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Dawa

Spun bearings. causes and prevention? (L20b)

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i recently acquired an L20b with a spun bearing, for a rebuild and install.

in my search, i also came across another l20b that also had a spun bearing.

on both engines, it is the 3rd rod bearing that is spun, if i remember correctly

 

i know there are general causes/preventions for spun bearings

but im wondering if there are L20 specific causes (design flaw, etc)

and preventions (specific 'better' bearings, etc)

 

thanks!

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99% of the time spun bearings are from no oil. Could have been at assembly- not priming the oil pump before install, or even worse assembling the bearings with no oil.

 

However you can spin a bearing from having a slightly bent rod. The slight bend can be from hydrolock (trying to start and engine full of water) or a stuck valve impact. With the rod no longer straight it puts weird loads on the bearing and knocks it out of its groove.

 

The last reason is somone putting STD bearings on an undersized journal, or the opposite putting oversized bearings on a STD journal (or one that hasn't been undersized enough). That's why you ALWAYS plastigauge EVERY bearing. Don't trust that it's right.

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sigh, i really wanted to believe that datsun owners werent spinning bearings simply because of lack of oil.

thanks for the input so far, gents!

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It doesn't matter what the make is. People still treat cars as disposable. I've seen cars go 30,000 miles between oil changes. Light comes on, they add a quart. Hell, I remember someone complaining that their car didn't make it to the warranty before the engine siezed... having never even CHECKED the oil, let alone change it.

 

 

I've never spun a bearing. I probably have 500,000 miles in Datsuns. Lost rings, yes, but that was a car that the engine was unsiezed by dragging it down the road and popping the clutch... and it still lived about 5000 miles after that. Even better, I had an L20B that had the main bearings in the wrong order... center (#3) was on the rear (#5), #1 and #5 were on #3 and #4, and #2 and #4 were on #1 and #2. So ONE bearing was in the right place. And it ran... until it ate the thrust washer. Then the crank slop got so bad the timing changed on hills. It had over a quarter inch endplay. But it survived, and I simply unbolted the crank, slid new bearings in the RIGHT order, and buttoned it up. Good as new. Didn't even pull the head.

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The L and Z series are the same rod bearing size, L20B and up mains are the same but L18 and down are only a different diameter. In all cases the mains are hugely wider than the KA which makes 50% more power! I would say that the L and Z motors are very very over built for bearings and will support a very large amount of horse power or abuse or hundreds of thousands of miles. If a bearing spins it's from neglect, either yours or a previous owner.

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oh i agree the make doesnt matter, but they werent just car owners, they are ratsun members, whom i hold to a higher regard.

 

 

awesome, good to hear that the engine is actually OVER built in some aspects.

 

looks like the riddle of spun bearings is solved, then. i just wanted to ensure i wasnt going to be rebuilding something that is a possible ticking time bomb.

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L and KA main bearings. Guess which one handles over 150 horsepower???

 

L20BcrankinKAE002Large2.jpg

 

I've pounded a bearing and it's LOUD like a sledgehammer pounding on the block. No way you can not notice whats happening and stop before the damage gets worse. I've spun a rod bearing before on my '66 Pontiac and it was a LOUD screech. It started as a tapping that got worse. (I should preface this by saying that it was common in Ontario back in the day to have a cheap 'winter car' for the bad weather and I didn't care much for it. Drove it for weeks with the pounding rod)The bearing was pounded so thin it slipped under the other one. At this point the oil hole can be blocked by the bearing shell (if there is any oil to begin with) and friction will heat the rod/bearing and crank to the melting point and they fuse together. If driving slow enough the motor just seizes. If your foot is 'in it' there may be enough power to snap the rod off below the piston. Now the bottom half swings free and punches through the side of the block. Saw this happen to a new '70 Camaro I was chasing at 120. 4 quarts of oil on the road from a rod through the oil pan.

 

I'm saying bearing don't just spin on the crank for no reason and they sure are not quiet about it. I'll bet both these L20B owners with spun bearings had LOUD stereos.

.

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L and KA main bearings. Guess which one handles over 150 horsepower???

 

L20BcrankinKAE002Large2.jpg

 

 

.

 

I read an article many of the big name stock car racers are cutting their cranks down to fit honda size main bearings to gain extra horsepower and superior oiling ..crank has less far to go to get back to the oil again and the reduced friction is an automatic horsepower gain.. The physics of it ( with charts and graphs in the article ) seemed to make sense and those cars go 400 miles at 9000 rpm so there might be something to it..

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DAWA:oh i agree the make doesnt matter, but they werent just car owners, they are ratsun members, whom i hold to a higher regard.

 

1st mistake!!!!!!!!!!!

 

I had 2 blocks redone and all was needed was polish the crank and standard size bearing put back in.

rods and cranks are Forge as alot of Japan made stuff.

 

just head gaskets are a proplem over: heating

L20s the center water passage hole where the headbolt goes thru needs to be check. Thay start cranking there alot soe soem reason. went I went thru 2 block before he found a 3rd good one

I had 1 bad one with only 80k miles on it. then had to get another block. Wheither or not if it really made a difference if still good but its just info to let you know

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I read an article many of the big name stock car racers are cutting their cranks down to fit honda size main bearings to gain extra horsepower and superior oiling ..crank has less far to go to get back to the oil again and the reduced friction is an automatic horsepower gain.. The physics of it ( with charts and graphs in the article ) seemed to make sense and those cars go 400 miles at 9000 rpm so there might be something to it..

 

Yes the larger the diameter the faster the surface is moving relative to the bearing surface. A smaller diameter would turn slower but then there is less surface to put a load on. At some point there is diminishing returns. The two bearings in the picture are the same diameter I believe, only the width is different. All things being equal a wider bearing should carry a larger load.

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so finally got the engine to the machinist. they say the crank looks good so thy dont think there was a spun bearing.

 

they did mention that the bores are about 20-22 over amd with stock size pistons and the pistons show aome scratchs from piston slap so i feel like that is what couldve caused a knocking sound for the previous owner

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They probably had a machine shop do service overbore ("just clean it up") but then put stock size pistons back in. Now all you need to do is find four new pistons the correct size. 0.20" overbore is 0.5mm overbore piston. Nissan pistons came in STD, 0.5, 1.0 and 1.5 overbore

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22? like 0.022" over? Well this is likely concentrated about a quarter inch down from the deck at the highest point of the top ring travel. Looks like over bore and new pistons and rings. To have a good idea of wear you should measure in 3 places down the bore and at 90 degrees to them... 6 in all.

 

This will show taper and oval if present.

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I read an article many of the big name stock car racers are cutting their cranks down to fit honda size main bearings to gain extra horsepower and superior oiling ..crank has less far to go to get back to the oil again and the reduced friction is an automatic horsepower gain.. The physics of it ( with charts and graphs in the article ) seemed to make sense and those cars go 400 miles at 9000 rpm so there might be something to it..

 

The old Chevrolet DZ302 small block was built specifically to run a small journal 327 crank in the Trans-Am series cars of the time. It was done for drag. You can still buy block main cap spacers to run a small journal crank in a big journal block. Nowdays the oil film strength is so much better that the smaller bearing doesn't notice the difference. That and tighter bearing clearances.

 

In any case, almost all of the time, a spun bearing in an L series is from lack of oil.

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They probably had a machine shop do service overbore ("just clean it up") but then put stock size pistons back in. Now all you need to do is find four new pistons the correct size. 0.20" overbore is 0.5mm overbore piston. Nissan pistons came in STD, 0.5, 1.0 and 1.5 overbore

 

.50mm = .020"

1.0mm = .040"

 

Just to be sure...

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Yep, approximately

0.20" overbore is 0.5mm overbore

.50mm = .020"

 

Nissan pistons only come in metric sizes

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mike i wasnt there when the bores were initially mic'ed so im not sure if he made the six measurements per reference, he showed me the top and bottom for each bore.

 

with the info of .020-.022 over i needed to make the decision of whether it was cheaper to source another block or find pistons. i found beck/arnley pistons on rockauto, .020 over and .030 over.

i got the 30s

i know nissan = japanese = metric

 

.030in to mm is .762 so idk....

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Its all rough conversions between those 2. Being a metric motor, they are probably .75mm

 

I put in ZX 40 over pistons, which was 80 over on my L18. Machine shop said they bored it out exactly 2mm (I think they meant that the block hadnt been bored before)

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The L20B along with all 4 cylinder motors is fed with only 2 mains where as the L6 motors have one rod oiled for each main.

 

One problem that I guess you could compare is with Triumph Spitfire motors. The small bearing 1300 worked fine but when they went to the 1500 they increased bearing size without touching the oiling system. #3 rod I think it is, gets starved, heats up, and always dies one way or another (3 main motor). When Kas Kastner went racing, their 1300 motors would last just fine but the 1500 wouldn't make it past practice even though oil pressure was reading just fine on the gauge (taken from the main oil galley). They wound up drilling into the snout of the crank, installing a fitting with a gauge, and running the motor. At idle it was fine but as the RPM increased, the pressure dropped within the crank.

 

Those motors were oiling two rods with 1 main, just like ours. Maybe some insight.

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so whats the easy solution

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easy solutions is to make sure there is oil in the damn thing lol my motor went 160k before it died and it only died because it lost compression it still has the original bearings in the lower end. i have all service records since new on this engine and i know for a fact it always had oil in it

 

basicly make sure the oil is full and it shouldnt happen again

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easy solutions is to make sure there is oil in the damn thing lol my motor went 160k before it died and it only died because it lost compression it still has the original bearings in the lower end. i have all service records since new on this engine and i know for a fact it always had oil in it

 

basicly make sure the oil is full and it shouldnt happen again

 

i wasnt asking the easy solution for how not to oil starve an engine, you silly guy. that was borderline insulting, although i know you meant well. 6 cars & a bike and ive never had an oil problem, it wasnt me that spun that bearing :)

 

i was asking josh817 what the simple solution for the problem he id'ed

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Make sure your tolerances are right and your parts are squeaky clean during assembly! Use proper lubes for assembly and follow break in procedures. Do not let the car detonate during break in as that can start pounding the bearings out!

 

If possible, always remove galley plugs and thouroughly clean everything like you would a gun!

 

Bad tolerances, dirty parts and inproper procedures are sure ways to kill an engine.

 

There is nothing wrong with the L engines and when built properly will last for a long time.

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