machining operations to cyl. head, cylinder height, crankcase spigots, etc), an equal amount should be removed from the chain box mounting surface in order to center the camshaft in the chainbox.

Sherwood Lee
http://members.rennlist.org/911pcars