How would you shim these cylinders

[wndsnd]

I am building a 2.7 magnesium case engine.

I have a straight edge ground to .001" or 0.0254mm

Placed across the spigots on the right side I can measure - 0.1778-0.1905mm
on the center bore.

Placed across the spigots on the left side I can measure -0.1524 - 0.1651 mm

Base gaskets are available in .25mm and .50mm and 1.0mm

My questions are, should I use an additional .25mm gasket on the center, compbined with what I decide I need for deck height clearance?

and, is that severe, moderate, or minimal distortion for a mag case.


Thanks for your input!
John

[safe]

You should machine the case straight again, not shim the cylinders.

[wndsnd]

Too late for that Magnus.

John

[The cap'n]

Considering the problems you'll likely have with doing it wrong, it's never too late to do it right ...

The Cap'n

[wndsnd]

Well I made the financial decision not to do the machining because I thought 15-19 / 100th of a mm was minimal when base shims start at .25

I know there are other custom sizes out there to be found as well. Wayne describes extreme in his book as .50 mm.

So since the Short block is complete, I will be using shims.

What about if I went for a 1.00 mm shim, on all spigots, so as not to stack. Will I get enough compression to even things out at specified head bolt torques do you think? Or go 1.00 in the center and stack .50 and .25 for a total of .75 ea on the ends?

Thanks
John

[safe]

I think you misunderstand the usage of shims. They are NOT for correcting warped cases, they are used to get the correct deck height.

0.2 mm are a huge difference. The difference between the two cylinder height groups are 0.025 mm and you can't mix the two on the same side.

How's your crank, does it turn smoothly over 360 degrees?

[wndsnd]

Yes, I am running a Std/Std Crank with bearings. We did do the head bolt inserts and the intermediate shaft bolt insert, and restudded the entire case.

There is not one pulled stud on this case and it reportedly was original without thermal reactors. Everything is very smooth, infact there was no evidence of any head leaks or cylinder leaks upon teardown.

John

[Steve@Rennsport]

Hi John,

Magnus is offering you 24kt gold advice and I'd strongly advise you to follow it otherwise you are in for some serious trouble with your engine.

I tell my clients that when they don't have the money to do something right, they always have the money to do it all over again. There is nothing cheap about correctly rebuilding a 911 engine and especially the mag-cased ones.

[wndsnd]

Thank you Magnus, Steve, for your advice. This is exactly why I posted and exactly not what I wanted to hear!

This is not going to be a race engine, nor will it be in a concours car. I know what should be done, and with an unlimited budget, I would be doing it. I was hoping that there was a little leeway here. And, I could make a decision based on the trade offs.

Considering this was a running engine that turned out to be a POS core, I thought well, it has been cleaned, measured, case inserts done, rebearinged, sealed, pistons, induction, head work, etc, it has be to be a decent build, not a perfect build. Believe me I am not cheaping out on this. I wish I lived out west where good machine shops flourish. Up here in the Northeast there was no one I could find that was experiienced in this type of case work. Everything gets shipped somewhere and returned.

So I can tear it all down, and clean it up, and ship it all to one of the other great west coast machine shops, or, what exactly can I expect?

No one makes corrections, every case gets re-machined? I am not exactly slapping this thing back together. I guess what I am asking is what specifically do you think will result? I really am listening and am seeking advice.

Thanks
John

[jhg41977]

Nothing to add, just want to see where this goes. For what it's worth it is better to delay and do things properly than to rush and do it twice. I've been down both roads.

[tocobill]

There is a couple places to machine the 911 engine out here east. One that comes to mind is Verden Tool in Dover. Still have to ship but not to the west coast. I know they do head work but I would try to call and ask about your case.

Machine Shop Leads - NY / NJ / CT - Rennlist Discussion Forums

I would also Dawe Motorsports. They build alot of race engines. I would imagine they could or might have some info for local shops for you.

About Us - Dawe's Motorsports

[pete3799]

http://exotechpower.com/5601.html
No affiliation.......just 14 miles from you.

[Walt Fricke]

I had the same reaction as others when I read what you were thinking - that you could compensate for the fact that the tops of the spigots were not level, so to speak, but for either side some were higher, some lower. Middle ones were the most off? And you planned to compensate by thinking the copper gaskets would crush differently, so you'd end up within the 0.025 tolerance Magnus points to. But they really don't crush any appreciable amount.

If the spigot tops on each of the sides are "level" with respect to the crank centerline, but are at slightly different heights, I don't think that is apt to present a significant problem. Probably not enough to impair cylinder head or valve clearances, or to change the compression ratio enough to worry about.

The goal is to have the tops of the cylinders all in a line. If they are not, then when you bolt the cam carrier on and torque the heads, you are apt to find that the cam is binding. Many of us have felt a little bind when assembling a motor, and have counted ourselves lucky that loosening the fasteners and retightening them solved the problem. But with things dimensionally wrong you can't solve that problem.

I'm now supposing the tops of the cylinders are where you measured (maybe you said that), as it is much easier to do with the cylinders slipped in place. If the copper gaskets could be had in 0.025mm increments, maybe one could fiddle with them to get the cylinder tops right.

Alternatively, one could hunt around for different height cylinders, and see if mixing things up might bring things into tolerance.

Or one could have a machine shop remove just the right amount from the lower mounting surfaces of cylinders, sinking the tall ones a bit. I don't think you can take much off the tops of the cylinders, lest the CE ring groove gets too shallow.

And, of course, for a cylinder a bit too tall you could have its mating surface turned down a little. I suspect a good machinist could come up with a scheme using these alternatives to get the tops of the cylinder heads, on which the cam carrier rests, to be parallel with the crank centerline.

But none of these things really makes any long term sense - you end up with a bastard motor. And I suspect a good machinist would try to dissuade you from trying something like this. I think it would take less machinist time to set up the case and machine the spigot tops true.

The benefit of using a shop which has experience with this kind of thing is that such a shop knows from experience how these mag cases do or do not squirm around, or change shape when all torqued together, and so on. Not something us home brew motor assemblers really know about. You knew enough to put the straight edge on, notice the problem, and ask about it.

There are an awful lot of good Porsche race shops in the NE. I bet most of them would take a call and tell you what machine shop they use to true case spigots, if they don't do that kind of work themselves.

[wndsnd]

Walt,

Thank you for the dialog. I remeasured today after reviewing my initial notes and I got slightly better measurements though not even. Yes the center spigot is lower and I am measuring the spigots.

Before I tear this down I was thinking I would build up to the heads with .25 gaskets and retake the measurements at the heads. I can not do that yet as the head work is not done yet. What I was getting at was what kind of problems could I expect if I continue on with this build as is. From a scale of let's say one being an oil leak to five being a pulled stud to ten being throwing a rod? I was thinking of uneven compression ratios, head gasket leaks etc. Cam binding did not occur to me. But if that happens, I can always take it apart then. An oil leak or two I can live with. Blowing up my new parts not so cool.

All my cylinders are in the 6 tolerance group. I suppose I could put 5 's on the ends but I think that would be over compensating and as Magnus points out it is not recommended to mix on one side.

I know the various gaskets are intended for deck height adjustments, and I would assume that you don't mix on a side, but I did not remember seeing anywhere that you could not vary per cylinder.

. Thank you for answering the question on gasket crush. That is what I expected.

Knowing what I can expect for results I can weigh my options and make a decision.

BTW, one of the shops mentioned did the inserts and never mentioned spigot variances after working the case on their mill. I asked for a thorough inspection at the time. I am not placing blame or pointing fingers, but if there was an immenent problem, I would guess they would have picked up on it, but then again we are talking a variance of about .0015"

[Walt Fricke]

Since you have the case buttoned up and the head studs in, I guess it might make sense to:

A) install the copper gaskets and cylinders (no need for pistons), and torque them down. That is tricky if you don't have some tubular pieces to fill in and act like the heads. But you could cut some tubing and put washers on the lower end so you don't push rough cut tube ends into the head material. And then use your straight edge and feeler gauges or whatnot, and measure variance up where it counts.

B) Then, maybe no matter what, put the heads on, and the cam carriers. I like to give the heads a light torque, then torque the carriers, then insert a cam, then do the full torque on the heads, rotating the cam by hand from time to time as I go. You can see how the cam feels this way. I seem to recall slight binds at first going away as I work through the tightening sequence. Because no sealant, no cam gears, no chains, no pistons, no rockers, etc., this is actually not that time consuming to do.

If you don't get a bind, and persuade yourself that the height variance you measured isn't going to be that big a deal, you can figure out if you want to go on.

The seal I would worry most about is not the spigot seal and some oil weeping. It is the cylinder to head seal. I'm not qualified to say if the differences will lead to CE ring blowout, and perhaps eventually a flame cutting through to the outside, ruining cylinder and head, but probably not blowing up the engine.

But 0.025 mm = 0.000984249 inch, or a bit less than one thousandth. 0.0015" = 0.038 mm. That is rather a large percentage change. I don't have the hands on experience with hundreds of older motors, so I don't know if that change of half a thousandth is or is not that big a deal.

You can't get a regular shim which is less than 0.25mm, right? So to get which is 1.5 thou, or is 0.25 plus 1.5 thou (0.15mm), you are at 0.40mm. So with a 0.5mm shim, you are 0.1 off the other way. Know a custom shim maker? You could possibly have a shop take shim stock of the right thickness and punch out two shims.

But Steve, who has that experience, gave you an opinion. I don't know Magnus other than from these pages over some years, but he sounds like he is a careful, knowledgeable fellow.

It is a shame that, since the machine shop had the case on its mill, the spigots weren't decked at that time. But if they don't do a lot of mag case 911 motors, they may well not have known this has become something to check, and if needed, fix. And the setup for something simple like drilling existing holes and tapping is probably rather less complicated and exacting than that for truing spigot surfaces.

[jhg41977]

Mark at Exotech is a good guy and a well experienced shop. He can be a bit gruff but dont let that sway you away from him. He did the heads for my 993, the work was great. Stop by his shop and present you problem. If he believes it to be a problem he will tell you so. Im not positive he can do that machining there. He will know a place though.

[tharbert]

There are any number of threads here of folks asking whether something atypical will work...like shimming spigots to account for case warpage. I think you are luckier than most in that you've gotten several answers, mostly which you were looking to avoid.

Fact is, you've saved about $1k by not having the "normal" mag case machine work done: case halves resurfaced and align bored then the spigots decked. Nobody can really answer when you ask what you can expect from going your own route. The successful builders simply don't work this way. Now, it's simply a gamble on your part to continue. Please keep us posted on whether you set the example or you end up being one.

[wndsnd]

Sorry, don't want to be an example good or bad.

I am not a builder, just an old hobbyist that has restored a few cars which all happen to be Porsches.

I ask questions so I can learn from others. But, in the end the decisions are mine and I own them.

However, I am guessing that Porsche's manufacturing tolerances are quite different than assembly tolerances. One is done in a controlled environment using precision tooling, and the other you try to control the variances. These engines are not swiss watches and have proven to be dependaple, reliable, and rugged over 50 years.

I did not expect anyone to put their reputation on the line.

I enjoy the different opinions and ideas.

I always enjoyed my fathers advice and sayings. But his strength and knowledge did not really include Porsche engine building based on application.

John

[wndsnd]

Originally Posted by Steve@Rennsport

Hi John,

Magnus is offering you 24kt gold advice and I'd strongly advise you to follow it otherwise you are in for some serious trouble with your engine.

I tell my clients that when they don't have the money to do something right, they always have the money to do it all over again. There is nothing cheap about correctly rebuilding a 911 engine and especially the mag-cased ones.

Well, I never really liked how I ended this thread and wanted to apologise to everyone that gave me good advice that I did not want to hear at the time. I was a little peeved the the shop that I had do the case savers did not mention or offer advice on the spiggot height and I was rushing into the build. I finally realized that that Steve's words above were right on, and I did tear the case back down.

A year later, the case was done properly in AZ, and I am progressing with the build. I do not want to do it over again, so it is being done right. It is now done up to the heads, and I got great deck height with one .50 shim on top of the barrel shims provided by the machine shop. For 2016 I am resolved to slowing down, and not being as snarky. Thanks guys for setting me straight before I hung myself.

[Jeff Alton]

Glad to see it going together properly. Sounds like the case went to George and Ike, a great choice!

Cheers