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Rod bolt assembly tips

I received a telephone call this morning asking about the assembly procedures when fitting new connecting rod bolts. This was on an earlier 911 air cooled engine. The customers rods had been reconditioned and he wanted to fit the new bearings and check the clearances.

The question asked was, “should I use the old bolts when doing this”? The answer is NO.

My question to him was, “why would you remove the new bolts and fit the old bolts to do this”? His answer was, I have yet to fit the new bolts.

Here is the mistake.

When reconditioning rods big ends, ALWAYS fit the new bolts first. The new bolts will place a different amount of crush on the BE as the bolt will have a different amount of stretch and tension. This can change the overall size of the BE.

The rod bolts can go through multiple stretches, as long as the bolt returns to its relaxed length and is not stretched passed its yield point. If using a particular bolt, use the grease that is provided from that supplier. Use the stretch method over a torque value as friction will change the tension of each bolt. If using the torque value only, make sure, same as with the stretch method, you use grease on the threads, and the beam side of the rod under the nut to help lower the friction.

Also, if using “plastigage” to measure the clearance, this should only be used in the vertical direction. Place the strip of plastigage either on the cap or beam at the 12 or 6 o’clock position.

Remember, you are measuring the overall clearance value between 0.0018” -0.0028” typically, so any difference in bolt tension can make a difference when measuring a small clearance of 0.0028” or less.
Old 11-12-2018, 09:55 AM
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Hi Neil,

I read your post with interest. I am a week or two away from assembling newly machined rods on to the crankshaft on my 1989 911 Carrera 3.2.

In Wayne's book, he specifically states to use the old rod nuts and bolts when measuring clearance via plastigage.

Has something changed in the technology of my engine vs. the older one that you discuss? I am just using factory bolts and nuts (Not ARP, etc.) for final assembly.

Is the concept of using the new hardware for measuring clearance the current conventional wisdom of the Porsche community?

Thanks,

Mark
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Old 12-06-2018, 06:51 AM
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Hi Mark,

While I'm not Neil, perhaps I can add something of value here for you.

First, I wholeheartedly agree with what he wrote; those procedures are SOP around here and have been for 40+ years.

Second, you'll never find plastigage here, ever.

Lastly, using ARP bolts is a prerequisite for Neil's comments since factory Porsche bolts are one use only and cannot be stretched repeatedly while the BE's are being measured and sized as needed. Quite obviously, you'll need a stretch gauge to measure this accurately to tension the bolts.

Hope this helps,
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Old 12-06-2018, 09:11 AM
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Quote:
Originally Posted by Steve@Rennsport View Post
Hi Mark,

While I'm not Neil, perhaps I can add something of value here for you.

First, I wholeheartedly agree with what he wrote; those procedures are SOP around here and have been for 40+ years.

Second, you'll never find plastigage here, ever.

Lastly, using ARP bolts is a prerequisite for Neil's comments since factory Porsche bolts are one use only and cannot be stretched repeatedly while the BE's are being measured and sized as needed. Quite obviously, you'll need a stretch gauge to measure this accurately to tension the bolts.

Hope this helps,
Steve,

You make the comment "Second, you'll never find plastigage here, ever." Can you please expand on that?

Thanks,

Jeff
Old 12-06-2018, 09:54 AM
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Complete agreement..........

Quote:
Originally Posted by Neil Harvey View Post
I received a telephone call this morning asking about the assembly procedures when fitting new connecting rod bolts. This was on an earlier 911 air cooled engine. The customers rods had been reconditioned and he wanted to fit the new bearings and check the clearances.

The question asked was, “should I use the old bolts when doing this”? The answer is NO.

My question to him was, “why would you remove the new bolts and fit the old bolts to do this”? His answer was, I have yet to fit the new bolts.

Here is the mistake.

When reconditioning rods big ends, ALWAYS fit the new bolts first. The new bolts will place a different amount of crush on the BE as the bolt will have a different amount of stretch and tension. This can change the overall size of the BE.

The rod bolts can go through multiple stretches, as long as the bolt returns to its relaxed length and is not stretched passed its yield point. If using a particular bolt, use the grease that is provided from that supplier. Use the stretch method over a torque value as friction will change the tension of each bolt. If using the torque value only, make sure, same as with the stretch method, you use grease on the threads, and the beam side of the rod under the nut to help lower the friction.

Also, if using “plastigage” to measure the clearance, this should only be used in the vertical direction. Place the strip of plastigage either on the cap or beam at the 12 or 6 o’clock position.

Remember, you are measuring the overall clearance value between 0.0018” -0.0028” typically, so any difference in bolt tension can make a difference when measuring a small clearance of 0.0028” or less.


Neil,

I completely agree with your above statement. But how about people using OEM Porsche connecting rod bolts which are not stretch bolts. How would they be able to measure the clearance/s with non-stretch bolts? As far as I know, these after-market stretch bolts were not commercially available 2 decades ago or longer.

Every time I read your posts I find additional and new information. Hope you continue to share your experiences with us. Thanks.

Tony
Old 12-06-2018, 10:16 AM
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Originally Posted by jkb944t View Post
Steve,

You make the comment "Second, you'll never find plastigage here, ever." Can you please expand on that?

Thanks,

Jeff
Hi Jeff,

Not much more to say really.

I've found it to be too unreliable & inaccurate, compared to micrometers and other professional measuring instruments.
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Old 12-06-2018, 02:33 PM
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^^^ I agree

In almost 30 years of building aircooled engines I've only used plastigage once, when I was a rookie.
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Old 12-07-2018, 11:22 AM
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Quote:
Originally Posted by boyt911sc View Post
Neil,

I completely agree with your above statement. But how about people using OEM Porsche connecting rod bolts which are not stretch bolts. How would they be able to measure the clearance/s with non-stretch bolts? As far as I know, these after-market stretch bolts were not commercially available 2 decades ago or longer.

Every time I read your posts I find additional and new information. Hope you continue to share your experiences with us. Thanks.

Tony
OEM bolts are "stretch bolts", basically all bolts stretch.
OEM bolts can only be stretched once, ARP can be stretched several times. For OEM bolts the torque value takes the stretch into account, one of the reasons you shouldn't mess with the factory tolerances.
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Old 12-07-2018, 11:37 AM
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Originally Posted by boyt911sc View Post
Neil,

I completely agree with your above statement. But how about people using OEM Porsche connecting rod bolts which are not stretch bolts. How would they be able to measure the clearance/s with non-stretch bolts? As far as I know, these after-market stretch bolts were not commercially available 2 decades ago or longer.

Every time I read your posts I find additional and new information. Hope you continue to share your experiences with us. Thanks.

Tony
Tony,

I have no idea how rods can be rebuilt correctly, if the same fasteners are not used in the resizing.

In my opinion, stock bolts can be reused as long as they are not stretched past their yield point. If I'm wrong, could someone show me why they cannot be used in the rebuilding process?

When we do this we measure the un stretched length and always make sure they return to this length in the rebuilding process.

Relying on the torque value is not the best way. It does not take into account any friction. I guess rebuilding an engine using only the torque value is safe but not ideal.
Old Yesterday, 04:12 PM
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Quote:
Originally Posted by Neil Harvey View Post
Tony,
In my opinion, stock bolts can be reused as long as they are not stretched past their yield point. If I'm wrong, could someone show me why they cannot be used in the rebuilding process?
It's in the factory service manual that they're to be replaced.
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Old Yesterday, 04:21 PM
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Is it possible you are mistakenly taking "replace after use" for 1 time stretch?

I think so. I think it is meant, after the engine has run and its probably thought that "run" means for many thousands of miles.

As long as the bolt does not become elongated and not return to its normal length, it can be reused in the rebuilding process.

Rods need to be sized and measured using the fasteners they will be assembled with. You may be surprised how much this can make a difference in the final size.
Old Yesterday, 05:01 PM
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It's in the factory service manual that they're to be replaced.
One thing you need to keep in mind is the service manual was designed for Porsche techs at dealer service departments. Mostly, these people were NOT building new engines. They were servicing running cars.

So when a car came in that needed, say rod bearings, they would be directed by the manual to throw the old rod bolts away and not reuse them.

I agree with Neil; I don't think the intent was to throw away brand new bolts used solely for measuring purposes and not run in a motor.

As the holder of several patents where it pertains to the case and squirters, I am CONVINCED that Porsche, for the most part, never thought whatsoever about rebuilds when publishing the manuals. Why would they....they are a new car manufacturer.
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