Cylinder top machining?

[Scott Clarke]

I have sent a set of 90mm Nikasil core cylinders to EBS to be reconditioned and mated with a set of JE pistons. I was informed by EBS that it was good policy to have the top surface of the cylinders machined (where they meet the head). This represents an additional $100 over the $750 it costs to have the bores reconditioned. The cores I sent seemed to me to be in OK shape at the head mating surface. How critical is this surface? Would it be foolish to not do this?
-Scott

[911pcars]

I guess it's too late for you to check personally. It wouldn't necessarily be foolish not to do it, but I'd probably have them go ahead and do it. It'd be good insurance to have all cylinder heights the same. Make sure they use the bottom spigot (gasket surface) of each cylinder as a reference point.

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

[Wayne 962]

I'm not sure that this is a good idea, unless the cylinders are seriously damaged. Any additional machining of this area can lead to problems with tolerance build up (see book). You really don't want to bring the heads any closer to the cylinders than you have to. Shortening everything up significantly can cause problems...

-Wayne

[911pcars]

Yes, the key word is "significantly". They being core cylinders, are they from the same height group or mixed? If they need trimming ... they need trimming. The questions are .... do they need trimming, how much and what are the consequences? You'll probably double-check deck height and piston-to-cyl. head clearance just to make sure. You can compensate (equally) with thicker cyl. base gaskets.

If the cam/cam housings move too close to the crank center line, trim the chain box mounting surface to recenter the cams. In addition, the chains might be a little too long. This moves the chain tensioner plunger further outward and places the idler arm sprocket and chain closer to the chain box wall. You can counteract with a larger cam idler sprocket.

I suppose the question could be posed to EBS for a first hand opinion.

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

[Wayne 962]

These are 2.7 Nikasil cylinders, right? If so, they should have a sealing ring / gasket in the top which should help guard against leakage. If there wasn't a gasket there, I would think there would be a better argument for machining the cylinders.

I sent the Engine Book out to many, many experienced people for review, and none mentioned machining the tops of the cylinders - I just don't think it's a common thing to do...

-Wayne

[Scott Clarke]

Wayne-
I thought that they were Nikasil. They do have a groove for a head gasket. A current thread about Nikasil vs. alusil has me wondering, as the cylinder walls were deffinately aluminum-colored, not bronze-looking as shown in the photos of the above mentioned thread. They are Mahle. EBS said that it doesn't really matter either way.

Yes, I am concerned about tolerance build up. The spigots of the case have been surfaced, and the heads were from my 2.2, so they have been machined. The pistons are high compression to begin with, so I will want to have the capacity to adjust deck height to dial in the compression ratio. There is no advantage, in my case, to reducing deck height.

EBS said that there is a small nick in the top of one of the cylinders. They have a set, already reconditioned and machined, in stock. I wonder if this has something to do with their recomendation. I'm sure they get plenty of cores from motors that have had pulled head studs, and are significantly damaged.

Yes, Sherwood, they are from the same height group.

Unless anyone thinks that a slight nick represents a significant risk of head gasket failure, I think I will have them recondition the cylinder walls and leave it at that. I very much appreciate both of your comments.

-Scott

[911pcars]

You may be correct in that the nick poses no problem, but EBS brought this up, yes? I don't know where or how large it is. I would be concerned if the nick creates a path into the burn area.

Since these are JEs, you should be able to compensate/adjust deck height by machining the piston crown. I believe JE leaves plenty of meat to adjust CR in this manner. I would also assume EBS (they being responsible and knowledgeable) would be cognizant of this if the trimmed cylinder heights fall below factory specs.

Best wishes,
Sherwood

[Scott Clarke]

Well, I called EBS back, and John was nice enough to go get the cylinders and describe what he was seeing. Two were of concern: one had corrosion on the surface, and the other looked as if it had been subjected to a screwdriver as its head was being removed, leaving a scrape that continued on both sides of the gasket groove. John thought that these defects represented a significant risk of head gasket failure. He added that a shop would not have used them un-machined, but that I could assume the risk since I'm building the motor myself. So, I feel confident that EBS has my best interest in mind. They have been very helpful and informative. Boy, it would suck to have a head gasket failure. Off goes a check for $1860. Thanks, again, for your comments.
-Scott

[Wayne 962]

I hope that the cylinders by themselves weren't $1860...

It's impossible for us to judge the condition of your cylinders based upon your description. It is not normally performed on the cylinder tops.

-Wayne

[Scott Clarke]

No, that includes JE pistons and shipping. I understand that it's impossible to evaluate the cylinders based on a brief description. I just have to trust EBS. Since I have heard nothing but possitive comments about them, and they have treated me well, I feel comfortable with my decision. I do find it odd, however, that this procedure is not common practice. Intuitively, it seems like poor upper cylinder surfaces would lead to head gasket failures.

[andrew15]

I called them a few days ago about boring 90mm cylinders to 92mm and replating them (got a set of RSR pistons I'm dying to use). I spoke to John (I think) and he mentioned the machining as well - he seemed to think it was a good idea and the cost wasn't much - $100 for all 6

Regards,
Andrew M

[911pcars]

Andrew,
I realize $100 isn't much in the realm of 911 engine rebuilding, but $100 is out the window if it's not needed. There are a lot of opportunities to throw money at an engine rebuild.

How about calling a couple of other reputable builders and get their opinion? At what point would they trim, or is it something they regularly do?

Or think of it this way; $100 buys:
10 bottles of Motul brake fluid or
9 cases of 20-50 oil or
65 In-and-Out burgers or
50 bottles of "two buck chuck" Chardonay

I'm not an advocate of "it's usually a good idea" unless I see a need for it. In Scott's case, it might be due to the nick. I'm less convinced if it's a matter of standard operating procedure, especially if the cylinder sealing surface is okay.

Sherwood

[jgparker]

I had mine done by EBS, and they suggested the same thing, but I'll admit I did a fairly poor job in packing my cores, and it ended up maring the sealing surfaces. They came back looking great though. I did need extra shims in the end, but I had a lot of machining done, and this was probably the third rebuild on this case.

[Doug Zielke]

Sherwood sez:
"65 In-and-Out burgers"
Mmmmoney well spent!

Oh yeah....I used EBS for quite a few things since I've had my car. Good people.

[andrew15]

Hmmm - $100US = $135 CDN and $1 will buy 2 McDonalds pies so that would be 270 pies! There goes the weight reduction plan.

Sherwood - I'm totally with you with you on the cost / benefits. I'm not trying to advocate the machining - Instead I was trying to point out that they mentioned it to me as a matter of course - they hadn't looked at my cores or anything. The part about 'they are already getting work done & it's only $100 extra' was their justification not mine

Personally, I'm still looking for a set of cores and will take a close look at their condition before I machine them. I'm definately concerned about the closer tolerances machining would give - I'm going w/ 2.8 P&Cs on a short stroke crank and am hoping to keep it single plugged - copmpression will be touch and go.

Regards,
Andrew M

[Wayne 962]

Originally posted by Scott Clarke
I do find it odd, however, that this procedure is not common practice. Intuitively, it seems like poor upper cylinder surfaces would lead to head gasket failures.

What you are saying is correct - poor surfaces may lead to cylinder leaks.

However, let's think about the setup here. We have a very hard steel cylinder and a relatively soft aluminum head. Which one is going to be deformed the most? Typically the head takes all of the abuse, and the cylinder doesn't really get damaged. The exceptions are pulled or broken head studs where the head is flapping against the top of the cylinder. This often breaks up the gasket and pounds it into the top of the cylinder.

-Wayne

[Wayne 962]

Originally posted by 911pcars
How about calling a couple of other reputable builders and get their opinion? At what point would they trim, or is it something they regularly do?

I did this research already in writing the book. No one I spoke to mentioned machining the tops of the cylinders.

-Wayne

[Scott Clarke]

Wayne-
Remember, these are aluminum cylinders, not steel.

I can imagine that if one were to bore the cylinders to 92mm that the machining process would be more important, as the area that is between the gasket and the cylinder wall becomes quite thin. Sounds like a good idea for Andrew.

As for me, I had the brilliant idea of sending them two cylinders that have good top surfaces! If I were to have the machine work done, it would make sense to send 6 cylinders that need machining!

I appreciate the comments of the value of $100. A few of these little episodes stacked up results in considereable expense. I will, however, be spending my $100 at Wayne's place as opposed to a burger joint.
-Scott