Piston broken in half(top-bottom) 5

[dirk489]

I reconditioned a friend's engine which broke later on during running in.The piston broke just on the pin line,conrod bolts snapped clean off, smashed the whole block.
My question is: is this always due to overreving or is there another possibility?
Dirk/Antwerp

 

[Tmoose]

Once again, Need way more information.

Got pictures?
What type, make and model of engine?
What prompted the reconditioning?

Were you present during the running in?
Please Describe the running in procedure.
Was there always oil in the sump?
Were there issues with the cooling system, and Did it overheat?

Was any non-destructive testing (crack checking) done on the rod(s) and piston(s) ?

What was the torque value used on the conrod bolts?
What were the bearing clearances, piston clearance, and piston ring end gaps?
What size rings did you install?

 

[dirk489]

Engine is MGA Twin Cam 1958 built
Recon due to holed piston.
Alas I was not present at running in!
Breakage was sudden after leaving traffic lights and very sudden.
This coincides with the clean oil, no small debris. Sump was full. Water OK, no overheating.
No testing of rod. New OEM rod bolts 3/8" UNF Torque 55 lbs/ft(6.9 Kgm) Bolts are 55 Ton tensile .
Rod bolts were snapped trough straight,no sign of bending before breakage.
Bearings were free of seizure and running marks.
Pistons are NOS, clearance .004".No Marks on the sides,top two rings still on and more or less undamaged.
To me these are sure signs of overrevving but I'd want advice before confronting.
Thks,Dirk

 

[MikeHalloran]

It's a tractor engine, not meant for racing.

Is the owner a fanatic about matching numbers and such?

If not, maybe you can talk him into something more modern,
like a hopped up Pinto engine, or a Japanese four.
On balance, a stronger engine is guaranteed to break
something in the driveline, and upgrading that will
allow the engine to break something else, etc.

Maybe you can talk him into installing a rev limiter,
just to curb the occasional burst of enthusiasm.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Ouch.

I'd like to see more close ups of the rod bolts, especially the fracture surfaces.

 

[tbuelna]

Thanks for the pictures.

The 55 lb/ft installation torque you note seems high for a 3/8-24 UNF bolt having 110 ksi UTS, especially if the threads were lubed at assy. You might want to do a quick calculation to check if the combined tension and torsion forces at installation would be enough to overstress these bolts.

 

[Tmoose]

http://mgaguru.com/mgtech/books/tdb/tdtc_b_engine.pdf

second page says 600 lb-in for stock twin cam rod bolts, but with no mention of bolt size, pitch, or grade.

110 ksi UTS is more like a grade 5 bolt spec.
Hardness tests and some tightening experiments of surviving bolts, preceded and followed by length checks might help understand if the bolts were adequate despite the claimed UTS.

I'm also concerned that the bolts might have bottomed in the blind holes.

 

[tbuelna]

110 ksi UTS is not even grade 5. I did a quick check for 55 lb/ft torque on a 3/8-24 UNF bolt with lubed threads and it exceeded 110 ksi at the thread root. Given that both bolts appear to have experienced similar failure modes, where they parted right at the thread root section closest to the mating internal thread, and right where you would expect to see a stress concentration, it seems like this might be the result of excessive preload.

 

[dirk489]

Hi all,
first thks. for reactions.
I tried to make some closer fotographs.
No bottoming,holes go straight through.Torque is factory one.New ones are not marked,an old one shows 55T65;I assume this is 55t/sq.in.,which is not bad.The conrod and piston assy is quite heavy;combined with a long throw(89 mm)will put a lot of pull when overrevving.MGA twin cam conrod bolts were a failure cause in the past for race engines.Would have loved to put ARP in but they do not make them in 2".
Dirk

 

[GregLocock]

Grade 8 3/8 UNF recommended torque is 49 lb ft, dry, half that lubricated. Grade 8 bolts are 75 tons per sq inch. You fitted 55 tons per sq inch bolts and then torqued them to 55 lb ft.

They broke.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[dirk489]

Actually it was 50 Lbs(I used the 6.9 Kgm setting)Bolt was dry as out of pack.
I cannot believe that using the factory spec would cause this.I will try some of the remaining bolts for strech/breakage
Dirk

 

[dirk489]

Well,I did some tests with oiled bolts:
Static length was 61.47 mm,
At 70 LBS/ft 16.59(overloaded 20 lbs/ft to spec.
At 80 Lbs(serious overload)61.62 mm
When released, bolt measured 61.49,just within elongation spec?
At 90 Lbs/ft,bolt broke. Other one broke at 85 when torqued up slowly,showing narrowing and torque damage

 

[dirk489]

I showed the bolts to a metallurgist and he immediately used the words: IMPACT SHEAR, without knowing what happened.
I always wondered why the conrod bolts would have gone first,as there is clear evidence of the piston broken in tension.
Shure enough,some searching on the internet brought the following: "Impact shear of conrod bolts often happens when there is a blockage of the rod in an engine due to breakage of other components,like camshaft."
This leads to the following:first the piston broke in tension(ewhaust phase). Top of piston is thrown against head,valve blocage and hence OHC broke.Next crank throws rod against remains of lower side of piston,smashing it to pieces and bolts break on impact.
I think this is a more logical line of thought.
Dirk

 

[LionelHutz]

If I was betting on it, I'd bet the rod bolts broke and then the rod was twisted when the crank came around and hit it which smashed the piston.

I do agree with the others that using 50 lb-ft is too high for a 55 ton-force or 110 ksi bolt.

 

[gruntguru]

I'm in the "piston failure" camp. Probably put it back in first gear instead of third during the traffic light drags.

je suis charlie

 

[tbuelna]

Just a couple other things I noted.

The pictures of the rod cap parting faces seem to show a fair amount of fretting. This would indicate they were separating and smacking back together for more than just a couple cycles. If the engine was over revved the fretting may have occurred after the bolts yielded from inertia loading. Or it may have been the result of the cap not being clamped tightly during assembly. As Tmoose noted, it is possible the bolts may have bottomed out in the blind tapped holes of the rod beam leaving a gap at the rod split face. Even a small gap between the cap and beam faces could produce significant impact loads on the rod bolts around TDC of the exhaust stroke.

The picture of the piston shows excessive scuffing wear at the inboard edges of the wrist pin bores. This is not normal wear and indicates significant bending of the wrist pin and/or piston structure along the bore axis, prior to the fracture failure.

 

[dirk489]

I have had this before on one of my own engines,with Carillo rods,ARP bolts. Piston was Powermax from Triumph Bonneville.
Thought I was in 4 th. gear,but I was in 3rd! Although much lighter piston and rod,the piston broke in two exactly the same.
Rod was twisted also the same,bolts (ARP2000) did not break.Block was also ruined. That was at around 9000 rpm.....
Dirk

 

[dicer]

Need to see all the parts.

 

[LadaTrouble]

BMC B block - rod cap is angled, not a horizontal cut. Rods not fitted backwards by chance? This would put tension on the rod bolts on the power stroke.

 

[dirk489]

I fail to understand why most of You want to point to the rod bolts.
1)OEM bolts,torqued to factory figure.
2)No bottoming of bolts,angled right way.
3)How do You explain that the piston broke in half after the rod let go as there is minimal damage to the cyl. head and top of piston.
If the rod let go first there should a lot of damage to the top of the piston and cyl. head
4) The top part of the piston shows no damage by the rod smashing into it,break line is practically untouched.
5)It is the bottom part of the piston that caused the blockage of the rod.(Nothing left,gone in smitheriens)
I'll have some left over bolts further tested and will keep You all posted.
Dirk