Parts tolerances and wear limits

[William Miller]

In Waynes book, the little spec book and the bently book there are specifications listed for tolerances and wear limits.

Exactly what do these terms mean?
The sizes are listed as a range and there is a wear limit next to them.

I see this for the crankshaft: main and rod journals
Camshaft Journals
Piston Diameters
Ring Side Clearance
Ring Gap
Cylinders diameters
Engine Case Bores
Rod bores top and bottom
Yadda Yadda.......


In none of these books does it explain what these terms mean.
My engine is a 3.0 930/16 but it has the european Fuel injection system on it. They original long block was replaced with the existing in 1996. I had good compression 180-185psi on all cylinders last fall. This spring one droped out (Rest still good with 95-97% on leak down test.) and upon tear down I found a broken ring on #4. No scoring ect. on the cylinder.
This top ring was badly worn the rest measure close to the new rings on all cylinders. I disassembled the whole engine and am now making decisions on what to replace and/or machine.

The car is an 83 euro cabrolet w' 250K and probably not worth a $15,000 or more perfect engine. (This engine is not that old.)
The measurements and the interpretation of the specifications are the determining point of these decisions.

Experience and knowledge appreciated!
Thanks!

[snowman]

First you measure things. Then compare to the sizes. Once you find the size range you are in ( the crank may have been turned and may be undersize by 0.010, 0.020 or whatever) You determine if the measurements are within the wear limits. The original size variation is for manufacuring, the wear limit is just that the limit before it must be repaired.

If anything is beyond the wear limit you need to somehow fix or replace it. Examples:
1. Piston Ring Side Clearance- Measure using a feeler guage. If its at or above the spec you need new pistons. The rings do not really wear much in comparison to the piston lands.

2. Crank shaft journals- Most importand is to check for roundness. IF its still round its probably ok, as long as its real shinney and dosen't have groves in it.

ANyway all these dimensions are important to very very important. All must be checked 100% and appropriate action taken if any are out of spec.

Based on your question I would suggest getting with someone who has done it before or an engine builder who is willing to work with you if you really want to do it yourself. Even better if your local jr college offers a class in engine building, take it first.

[William Miller]

Thanks, you answered my question. (That's the way I was interpreting the information.)

I have measured my crank. Although it's not new, it's shiny!
The measurments are below the tolerance range but well within the wear limit.

So to make the crank like new I need to go to 1 under on the mains and the rod journals. I understand Wayne's thory to be: A good used crank is better than a reground crank.

I believe mine falls into this category. It has some wear left, but it's not new.

Many of the other measurements revealed the same senerio.

The ring side clearances, with new rings are below the new tollerances, but not to the wear limit. I have Alusil Cylinders have not yet decided if I will replace them or not. (Probably will.)
Any advise for my situation?

I will do a complete valve job with new valves, guides, springs.
The seats look like they have enough meat to machine.

There are many other parts to measure and just as many decisions to make. My goal is not to build an engine to new specs. (I don't have that kind of cash at the moment.) My goal is to increase the longevity of the engine in the most cost effective ways. I doubt I could recoup the cost of a rebuild to like new condition in a car that has 250K + miles on it.

I hope to keep the car for several years, I expect I will tear it down again some time in the future.

Regarding your suggestions:
1. I like the idea about the Jr. College class. I'll look one up.
I had this thought the other day. Do they normally have machine shops? (I could take a class and use their machines?)

2. I have rebuilt a few engines before. Some carefully some not.
I built a twin carb bettle engine that screemed. Another one for a dune buggy I built. The simplest was in my 63 E-Type. (3.8 ltr. Straight-6 twin cam. (The camshafts are mounted directly over the valves. No rockers, no intermediate shaft.) Valves were adjusted with various size shims.) Sent the one head for a vlave job and planing. Everything measured Ok so Honed the cylinders, replaced bearings. Except for waiting for the head work this took about 8-10 hours. (I forgot, I had to order the correct size shims.)
This was for 98+ point concorse car. Spent more time polishing the aluminum valve covers ect. than working on the engine.

So I have some experience, my knowledge is growing every day.
Waynes book is very valuable.

Even though there are strict tollerances and wear limits everyone has their opinion on what should be done. I like that because usually within all the opinions there is some concensus and a ton of information. I will listen (read) carefully to all of it and make my decisions. Fortunately for me, I am doing the work for myself. It will be a ***** if it all fly's apart some day, But I don't have to worry about my reputation and warrantee claims.

I appreciate everyone's comments, Let um Fly!
snowman thanks for taking the time to respond. You are one of the reasons this site, and the internet in general are so valuable!

I hope some day that I know enough to help someone else out, that's what it's all about right?

[ChrisBennet]

If you haven't picked up Wayne's engine book, do so. The book is so inexpensive that it will easily pay for itself.
Measurement is a tricky business. One way to test your measurements is to see if you can repeat them.
Assuming you have a background in "normal" motors, here are a few things that are probably different than you are used to:
In general, Porsche cranks are usually replaced (with a good used one) rather than repaired. (Repairing a Porsche crank is a bit more involved than a repairing a Chevy one.)
Cases that are out of spec can be machined to bring them back to spec without resorting to oversized bearings.
Gasket usually go on dry.
Some fasteners aren't reusable (rod and flywheel bolts).
-Chris

[William Miller]

Thanks Chris!
I have Waynes book. Read much of it. Not all of it yet. I see that you and Wayne concur with replacing a bad crank with a "good used one".

I'm trying to get help to define what "a good used one" means.
Does that mean:

[William Miller]

Sorry hit a wrong button.
Does that mean:
Not Cracked
Not Bent
Not worn at all, or not worn more than the wear limits?
I may already have a "good used one" It may not be bad at all?

[ChrisBennet]

For my purposes, I want a crank that is straight and measures within spec - what they call STD/STD (rod and main journals are standard size, not undersize).
I get the impression, and I could be wrong, that 911 cranks don't wear much. I think they are more likely to go out of spec from damage more than wear. I did measure a 3.2 crank that was way out of spec (from MM!) but I wonder if it wasn't damaged somehow and then polished.
One of the lesson's I learned with that cranks is Trust No One, measure everything yourself..
-Chris

[William Miller]

Mine is STD, STD
The Tolerance for a new crank mains is listed as a range STD=(59.971-59.990) Wear limit 59.960. I don't have my notes at the moment, but all the mains were larger than the wear limit.
The Rod journals tolerance are listed as STD=(52.971-52.990) wear limit 52.960 These measured larger than the wear limit.

#8 was the same sererio above the wear limit.

Taper and ovality were not perceptable

[ChrisBennet]

William,
It sounds like your crank is fine to me.
-Chris

[Alan Cottrill]

William,

you asked for advise about reuisng your alusil cylinders.

have you made a decision?

I'm faced with the same decision.

I've heard success stories and horror stories on this issue.

[William Miller]

Regarding the cylinders, I have had one measured. (It was #4 were I found the broken ring.) At first I took this one off by itself to inspect it.
I didn't want to do a complete tear down at the time because the leakdowns on the other 5 cylinders were very good. 97% and better. The compression was 180-185 across all cylinders until this top ring on #4 broke.

The valve guides on #4 were really worn so I decided that I needed to tear the top end down to check the others. My local mechanic first pointed me to measure the top ring side clearance on the piston. I did.
He said it was bad if the measurement was above 0.4mm. This one piston measured more than 0.4 so his opinion was that it was no longer servicable. We did measure this one cylinder. I recall he said it was marginal. (I need to find the notes.) We went no further because he, (I've seen a lot of consensus on this.) said if the top ring side clearance is larger than specification than the rings will wear very quickly and break.

This is why and where I started this thread.
When I continued my reaserch I found in the specs. in the books included both the new tolerances (A range) and a wear limit. (A specific number.) snowman confirmed my interpretation of the specifications.
The tolerances (the range) is for new parts. (Also, I assume machining specifications for over or undersize.) The wear limit defines when the used part is no longer serviceable and should be replaced or machined.)

With this clarification of the intent of the numbers, it appears that although the side clearance exceeds the tolerance it is within the wear limit. (Top ring:installation size 0.2-0.4 mm wear limit 0.8)
I don't have metric feeler gauges, but they do state the metric equivilent on the blade. The top ring clearances (With new rings) measured somewhere between .5-.6mm the next blade larger (Less than .8) would not go. This tells me that they are not new, but do not exceed the wear limit.

I was told of another way to measure the side clearance. I did mine with the rings installed. I was told to take it off and stick the outside edge of the ring into the grove. (Roll the ring arround the piston as you measure the grove. This will help see if there are any variations in the grove size.
These variations would cause the ring to flex. Maybe break from metal fatigue.

Anyone have any comments on the measuring method for side clearance?

Anyway, I'm going to double check them all before I decide if the pistons are good. I already confirmed that the diameters of the pistons are in the largest group (Code 3). They are close to the big number in the tollerance range. (Hardly worn.) I did notice a 3 stamped on the top of the piston so I guess that was stamped when the sorted the pistons into the size groups.

Once I have confirmed the pistons are OK. (I hope.) I will turn my attention to measuring all the cylinders. Diameter and Ovality. Then I need to subtract the diameter of the piston to determine if the clearances are in within the tolerances. (The little spec book 82-83 lists this to be 0.023-0.044)

Once I have these numbers I can decide if I need to make the 2500 investnment in new mahle P/C s. I am expecting I will have to. I might do it for a piece of mind. It might also depend on what else needs to be replaced and machined.

If I keep them at least I will have documented what condition they are in for future info. Part of my problem with this car was a lack of knowledge of what it was. It's old and has had a lot done to it. Much of it was done the wrong way with some of the wrong parts.

I don't have a lot of experience so I hope someone with more of it can confirm my thoughts above.

Like everyone, my budget is limited and I have many other things to attend to on this old car!

[Superman]

My Alusil cylinders were well within spec at 182k miles. The greatest wear was about 1.5 thousanths (miniscule), so I reused them. They got washed thoroughly first (strongly recommend this), and they work fine. Never smoked.

[snowman]

Sounds like all you need is a new set of rings. I strongly recommend the use of a ball hone on the cylinders (see other posts) except for the Si cylinders of course. this will insure proper seating of the rings. Rember you need new rod and flywheel bolts. check the ring end gaps. Good luck.

[William Miller]

Months later.....

I almost have it back together!
I ended up re-unsing the still in-spec pistons and cylinders. I cleaned them using scotch brite padding on a 3 arm hone. This put a very slight criss cross pattern on them. Hardly any metal removed.

Because the bottom end shafts were std/std and within specs (wear limits.) I only replaced the bearings.
I also replaced the wrist pin bushings on the rods.

I replaced the timing chains and sprokets.

On the top end I did a complete valve job with new valves, guides and springs.

The camshafts journals measured fine and I had a local porsche dealer mechanic look at some slight pitting on two of the lobes which he said were manufacturing defects and the shafts were fine to reuse.

There has been no machining done on the case, cylinders, or head flycut. Cylinder heights measured within specs.

I put it back together with the standard base gasket. Measured the deck height using waynes method in the book. The solder crushed to a little less than 2mm. This is on a stock CIS domed pistion. I think Waynes method really measures piston to head clearance. There was a flat lip all arround the top of the piston. I measured from the top of the piston to the top of the cylinder which was 1mm.

I installed the rest of heads, cam cases, shafts and sprokets.

I began timing the camshafts the other night.

I set the initial timing for #1. It was substantially off, because when I turned the crank ( gently and slowly) the pistion touched the valve.

I set up the dial indicator. (Interestingly I learned that the probe was not quite long enough to get the 10mm preload. I could only get about 8mm which was OK for this anyway.)

I turned the crank to dial in the 1mm intale valve opening, pulled the pin and turned the crank to tdc. The left side went great. I turned the crank (slowley and gently) no more resistance or valve hitting the piston.

The right side #4 went smoothly except I couldn't get it to exactly 1mm.
(I could not get the pin in any of the holes at exactly 1mm.) having to make the choice I chose the hole which would advance it a hair.

With no machine work which would reduce the stock clearances I expected no concerns with piston to valve clearance.

Since I was working on #4 I set up to do the "quick check" described in Waynes book. I screwed in the adjusting valve to touch the valve stem and added 1 turn. (1mm) I turned the crank (Slowly and gently) No reisitance. I screwed in the adjuster screw in 1/2 turn more and found that the valve touched the piston several degrees before tdc. I believe this is the closet point as Wayne showes on his graph in hi book. (The valve opens quickly before the piston starts ittravel down the cylinder.)

I repeated the step with a 1/4 turn less and found no more resistance.
Concerned that this was a little tight. I retimed the right camshaft moving the pin one hole to retard the timing slightly. below the 1mm spec. With this setting there was no valve to piston touching with 1.5mm or 1 1/2 turns on the screw.

I will move on to check the exhaust valve clearance tonight. I hope.

I have a few concerns:
Backing up a moment.
This is a 930/16 block that was supposedly rebuilt in 1996. It goes in my 83 euro cab. The fuel injection remained the euro spec. which ultimately got some wrong parts on it. Currently it has the original euro spec air box, big intake runners, throttle body, mixture control unit. It had a 83 USA fuel distributer with a bolt in the port where the frequency valve pipe would normally go. I replaced this with a 79 fuel distributer and control pressure regulator along with the 79 aux air regulator and 79 aux air valve. These seemed to be the closest match for the euro parts which I couldn't find at the time.
I forgot to mention that I bored out the intake ports on the heads to match the larger intake runners. (Previously there was a step from the larger runners to the smaller diameter bores in the heads.)

Back to my concerns:
I took the engine apart because of low leakdown on #4 and ended up finding a broken top ring. (No dammage to the cylinder or piston.)
The top of the #4 piston has a mark where the intake valves were kissing it. #1 has a very slight mark. The others had slight marks in the carbon on top of the piston, but nothing when I cleaned them off.

My conclusion was that #4 intake valve may have been adjusted tight at some time and was kissing the piston. This caused it to rock slightly in the cylinder each stroke which created excessive wear in the top ring to the point it broke. None of the other rings showed this excessive wear.
The valve guide on this one was worn more than others.
This I assume could also been caused, I assume by weak valve springs or overrevving.

I am realizing now that since my piston to valve are tight with the euro spec. cam timing. The 1mm (mid range setting) setting I used. It would be substantially tighter with the USA spec. Valve timing 1.55mm mid range setting.

I began to wonder if there is another reason the valve timing on the euro cars was retarded to 1deg. Could it have been that 9.8:1 pistons had less valve clearance? I have never figured out how to find what pistons are actually in the engine. They look like stock CIS pistons what's the difference between the USA 9.3:1 and the 9.8:1? Is there anyway to tell visually.

Another thought: If the valve seats were replaced on the last rebuild, the valves could have been sticking below the dome of the head enough to reduce the valve clearance. Is this a probable senerio?

I'm not sure there is a problem because I was able to get the recommended clearance with the cam timing set very close to the optimum 1mm. BTW I forgot to mention the clearance on #1 was fine the first time at 1.5mm on the adjusting screw. Actually it was a little more. The timing on that was right on the money at 1mm.

Sorry for writing a book. Lots of details to get my piont across.
Can anyone confirm or correct my thinking. I'm pretty sure I understand the concepts but don't have the experience. I'm gaining a lot because I really got a basket case on my hands.

[Wayne 962]

It's difficult to tell visually between the USA and Euro pistons - I can't tell unless I have them side-by-side. If you check all your clearances, as detailed in the book, you shouldn't have any problems this time.

One note on the original post, most people don't have the proper tools to make these precise measurements, which is why I recommend sending a lot of your stuff to the machine shop, where they can accurately measure the parts for you.

-Wayne

[William Miller]

Wayne, when you look at them side by side, does 9.8:1 piston have a higher dome at the flat spot opposite the intake valve?

Do you know if there is any relationship between the valve timing and the shape or height of the euro 9.8:1 pistons? (Could the theory I outlined possibly correct?) I might possibly have the euro pistons. If I time the cam to the euro spec I have the 1.5mm clearance you recommend. If it was to timed to the USA spec. , the clearance would be less than your recommended 1.5mm.

This is because with the USA spec. the valve begins to open 6deg. sooner than the euro spec and thus is open farther at the critical point.