Newbie '73T 2.4 MFI Engine Rebuild

[Inkblot]

Regarding the question of how to measure the clearance between the rod and the crank, I found this in an older thread:

Originally Posted by Henry Schmidt

We use a general rule of .007"- .012.
The truth is after 30 years of building these engine you can actually tell pretty close what the clearance is by feel or more accurately, sound. Rattle the rod back and forth and you can hear the clearance. It is very difficult to measure to rod journal on the crank so this method proves more than adequate.
The key is to start with a rod in which you have complete confidence.
That means rebuild it (all of it) and then measure it.
If a rod feels/ sounds too loose we use a feeler guage in multiple places around the rod to verify thrust.
If the engine is intended for racing then we lean towards the looser end of the spec and run higher oil pressure.
We also prep the big end thrust surface with dry film lubricant.
We are experimenting with DLC (Casidium) in place of dry moly but at this time a lack of competent coating suppliers has left this process very expensive.

Here's the whole thread, which is a great read:
Bored? You'll think about stroking when you see these rods.

I really appreciate the idea of using "a rod in which you have complete confidence" - and not just because I have an immature sense of humor - but because it's clear that side-surface of the rod can vary a little bit (depending on the quality of your machine work). So, measuring the gap at one position might lead to a different value than measuring at a different position, which might cause you to overlook a problem.

I also love the idea that experienced engine builders, like Henry, can develop a tactile sense of spec over time. I have no doubts that developing a memory for the sound an in-spec rod makes when it's jiggled in place would grant Henry results on par, or superior to, spending all the time to dork around with feeler gauges.


- Jake

[Inkblot]

Originally Posted by Neil Harvey

There are many ways to do something, but I would like to suggest you re think your method here.

When rods are rebuilt/"resized" the fasteners the engine is going to run with have to be used in the resizing operation. The free length should be recorded, then stretched. To be sure you have not gone past their yield length, they can be loosened and remeasured. As long as they go back to their original free length you are Ok. Factory spec's will not over stretch them.

They (BE) should be measured then (after honing when cold), disassembled and cleaned. The same stretch should be done when using your method of checking the bearing ID size. Changing fasteners more often than not, changes the bore size.

Not sure why its said you can stretch these bolts only once? Too much fear is placed into the minds of many about rod bolts.

Originally Posted by Trackrash

Neil. Good advice. I think the "gotchya" here is the fact that you will need to know the length when the bolts are new and unused. So old bolts coming out of a motor are an unknown. But when using new bolts it doesn't make sense to only allow one torquing as you mention.

Also good advice on using new bolts to measure the big ends. I had, on my recent build, one BE that measured at the limit of spec. I put in new bolts and it measured perfect.

@Neil, @Track: Ok, thanks for weighing in! My lack of experience is showing here... I've just been "going by the book". I completely agree that, logically speaking, it doesn't make sense to me that you can't torque a bolt more than once- assuming you don't over-torque it. Unless, of course, the spec is actually designed to over-torque the bolt, as a one-time only operation... according to Wayne's book, pg 123-124:
"Again, it is important to note that the stock rod nuts and bolts can only be torqued down once. Once they are tightened, they are permanently deformed, and cannot be reused."
... and a paragraph later:
"Make sure that you don't reuse the rod bolt, as you can only torque them once."

So, in my case, knowin' nuthin' from nuthin', the message seems pretty clear!

I guess the question (as you point out) is not so much about final assembly as it is about measuring tolerances- i.e., are the big ends of my rods actually round, as the roundness was measured using the old rod bolts. Wayne's book would seem to suggest that either:
a.) It's ok to recondition and then measure your rods using old bolts, prior to reassembly with new bolts, or:
b.) You have to use a set of sacrificial bolts, using them only once, to do the reconditioning work, or:
c.) Wayne's concern over the stretch of the rod bolts is misplaced, and you can re-use them (assuming you double check their lengths against a pre-torqued baseline).

I'm not sure where to go with this. What does the forum suggest? My instinct is to choose a.) "call it good", following the book's instructions, and thus assuming that the roundness won't vary beyond spec when moving from the old rod bolts to the new ones.

If time and $ were no object, it might be fun to buy a set of sacrificial rod bolts, and do a little test to see if I can measure any variation between the old bolts and the new.

I'll cogitate a bit and see if any more good feedback comes in...

- Jake

[Trackrash]

A number of us chose to buy ARP rod bolts. Those eliminate any doubt.

[Inkblot]

@Track How do ARP bolts solve the issue? Is the idea that they are more resilient to multiple torque cycles?

[Trackrash]

Originally Posted by Inkblot

@Track How do ARP bolts solve the issue? Is the idea that they are more resilient to multiple torque cycles?

Yes. Additionally the preferred method of installation measures the stretch of the bolt to more accurately determine the tightness. Torque wrenches only approximate the tightness of the fastener.

[Neil Harvey]

"""I'm not sure where to go with this. What does the forum suggest? My instinct is to choose a.) "call it good", following the book's instructions, and thus assuming that the roundness won't vary beyond spec when moving from the old rod bolts to the new ones."""

What book are you referencing?

[Inkblot]

@Neil I've been using Wayne Dempsey's "How to Rebuild and Modify Porsche 911 Engines 1965-1989" as my primary guide, with a handful of others as reference.

[Trackrash]

It looks to me like you are good to go. If you want to take it to the next level get a micrometer and verify the specs.

Did you use plasti-gauge on every rod? If so, and they are all good, put it together. I am assuming the guy who did your rods knew what he was doing. Double checking is good, but no reason to over think it.

90% of the pros out there never double check specs and use a torque wrench to put stuff together. If you follow Porsche's specs you are good to go.

[Inkblot]

@Gordon Thanks for the reply, and pic. Yes, measuring to a stretch distance, instead of torque, makes a lot of sense. Yeah- I did the Plastigage on each rod.


So, the picture is getting a little clearer...

[Inkblot]

Interestingly, I can't find any rod bolt stretch specs for the OEM 911 rod bolts. All references seem to point to the torque specs.

I have a copy of the Woods/Anderson 911 engine overhaul guide, which agrees with Wayne's book. In short, use the old rod bolts to recondition the rods, and use the new rod bolts (oem is the assumption) only in final assembly, torquing them down only once.

So, my rod bolt debate goes on... the best way I can sum it up now is to say that there are really two distinct approaches:

a.) Use new OEM rod bolts, and torque them down only once, using the published torque spec.

b.) Use aftermarket (ARP, e.g.) rod bolts, and torque them to a specific stretch measurement.

It seems clear that option B is the more precise one, but it requires the more expensive bolts, and an appropriate stretch tool setup.

As I'm building an essentially stock street motor (probably will end up w higher compression than stock, but haven't figured that all out yet...), and as I've already purchased the OEM bolts, I'm going to go w option A. Assuming I survive this rebuild, my next rebuild will be something much hotter, and I'll definitely revisit this question then!

Here's a cool article about using a torque setting vs. measuring stretch for rod bolts.

https://www.enginelabs.com/engine-tech/blueprint-series-measuring-rod-bolt-stretch-vs-torque-with-arp/

- Jake

[Trackrash]

If your new bolts have not been torqued yet, measure and record their lengths. Install and torque to spec. Measure the lengths of the installed and torqued bolts. They should all have stretched the same amount. That is the old school method of checking for a weak bolt.

With new bolts there is rarely an issue. The stretch method is just a more precise measurement and can serve as a check for the bolts integrity.

IF you remove the bolts you can verify they are still good if they returned to their original length.

[Neil Harvey]

There are many right ways. Everyone has their opinion of what is "their right". But lets look at some logic here.

If many go with the "right way" is to use their old bolts when rebuilding the rods, then fitting new ones after, on assembly, and no engines fail, you are Ok I guess. In the same manner, many do not even measure the bearing clearances, and those engine live too.

But as a business we cannot and will not take chances. Its that same old line I read here all of the time. The inter-web is rife with this. A DIY makes a bone head mistake and the internet is full of sympathy. A business does the same and they are ripped to pieces.

I have not seen anything from Porsche that states these bolts are a 1 time use only. It has been brought to my attention what was written in some of these "how to" book. In my opinion, these are wrong.

Lets look at this from the bolts point of view. You tighten the bolt so the clamp faces are against one another. Nothing more will change with respect to those faces getting any closer together no matter how much to turn that bolt. What does change is the length of the bolt. It stretches and in doing so, it comes under tension, which helps hold the two clamped joints together. Another issue that can happen is, the under head face of the part can distort and gall. You don't want this
to happen, so the use of good anti friction grease and some prior development is required to make sure the suggested stretch does not allow this to happen. If the bolt in use did not stretch the clamping faces would distort. Something has to give here.

All bolts stretch. Some are designed to do so and will not return to their pre stretched length. These are one time use fasteners. But Porsche rod bolts are not. In the later air-cooled engines, Porsche actually refer to an over stretched length that require the bolts to be replaced.

If you wish to replace the bolts with new on assembly, do so. I'm sure many have done this before and I cannot think that the engine will fail if you do. Use the same grease each time, but different grease will yield a different stretch v torque. Put it under the head of the bolt, and on the threads and nut face. Pre measure the bolt and either stretch it or use the factory torque specs. I think its 15 + 1 x 90°. Better check this to make sure. Torque each nut to seat it. 15 or 20 Ft /lbs. Then loosen 1 at a time and do the 15+90 if that is what you are going to use. Then loosen the other nut and repeat.

But as for the right way to do this, our right way is to replace the bolts on rod recon and pre measure the bolt before tightening, and re check after on disassembly. We do this knowing the bolts can be reused if not past their yield length and the BE bores do change when replacing the bolt. If you have set a tolerance of 0.0000" +0.0001", in your rebuilding specs, you have no choice but to do this.

[Inkblot]

@Neil, @Gordon: Thanks again for your thoughtful responses- it's really helpful, and part of what makes this forum so great.

Originally Posted by Trackrash

If your new bolts have not been torqued yet, measure and record their lengths. Install and torque to spec. Measure the lengths of the installed and torqued bolts. They should all have stretched the same amount. That is the old school method of checking for a weak bolt.

This is a great suggestion, I'll definitely do it.

Originally Posted by Neil Harvey

...But as a business we cannot and will not take chances. Its that same old line I read here all of the time. The inter-web is rife with this. A DIY makes a bone head mistake and the internet is full of sympathy. A business does the same and they are ripped to pieces.

I totally get this. If you're a pro, the stakes are completely different. All the more reason why I appreciate your input.

Originally Posted by Neil Harvey

I have not seen anything from Porsche that states these bolts are a 1 time use only. It has been brought to my attention what was written in some of these "how to" book. In my opinion, these are wrong.

This is interesting. I wonder where this notion began... My reference materials are all written by reputable folks, but that doesn't make them right Makes me wonder if they're just referencing each other. I guess it doesn't really matter; as you suggest, folks have found what works for them.

Originally Posted by Neil Harvey

But as for the right way to do this, our right way is to replace the bolts on rod recon and pre measure the bolt before tightening, and re check after on disassembly. We do this knowing the bolts can be reused if not past their yield length and the BE bores do change when replacing the bolt.

Do you always use aftermarket stretch bolts (ARP, etc., which have a clear spec), or do you have a stretch spec for the OEM bolts? I haven't been able to find that.

Thanks again guys.


-Jake

[Neil Harvey]

Regardless of what is said here, you have to go with what makes mores sense to you and makes you comfortable. You are the assembler.

"""This is interesting. I wonder where this notion began... My reference materials are all written by reputable folks, but that doesn't make them right Makes me wonder if they're just referencing each other. I guess it doesn't really matter; as you suggest, folks have found what works for them."

If the references you are thinking about are the two most common "rebuild" books, neither of the authors were engine machinists, engine builders or even assemblers. One did have the knowledge and sense to co write with my respected colleague, Jerry Woods. One if I am correct, attended the others assembly class before writing the book. Go figure????

We are often asked to recon factory rods with factory 9.00mm and 10.00mm fasteners. We use the same process as we do with ARP, SPS or similar aftermarket fasteners.

Remember, change the grease used and the torque and or stretch value will change. We use a grease that seems to be more consistent in both values. Make sure the Torque wrench you use is calibrated correctly, if you do it this way.

As long as you are resembling a street engine with street parts and do not run any higher RPM limit, use the toque method. Measure the fastener lengths before torqueing and remeasure after torqueing. They will probably be different. Take the longest stretched length and equal each shorter fasteners to that stretched length. Or take the mean average and see where you land. A lot has to do with your assembly process and care taken. This way you remove any frictional errors.

Too much is said about rod bolt failures. It has put a lot of unnecessary fear into many. Just like the cam timing fear. Failures typically happen due to poor assembly practices. Use of un cared for torque wrenches, friction over coming the torque value due to poor assembly, excess RPM for that bolt, changing to heavier pistons without calculating the bolts ability, etc.

I wonder why there is not as much consideration for the through bolts on these engines. Lowering the friction here changes the torque value and the clamp on the main bearings/ clearances. Oil pressure can be affected here.

Its always the poor rod bolt that takes the brunt of all this.

[Trackrash]

I hate to be the one who brings this up but in the original Porsche Manual here may be where the one torque rule comes from. I am thinking they mean after being used in the motor. It doesn't specifically say one torque and done.

[Inkblot]

To satisfy some curiosity, I measured the lengths of all of my old/used rod bolts. I thought it might be interesting to get a sense for the variation.



The 12 bolts were between 2.7128 and 2.7302 for a range of 0.0174" and avg of 2.720". This doesn't really tell me anything, as I don't know what they were to begin with. Still, I'm hoping it'll give me some context as I move ahead...

I'll measure all of my new bolts before and after torquing them down.

But, before I get to that, I'll next weigh all the rods and hardware to balance everything out.

Jake

[Inkblot]

So I was planning to weigh all the rods and hardware to group them into balanced pairs. I started by weighing the rods and caps (with the new bearings still pressed in from the Plastigage test).




They were all pretty close, only 1.8g between the heaviest and lightest. But, to really balance everything, I need to include the pistons, pins and clips.

This presents a bit of a problem, as I haven't decided which pistons to use. Earlier in the thread I started down the road of figuring out what the compression ratio would be if I used the 2.2 S pistons I picked up, but I never found a conclusive answer. Here's a link back to that discussion (towards the bottom of the page):

https://forums.pelicanparts.com/911-engine-rebuilding-forum/969543-newbie-73t-2-4-mfi-engine-rebuild-2.html

In short, the only way to know for sure would be to cc it all manually. So, before balancing my rods and pistons, now's the time to make sure these pistons are the ones I'll actually use! Time to for a tangent to figure out how to cc this setup...


-Jake

[Trackrash]

Do you have Bruce Anderson's Porsche 911 Performance Handbook? It has a section on how to calculate the compression ratio.



Also, and I hope I am not opening a can of worms here, I was told by an friend/machinist/race motor builder with over 4 decades of air cooled experience, that end to end weights of the rods should also be checked. FWIW, your rods are fine. They are within Porsche specs. But if you want to take it to the next level.....

[targa72e]

Your math on compression ration looks correct. In general going from 66 to 70.4 get you about a .5 increase in compression. So 10+ with 2.2S pistons. If you are going to use E Cams you might want to use 2.2 E pistons which would get you about 9.5 compression.

john

[Inkblot]

@Gordon Yeah- I just finished following Bruce's book to do the manual cc'ing, with some modifications to the process which made sense to me. I'll post my results soon.

I did have my rods reconditioned by a reputable machinist, so most of the double-checking I'm doing is more academic than necessary. I think it's fun to see how it's done though. Not sure I'll go so far as to check the balance of each rod, knowing that Ted @ German Precision already did it, and his work has all looked spot-on thus far.

- Jake