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.

 

[icrman]

I would like to know which half of the bearing see's the distress first on this engine? Or has it not been caught in time to see?

Do you have any bearing loading figures?

 

[MNRaptor]

Thanks Tigretr, that is good info. I hadn't really considered those aspects of the situation, What would be considered optimum for a high RPM application in relationship to TDC?

 

[Milanoguy]

MNRaptor

Can you tell us more about these spun rod bearings? Is it always the same rod journal that spins? Do you have an oiling diagram of this engine? and if so can you post it some where?
Do you still have the damaged bearings and/or rods? I read an article a long time ago which showed how by examing a ruined plain bearing you could tell why it failed (i.e. oil too hot, not enough oil, foreign object damage)

Bye for now

 

[MNRaptor]

As I said before, not an issue I have had personally. I own a speed shop in St Paul and I am building one of these engines for my personal car, but it has not had any bearings spun. The reason I ask about this is that I will be building these engines on a regular basis from here on for customers as well and I want to make sure I do any possible research I can to prevent what for this particular engine style seems to be a real issue based on the amazing number of occurances. I will see if I can get an oiling diagram, shouldn't be a problem. Also, I will see if I can compile some data as to which bearings are the most common failure. Thanks again for all the help, it is appreciated.

 

[richdubbya]

I completely agree with everyone that has posted, spun bearings are a lack of lubrication issue. The bearing gets hot and sticks to the crank. Adding tangs or other methods would only add a slight insurance factor in a situation where lubrication or rod distortion happened momentarily and the bearing would suffer damage anyway. Adding tangs sounds risky as far as maintaining all the original specs for the bearing. Back in the sixtys we pinned rod bearings in big block chevs with brass pins and the bearing mfgs even had bearings designed for the pins. I'm just mentioning this because it is what you are asking. Pinning the bearings is just a band aid and isnt the answer but it will help in very limited situations.

 

[Tmoose]

The guys that really needed pinned bearings used aluminum rods, which lose some of their initial bearing crush when they get hot. There were articles showing how British motorcycles with aluminum crankcases and rods could start off with nearly zero bearing clearance on a cold day. Similarly The Corvair SAE papers plotted some wild main bearing clearances with respect to temperature.

In regard to the Mitsubishi engine, I wonder if there are some popular modifications that might often accompany the first rebuild and might be inferior to some of the factory parts. Looser clearances, Hot Rod oil pickups, changing rod bolts without re-sizing the rods, etc.

 

[MNRaptor]

The engines being rebuilt havn't had any specific popular mods that would be responsible for the bearing issue. After digging into this a bit more, I have heard that a lot of them are putting fresh bearings in between 60 and 100K to prevent the issue. With the bearings wearing enough to justify that, I agree the problem lies somewhere in the oiling system. I am getting one that had a bearing spun this week, I will see what I can find from it, maybe there will be something fairly obvious.

 

[tigretr]

MNRaptor,
The "optimum" position of the oil holes on the rod journal is not necessarily a fixed number of degrees before or after TDC. The angle of maximum force on the bearing is a function of camshaft, timing, fuel, ... etc and is obviously motor specific. However as a general rule we use a Chevy small block crank as our model for re-drilling cranks. The key is to make sure oiling is not happening too late.

I look forward to hearing your report on what you find in this motor coming to you soon.

 

[DesignOutsourcing]

Since it hasn't been mentioned yet, I've heard that some big end problems are due to the cap ends rotating around the bolt sleeves. This is avoided in some rods by dowel pins either side of the bolts or by a serrated interface between the cap and rod.

I was told (by a Japanese Mitsubishi mechanic in my old rally days) to keep the bearings super clean and dry (after cleaning with solvent) so that the bearings "gripped" properly and transfered heat well.

 

[willeng]

OpenSourceCar makes an often overlooked point, i have seen many rod resizing jobs that have the internal bore finish on the rods to coarse & transfering heat away from the bearings is then a problem with high rpm race engines.

Tigretr makes the point about oiling angle, this should not really be an issue because you should have 360deg oiling to the rod journals, if not your not racing & if not you better fix it!

 

[icrman]

Tmoose, you mention Corvair bearing clearances with respect to temperature, do you or anyone else have any information on the VW Aircooled engines?

 

[patprimmer]

I remember that air cooled VWs had a locating dowel in the main bearings, and that it was essential to check the crush on the mains when reconditioning one. In most cases, the tunnel had to be cut and shut, or oversize OD bearings could also be obtained as to many cut and shuts ended up effecting the distributor drive gear clearance and deck height.

I will have the factory specs buried somewhere in the library if you really need them

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 thought of using a cerated interface on the cap is interesting. I have seen top fuel rods that way. I am curious as well if anyone knows what neighborhood the RA should be for the rod's internal bore. We have them resized after changing the bolts to ARP's. I am sure the machine shop we use knows what finish to put on them, but it would be nice to confirm it.

 

[SLIDING]

If detonation occurs, this can hammer out the bearing. This will lead to bearing failure. The bearing can then spin in the rod. Amazing the number of bearing and crank issues that are caused by detonation. This is about the only thing I can see know one has recommended to watch out for. Hope it helps..

 

[icrman]

I have seen alot of hydraulic locked engines with majorly bent con rods, with crankshafts that survived it just fine, and bearings also in okay shape from it. I think that is alot more stress than the average detonation causes.
And besides most top fuel engines are pretty much running in a full detonation range.
Hmmm I wonder why those diesel engines don't just kill bearings in a week or so. With that detonation like cetane knock?