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Oil feed to the crank big end journal

rolf j

Tuned and Synch'ed
Joined
Nov 2, 2008
Messages
70
My big end went tits up on the race track from obvious oil starvation reasons by the look of things. I use a windage tray. Ordinary big twin 78 mm crank hooket up to Carrillos in 88 mm cylinders. So the question is why? I can't exclude a slightly low oil level as part of the reason, but still.... Best quality oil money can buy was in the sump. The oil pump I first suspected is a type with 16 mm high cogs and looking good visually. I do not have a mechanical pressure gauge (next year for sure!). No flicker or light from the oil lamp, the switch appears to be fine, but I have not tested the pressure relief valve.

The engine is raced in anger, only used on the track. Normal shifting rev is 8-8500 with an occasional push to 9000 to save a gearshift, but never beyond that.

I have a number of cranks lying about in the garage for replacement and I noticed a slightly different feature of the failed crank compared to the others:
The edges of the oil feed holes in the big end journal are slightly bevelled or trimmed down on the SPARE cranks, but the one that failed have holes with an absolutely SHARP edge! I have noticed that tuners of americane race car engines are hysterically concerned about making bevelled edges of the oil feed holes in order to aid flow when taking the revs beyond the "normal" for a street engine. There are youtube shots out there showing how they grind down the edge of the holes "downstream" for the oil feed in order to aid flow into the bearing.

Has anyone heard of this issue in Guzzi engines? Both me and my friend and country mate Rolf Halvorsen wonder about this. Could ommission of this detail be part of the reason why I hear about so many reground cranks not doing too well? I.e. an oil feed problem reaching further than the nitrite surface issue. Incidentally my failed crank was not reground, but the sharp edges of the oil feed holes look like they would look like if somewone reground the crank without giving the edges of these holes any attention. You can clearly see that these holes have been slightly trimmed down on my spare cranks.

Anyone with knowledge into this? Should these edges ble trimmed down even FURTHER for racing use?

(Obviously the above is not an issue for the front and rear bearings that have their oil fed into the bearing by different means through a deep groove in the bearing).

Rolf Johansen
 
What is your big end bearing to crank clearance ??

What is your oil pressure relief valve set to ?? For racing should be 10lbs per 1,000 rpm

If you are running too lean that can cause pre ignition and heat that seems to affect rod bearings first which could then affect your crank ?

The bevel on the oil feed holes helps but I doubt it is critical.....
 
When I blew my Le Mans up, it was due to the main bearing oil hole "locating dowels" were missing.
The bearings didn't turn out of alignment, but I would have had low oil pressure to the bottom end and high pressure to the cam and above.

At the same time the engineering shop had a Chrysler classic with 60,000 miles and a spun bearing.
One big end had no oil holes at all. 1 careful owner = no problems until traded in, young salesman took it home for the weekend, ......................... and bang!

1. Things can be missed at original factory machining and assembly.
2. Engines can survive regardless of the above, until raced.

Advice, do everything possible to make it stronger and more reliable.
 
Thanks Pete, looking forward to your views.

rolf
 
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