virtual fishbone

[ivymike]

this ought to be fun for some out there -
What environmental/application factors can contribute to increased alternating stress in connecting rod bolts, while running in the engine?

- impending piston siezure, leading to increased tension
- engine overspeed giving higher tension
- rod large end lateral deformation due to _x_ leading to bending
- etc...

 

[MikeHalloran]

Where "_x_" might include..
.. improper setup, terminal wear, or outright failure of the engine's main thrust bearing, allowing thrust loads to be applied to the conrod(s).


Mike Halloran
Pembroke Pines, FL, USA

 

[ivymike]

good one.
how about detonation... it's hard to picture how, but do you think that could lead to big end deformation?

 

[ivymike]

in the same vein, how about if the bore spacing were incorrect... resulting on thrusts in opposite directions for a pair of rods?

 

[evelrod]

Are you picking on me, Isaac?

Bore spacing, at least on the production engine would seem a stretch...A 'one off', perhaps.

Detonation usually leads to other failures first in a properly designed engine...However, I have had the bearing shells torn up by det a couple times. Perhaps left to it's ultimate conclusion, a rod bolt failure would be possible.

Most of my rod bolt failures have been the result of over-revs and or too long in service....too many races on that engine, etc.

Rod

 

[ivymike]

yeah, the bore spacing seems a stretch.

 

[patprimmer]

I am sure I already posted "how about extra time with the piston top under vacuum on a trailing throttle"

Regards
Pat
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[evelrod]

You have a point, Pat.

EVERY TIME, yes, each and every time I've had a rod separate, had a pin pull out or a rod bolt break, it has been when I lifted to shift or at the end of a long straight. Trailing throttle problem, for sure!

Rod

 

[ivymike]

you guys are right, of course, for that application. at the moment I'm primarily thinking about a turbocharged constant speed / constant load engine. still, it's a useful thing to bear in mind.

 

[malbeare]

how about a learner driver kangaroo hopping at low speed after letting out the clutch too quickly. alternating full throttle and let off

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

http://www.sixstroke.com

 

[Racer704]

I would have to agree that most rod issues are from the decel and not the accelration....when useing a direct drive like a standard trans....autos are not as bad.......fastner companys...i would only use the best...so see what the rod company uses and offers...

Engine detonation is also right as it will beat a rod brg. into submistion..it wasnt that many years ago methanol and nitro burning engines we tuned by the rod brgs.....yes before data recorders....and even today with all the recorders i still read my brgs....

How said rod bolt is tourqed and is it done by strecth guage or just a tourge wrench thats 25 years old....strech gauge is the only true way of doing it corectly IMOP..This also is the biggest reason of bolt failure from decel because the rod bolt has been streched to far from over tourqe...i think ARP gives there specs at 75% of bolt capacity....Ktech are a good bolt and i like A-1 out of all of them...

Johnny

 

[patprimmer]

SPS also make some very nice bolts.

There are a huge number of engines that run the entire life of the engine with rod bolts tightened by a good accurate torque wrench used correctly. That is also how most main cap bolts and head bolts are installed as there is no place to measure stretch.

I agree stretch is a better method, but it is only really necessary if the bolts are marginal.

Poor workmanship or poor tools can result in a failure by either method.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[PJGD]

How about running with a non-standard piston; one that is heavier than originally designed for, such as might happen in an engine intended for moderate boost/cylinder pressure which is then uprated, and your piston supplier recommends moving from an aluminium piston to a heavier steel Monotherm or similar?

PJGD

 

[Racer704]

Pat I agree with what your saying,but a main and a rod are 2 differant forces...and yes done for years but why are there dimples in the center of any quality rod bolt?
Just like all the tourqe to yeild head bolts...but we are talking rod BOLTS....

Do you think Hendrick engine builders use a strech or a wrench on the rods?Any major nascar team...?

My aluminum rods are a cap bolts and I have no choice...I take great care in how i do them and what extentions add to the equation and the sockets as well....this may seem anal but my engines come down after every weekend and sometimes at the track so I have alot more to look at as I measure them on a service schedual that i maintain...

I have done many motors over many years also,I am just adding to what causes or can and the proper way which if you ask any rod manufacture is to use strecth because of the variables with wrenches...

 

[ivymike]

we do use stretch for all our rods, but typically not by measuring stretch itself (although that is done in certain cases). It is much more common to set stretch by using a torque+turn tightening method, where the initial torque gets the joint snug and the turn angle sets the stretch.

Hydraulic tensioning is common for large engine head bolts - the bolt is stretched hydraulically to a desired amount of stretch, and a nut is gently (as w/fingers) tightened, then hydraulic tension is removed. This is even done in the field on these engines.

 

[evelrod]

Johny---Stretch is the only way to accurately set rod bolt torque. The idea that I ck every rod bolt on every engine I build each and every time I tq them is a bit of a 'stretch' (pardon the pun).

Typically, I check the stretch on several rod bolts in a set and then use the tq determined by those cks as a final setting for assembly...USING MY TQ WRENCH! I see nothing good coming from ME cking the stretch and YOU doing the assembly with another tq wrench.

Isaac--- In the field, using bolt stretch to set final tq is pretty common. Handy little gadget...labor saving device, and assembly is a breeze...especially when the bolts are ~2" dia. Even at that, the stretch is determined in the "lab" and all we get in the field is the pressure settings on the gauge of the Porta-Power.


Rod

 

[ivymike]

re: " to set final tq "

The critical point is that bolt torque is not important. The desired outcome is a particular amount of stretch (which gives you a particular joint preload). The torque required to achieve that stretch can vary wildly for a given set of geometry based on differences in lubrication, surface condition, cleanliness, etc., even between repeated installations of the same hardware.

Yes, the required force (and therefor pressure) is determined in advance when the joint is designed. Luckily the hydraulic piston diameter doesn't change much over time.

There's a nifty tool which uses an embedded chip on each bolt head to make before and after length measurements.