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948 sprite chewed the number three rod bearing

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7

71MKIV

Jedi Warrior
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Not my engine or my overhaul thankfully.

But a friend of mine, 200 miles after a fresh overhaul.

Number three rod bearing trashed. As in blue, worn down the the shell, pieces of bearing material all over the place. Sure looks like no oil, but the oil pressure was 40 to 45 hot. The oil gallery in the crank was open. #2 was scratched, but not mangled.

I know Spitfire engines, not these little motors.

What was missed? How can there be no oil to the #3 if the gallery in the crank was open?
 
no zinc? what oil?
 
I would think if it was the oil all the bearings would be trashed. I can find out what was in today and get back to you.
 
If oil was present, sounds like a tolerance issue e.g. wrong/mixed bearings, machining issue, etc. Anyone use plastigage to check out prior to assy? Were the bearings pre lubed after assembly? Did the engine turn over freely by hand after assembly prior to start. Was oil pressure OK at turnove prior to start up?
Lots to think about
 
Check the condition and location of the little brass oil restrictor plugs. These occassionally will dislodge from their crank location and groove a bearing and cause a catastrophic bearing distruction. This is kind of a chicken and egg investigation. Did #3 go first or was there something somewhere else that happened and #3 was the weak link in the chain? Excessive side clearances (>0.025), oil pump issue at idle, blocked oil passage on #3 rod? Something is the culprit.

Also check to see if the rod cap was installed backwards. 948/1098 diagonal split rods can be installed backwards rather easily. Most of the time, they lock the crank down, but once in a while, not as tight as would be expected. Bearing tangs in the A-series engines are always on the same side of the rod.

Doubt it was oil type. Lubrication products are just too good and consistent today. Zinc would have only been an issue for the metal to metal interface locations, like the cam and lifters. Would have taken a while to show up there.

This was a internal mechanical or assembly related condition. A detailed autopsy is required to find the underlying cause.

HTH,
Mike Miller
 
The detailed autopsy is already underway. I will get back to you.

Thanks
 
Gosh, I hate it when that happens
 
Steve,
We're almost neighbors, I'm in E-town.
Hope you're staying dry today, lots of local flooding here.
 
Amazing who you find. Im actually in Leola. Staying dry so far but I dunno if I will get where i am going tonight. Dont seem fair. We're under water while Texas burns.
 
Spridget64SC said:
Check the condition and location of the little brass oil restrictor plugs. These occassionally will dislodge from their crank location and groove a bearing and cause a catastrophic bearing distruction. This is kind of a chicken and egg investigation. Did #3 go first or was there something somewhere else that happened and #3 was the weak link in the chain? Excessive side clearances (>0.025), oil pump issue at idle, blocked oil passage on #3 rod? Something is the culprit.

Also check to see if the rod cap was installed backwards. 948/1098 diagonal split rods can be installed backwards rather easily. Most of the time, they lock the crank down, but once in a while, not as tight as would be expected. Bearing tangs in the A-series engines are always on the same side of the rod.

Doubt it was oil type. Lubrication products are just too good and consistent today. Zinc would have only been an issue for the metal to metal interface locations, like the cam and lifters. Would have taken a while to show up there.

This was a internal mechanical or assembly related condition. A detailed autopsy is required to find the underlying cause.

HTH,
Mike Miller
Click to expand...

Thats pretty much cover anything I could add, Mike pretty much nailed anything that could have cause the failure. I would ake aprt carefully and pay close attention, like Mike said when you pull the rod cap, right tne check it orientation to see if it was installed correctly. The clearnece could hve been a isuue, did you guys check clearence in anyway duirng the assembly, blueprint, or plastigauge, was the crank or rod resized as part of this rebuild. I even measure the bearing shell thicknesses in all the motors I build, even though I never find much of a difference, +/- a tenth of thousand , but atleast I know I didn't get a mislabled bearing. Alway cehck rod side cleanrece as Mike mentioned, especially if you are swapping parts like rods and crank that never have ran together before, most of the time it's good, the 1275 motor we finished yesterday had about .012" rod side clearnece across the board, I like to see alteast .010" an no more than .020", and I have had to dress the thrust sides on connecting rods before in a belt sander to get it up a bit, but most times it fine and within spec.

Also important , A series thrust bearings can be a bit testy, I like to check my crank end float at teardown and if I see them in spec, then I reuse the thrust bearings, A series thrust bearings noramlly can last several rebuilds, and if you get new, they can often time be too tight, and need sanding down a bit to get proper crank endfloat, if any of this sounds familar in the rebuild, then pay close attention the center main as it is the oil feed for the #3 rod, so the problem could have originated there. Hope this helps
 
thanks,

I haven't made the time to get ahold of my friend, but I will try and get to it in the next day or two.

He's a preacher, so Sunday's are kinda out for this kind of thing.
 
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