Connecting Rod bolts upgrade? (2012)

Hobot

Hobot

Joined
Oct 19, 2005
Messages
18,978
Country flag
I've ain't ever heard of crank shaft fasteners failing even with bent crank, but sure have read stories and seen results of rod bolt failure going slow sight seeing like me or flying over 120 mph like MacRae's Dayton take down. Both of us found corrosion decay darkened evidence working its way across metal grain seams so likely would of come apart by gravity alone in time.
What is the wisdom gathered on that weak link?

Connecting Rod bolts upgrade? (2012)
 
My Dunstall tuning book recommends polishing the shank of the bole by mounting it in a drill press and using fine emery, then metal polish to take any stress raisers out .

I've never done that, I just use standard bolts and replace them every time the con rods come apart.
 
What bolt would one order for a non Carrillo Al Norton rod or would the 5/16" Carrillo fit instead?
 
hobot said:
What bolt would one order for a non Carrillo Al Norton rod or would the 5/16" Carrillo fit instead?
Click to expand...

The Carrillo bolt wouldn't fit the Norton rod and I doubt it would be any better in the Norton rod even though I am sure it is a high grade material.

The Carrillo rod is stiff steel with dowels to locate the cap so the bolt sees mainly tension loads- as it should.

The Norton rod locates the cap with the bolt plus the cap and rod have different expansion rates. That along with the flexibility of the rod means the rod bolt has to deal with shear and bending forces along with tension. It has a much tougher job.

I don't know of a better bolt for a Norton rod. Jim
 
Re: Connecting Rod bolts upgrade? > Not

Ok comnos thanx for setting me straight on another inherently Norton radioactive component. MacRae racer and my Trixie both showed dark oxidation plane about 1/3 the bolt dia. that pre-existed the shiney fresh rupture fractures. One would illogically assume MacRae failure was Dayton top out stress but Trixie's same injury, let go almost coasting 50 mph to enter pasture for photo with yellar flowers proves otherwise. A microscopic surface fault let the outside in to decay over time more than stress.
I never thought to polish up a rod bolt as suggested by p john but next set, [if ever again] will be x-rayed sono grammed magnafluxed then if passed, cryogenic'd then prettied up then installed stretched to .0065" with a wrench wrapped in my rabbit foot lucky charm and sealed with a kiss.

Connecting Rod bolts upgrade? (2012)


Here's air cooled failure analysis photo booklet to ponder for our own show stoppers

https://www.gardnerinc.com/dealers/kohl ... debook.pdf
 
Magnafluxing the bolts would certainly give peace of mind, but it can show honest machining marks up as possible fracture lines, so my guess would be it would be best on polished bolts only, perhaps?

Here's a shot of one of my cranks under the UV lamp to highlight what I mean: the machining marks on the crank cheeks stand out more than a little.

Connecting Rod bolts upgrade? (2012)
 
Re: Connecting Rod bolts upgrade? > Not

hobot said:
Ok comnos thanx for setting me straight on another inherently Norton radioactive component. MacRae racer and my Trixie both showed dark oxidation plane about 1/3 the bolt dia. that pre-existed the shiney fresh rupture fractures. One would illogically assume MacRae failure was Dayton top out stress but Trixie's same injury, let go almost coasting 50 mph to enter pasture for photo with yellar flowers proves otherwise. A microscopic surface fault let the outside in to decay over time more than stress.
I never thought to polish up a rod bolt as suggested by p john but next set, [if ever again] will be x-rayed sono grammed magnafluxed then if passed, cryogenic'd then prettied up then installed stretched to .0065" with a wrench wrapped in my rabbit foot lucky charm and sealed with a kiss.

Connecting Rod bolts upgrade? (2012)


Here's air cooled failure analysis photo booklet to ponder for our own show stoppers

https://www.gardnerinc.com/dealers/kohl ... debook.pdf
Click to expand...

I doubt that there are a lot of cracks in new bolts.
Polishing the bolts isn't a bad idea on an engine that is going to be stressed but I think the most important thing is replacing them as part of maintenance when they see bending loads. They can only be bent so many times before surface cracks appear and they will rapidly grow till the cross section of the bolt is so small it can no longer hold the load and you know what happens then.

A bolt that is loaded in tension only like most racing rods will never flex as long as the preload of the bolt exceeds the tension applied to it. I had two pair of Carrillos that I rotated through my engines when I was racing. I sent them back to Carrillo several times for reconditioning and they never replaced a bolt. They said there was no need to. I always though that was quite a feat when I looked at those little 5/16 bolts that were necked down to .270 in the middle. Jim
 
Ok silly me thought I could get away with just new nuts on Norton rod bolts but not yet convinced its mechanical stress failure as primary cause but metallurgy fault or invisible surface fault that lets oxidation pressure dissolve the grain interface bonding. I'm sure one can rev crank high enough to flex-tip journals to leverage-wedge rod caps against bolts at skewed angle but sure wasn't hi rev's or loads that did Trixie's in. Anyway at least Norton rods are indestructible as Z-plates until something else gives up first. Last ride before tire cords appeared had encounter with hot rod PU who though 85 mph would prevent me passing in a short open but that's just a bit below 'Trixie's 2S power band so disappeared ahead of him at ~115 before the next turn.
 
I am always surprised at the tiny details everyone seems to dwell on here. The Norton bolts are 3/8 diameter and very robust. The Triumph bolts are 5/16 dia. I have done dozens of rebuilds on these brands. I almost never replace the rod bolts but always the nuts. I have never had a failure by using the old bolts. For normal street riding there is no problem. If you are that worried then sure, spend $1000 and buy new set of rods, but if you apply that logic to the rest of the bike then you would need to buy new everything. There is a balance that makes sense. Be sensible.
 
One Thing
they used to prattle on about
Back In The Day !

Was some NEW were shaving the rod hole with the hard edge under the bolt head .
the aluminum would be under the bolt head, when tourqued. Later it'd compact ,
And KAPOW . rattle BANG .

SO , " Remove all burrs & sharp edges " was Std. advice . as in
Check the underside ( First In to the rod ) of the bolt head .
De Burr on / with a stone etc , if its a horrible nasty swaged finish / fit .

Would pay to oil all that shirt & then wash thread before locktite & nut fitted .
2 drops of the ded super stud lock or somesuch .

Real Tecnical types measured the BOLT LENGTH and used the STRETCH to ascertain the correct tightness .
 
Matt Spencer said:
snipped

Real Technical types measured the BOLT LENGTH and used the STRETCH to ascertain the correct tightness .
Click to expand...
Yes, using bolt stretch would be best. Anyone know how many thousandths of an inch of stretch would be correct for a Commando rod bolt?
 
In connection with the thread "Resizing Connecting Rod Big Ends", I watched a video where ARP rod bolts replaced the OEM bolts.
After torquing the bolts (which were verified against bolt extension), the bore went oval and out of factory limits.

Lesson learned: If replacements are necessary, fit new OEM bolts, or aftermarket bolts _prior_ to resizing.

- Knut
 
Just for example the 500 unit triumph , bolt stretch are 0.004 /0.005 , but they are 5/16, not 3/8.
 
Deets55 said:
.006” New -.0055” Used is what I have in my notes
Click to expand...
Pete, that seems to be within the realm of possibility. Do you remember where you got that info? Perhaps I should measure the rod bolt lengths before & after removing the nuts to determine bolt stretch?
 
Tuna,
More than likely I got that from Jim Comstock. Either on the forum or verbally while I was rebuilding my motor.
Personally I would just measure them with the nuts off right before the install. Depending if they were new or used would determine what number I would use. I think the reason there is a difference is that once the bolt is torqued and then loosened it doesn’t return to it’s “new” length. I‘m pretty sure .0055” is a safe number regardless.
 
You must log in or register to reply here.
Back
Top