Any input on this idea for reducing spun rod bearings is welcome. 1

[MNRaptor]

I am going to be destroking a quad cam V6 soon and I am concerned about this engines history of spun rod bearings. The bearing speed will be lower due to the offset grinding to destroke it. I would assume that itself would help some. I was considering having additional notches cut in the rods and caps at the separation line and modifying the bearings so they have 4 tangs total per rod rather than 2. I noticed this on some of the alluminum top fuel rods I have seen. I would think that would have to help, but I would like the oppinion of some others with more knowledge about this. Any info or ideas is appreciated.

 

[patprimmer]

If the bearing crush, clearance and journal shape and finish are correct and there is adequate supply of a suitable quality oil, bearing should not spin with only one tag per bearing.

Check crush. Check clearance. Check ovality, taper and finish of the journal. Check the bearings have sufficient extra clearance across the parting line so not as to wipe the journal clean of oil as the cap distorts at high rpm. Check side clearance.

Check oil pressure for at absolute minimum of 10 psi per 1000 rpm hot.

Use premium grade synthetic oil.

Make sure the pick up is always under good air free oil under all operating conditions.



Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMOKEY44211]

Additional tangs will do little to hold a rod bearing from spinning. They are there for locating the bearing side to side position and to align with the rod/cap parting line. Most bearings are manufactured with a larger I.D. across the parting line as oppossed to the size measured 90 deg. from that. Premium rod/cap fasteners torqued or stretched to manufacturers spec. is good insurance. Pat's advice about oil pressure is right on. As a suggestion you might do well to run mineral based oil for the break in period and switch over to the synthetic after. Synthetics lube so well it delays the ring to cylinder wall seating. With proper crush and an adequate oil wedge between the journal and bearing, you won't run into any problems.---------Phil P.S. Good assembly hygene is also crtical. Any dirt that somehow may find its way into the oil supply is a killer.

 

[patprimmer]

Another idea I DON'T like is when guys completely fill the oil pump with white grease or something similar. They do it so the pump will prime, but it restricts oil flow until it is completely flushed.

If the pump is any good at all, it will only need a slight smear of grease on the gears and internal surfaces swept by the gear to prime instantly. The best way to ensure oil pressure on start up is to have a good pump and spin it up with a drill and an old distributor bottom half and shaft with no gear.

As SMOKEY says. cleanliness is next to godliness when it comes to engine assembly.

Mineral oil will help the rings bed into the bores, but synthetic from the start with new bearings, but rings and bores that are already bedded in as you do not need to bed in bearings.

Are you sure the crank is being linished correctly. The grain raised against the direction of rotation can cause bearings to spin at times.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[willeng]

MNRaptor:

Pat & Smokey are on the money, is it a problem associated with a particular journal or is it random??

Cross drilling the crank will improve the oil to the rod journals & should be the end of your troubles!

If it is a particular journal all the time further inspection of the oil ways may be in order!

How much oil is going to the top end instead of where it belongs, it may need restricting??

 

[Tmoose]

which engine is that?

 

[MNRaptor]

Well, to start off, this engine is the Mitsu 6g72 found in the 93 & up stealth and 3000gt with the four bolt main and forged crank. The engines after rebuild have a really common issue with spun bearings. The engine I am building is at 100K and is in perfect condition now with no bearing issues. The crank is getting offset ground undersize to destroke it and use a different rod. The engine is being designed to rev with longer rods 2.14 ratio when done and 1.3:1 oversquare. The dry sump system is almost complete so oiling won't be an issue. The crank finish is also not an issue in this case, it will be taken care of by the best in the St Paul/Minneapolis area, Nitrided after grinding and checked for straightness,OOR etc. The rods are Crowers with ARP's and the bearings will be H rated ACL's. The block machine work is top notch, I have run 16 engines through this place this year and they do amazing work so I have no fear on that end. I always am very anal about everything being clean and tripple checking my clearances etc. So really, I shouldn't be worried, but I am just because of how common an issue this is with these engines. In a similar situation before, I believed because we are so careful, we wouldn't have the same issues with a specific motor everyone else was having, and it came back to prove me wrong. I just want to cover every possible base this time because I don't like to fail and also this one is mine.

 

[MNRaptor]

I missed the cross drilling answer above. We did discuss that, but the crank is already drilled all the way through and adding another hole is not something I feel safe doing. The HP expectations for this engine are pretty high, I don't want to create a new point of failure trying to prevent the other problem.

I guess I am not surprised by the answers above. I did think adding an additional tang may help and seeing it on some of the top fuel rods in my mind supported it. With the additional details I gave, do you all still feel the same? Do you think there is any other way to help prevent this? It is a bad enough problem that most of the 3S owners opt for a new shortblock from the factory as opposed to a rebuild since nobody can seem to overcome this with any consistancy.

 

[willeng]

MNRaptor

An extra tang should not be necessary, there is an oil related problem. Are the blocks being line-bored when rebuilding???

What main & big end clearances are you running??

What brand of bearings are you using, are they having problems with the factory bearings or just aftermarket types!

Are the oil holes in the main bearings as large as the oil holes in the main bearing saddles??

Have you tried to feed the mains with seperate oil lines direct from the pump!!

 

[willeng]

MNRaptor:

A couple of things i asked you already stated--DUH!!

The bearings you are using, are they tri-metal bearings or sintered alloy??

 

[patprimmer]

I cannot see an inherent reason why a new factory short engine should be any better than a used (read seasoned) short engine, so long as nothing is fatigued nor cracked, and so long as suitable quality of preparation and parts are used.

Have you inspected and measured new parts. Do you have specs for new parts, especially bearing shell dimensions and materials.

The bearings do not know if they are in a Chev, a Mitsubishi, A F1, a KB or a Gogomobile. They know what temperatures, oil supply, clearances, loads, dynamic changes to clearances etc they see.

People like Cleveite make a number of different bearings for the one engine, to suit different applications. They do this because it is necessary to optimise different properties for different applications, eg aluminium for long wear at moderate loads, or white metal coated for the ability to wear rather than grab in very high load applications where parts flex so much that clearances are more than used up for an instant.



Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MNRaptor]

The bearings used are all tri-metal. I don't know if the blocks with the bearing issues have been line bored or not, it is just one of those things that seems to be all too common like a crankwalked 7 bolt 4g63. I am probably already fixing the issue with the dry sump, I know there has been issues with the oil pumps and cavitation. This is just a known issue with this particular engine model so the specifics of what bearing clearances and types as well as what extent they each took the machining is unknown. I would hope that they were all within factory spec for clearances and I would guess the bearings would mostly be from Mitsu since that is the norm with this group but I couldn't say for sure. I am changing so many things about this particular engine that the issues everyone is dealing with probably don't apply. We are probably going to try to get a shortblock available for these owners that is more reliable than what they have available now so all the info will still help with that. Thanks again for all the info, if any other possibilities are out there, please let me know.

 

[willeng]

Pat:

If the bearings knew they were in a Gogomobile they'd want out of there!!

Wooden blocks would be fine for them!!

 

[patprimmer]

But then again, If I was a bearing, the last place I would want to be is in a KB in a Top Fuel car. 2 minutes life, including warm up and burn out.

Come to think of it, I think a Gogomobile is a 2 stroke.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

Any verified failures with Mobil 1? I believe it is remarkable stuff when "design" deficiencies have left me on the edge.

Any verified failures using an oil accumulator? They are a great band-aid/insurance policy for momentary oil loss.

 

[patprimmer]

Also, if the oil accumulator cures the problem, a dry sump as mentioned earlier, should also cure it.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MaxRaceSoftware]

Another idea I DON'T like is when guys completely fill the oil pump with white grease or something similar. They do it so the pump will prime, but it restricts oil flow until it is completely flushed.--Pat
==========================================================

i know that's probably not your problem ?...but like Pat stated , i would "never" assemble a Race Engine with a "LUBE" i couldn't fillup the pan with and run the engine for 100 miles. You can't fillup your oil pan with white grease and go run it for 100 miles or so, ..so why would you assemble an engine with white grease (Lithium) ??

How about your Rod Bolt Torque ??
which method are you using to Torque the rod bolts ?
Stretch or Torque ??

i just had an engine builder come to my Shop 2 nites ago with a LS-6 engine in pieces...he used the "Stretch Method"
2 Rods wanted 80 Torque , and all the others wanted around 60 to 63 Torque for same "Stretch"

the 2 that were at 80 TQ didn't fail, and bearings looked like the day you took them out of the box !

all the others at 60 to 63 spun, blackened big ends, and some rods came apart and destroyed the engine

i took one of the rods and it wanted 78 to 80 TQ by "Feel"
pulling with my Torque wrenches, instead of his 60 to 63 by Stretch

make dead-sure your rod halves "don't" have a chamfer on the parting halves....as the bearing tangs can get rounded-off in that chamfer ! The chamfer is death to the bearing tangs...it lets the bearing halves and their tangs move in and out into that area...if the bearing is moving around , it will "blacken" the oil film on back sides and the backs of the bearings will have a faint Skunk or Rear Gear oil smell

never grind a chamfer on rod caps parting lines
the bearing tangs need a dead stop to be against, not a wedge !








Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3

http://www.maxracesoftware.com/

 

[MNRaptor]

Thanks again for the info, I build high performance engines just about every day, so the specifics of proper assembly are not really needed, I am probably more anal about assembly than most of the shops you will ever see. I am one of the owners of a performance shop in St Paul MN, dealing with forced induction applications only. I use the stretch method for assembling rods with only ARP fasteners and lube. I have never had one of my engines fail from a spun bearing or any assembly error. The question mainly was, aside from the correct methods of assembly for a given application, what would help ensure an engine would have less chance of bearings spinning. The question about adding additional tangs to the bearing and rod was more the type of thing I was interested in. That has been covered by others saying that it would likely be an oil supply/quality related issue causing the bearings to fail on this engine family, and that may be true given the known issues with pump cavitation. The reason I believed that extra tang idea in the bearing and rod had merit was for the same reasons you give for not chamfering the separation line on the rods, the tangs need a positive stop, not a wedge. Why would that matter if it did nothing to help them from spinning? I believe the issues with this engine spinning bearings with the frequency it occurs is a design flaw from the mitsu engineers. Obviously they failed with the oil pump, the engines do spin bearings frequently even from the factory as well(not only from someone's failed rebuild) The bearings are considerably more narrow than most of the Mitsu engines I deal with and the diameter of the bearings are quite a bit larger. Could either of these things contribute to the bearing spinning? The additional speed from the larger diameter?

 

[motorsportsdesign]

If the problem is limited to rod bearings and is common to this engine design, I would look at the following:

1. The oil groove in the main bearings, as they feed the rods.
2. The design of the rod oil holes in the crank, I'm not familiar with this engine but they may not be well suited to racing engines. Check to see that the main journal opening of the rod oil holes is right on the oil groove in the bearing.

Some people carve scoops into the main journal opening of the rod oil holes with sucess.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[tigretr]

Have you looked into the timing of the oiling relative to TDC? In very high performance applications if the rod bearing is lubricated too early or too late relative to combustion, the oil film is not at its optimum thickness and thus bearing / journal contact is more likely to occur.

Also look into how that rod journal is getting its oil. I build performance Sunbeam Alpine motors, and they have a history of spinning #2 and 3 rod bearings. Turns out that 2 and 3 get their oil from main journal #3 which is also feeding the center journal on the cam as well as the entire upper end of the motor. Additionally the oil hole on the crank is providing the rod bearing with oil way too early, leaving the oil film too thin when combustion occurs at hight RPM's. These are the kinds of things that can make or break your motor.

Good luck