BIG 406 smallblock problems...help needed

[pismorat]

Well, I just blew through my second crank...spun the #1 rod bearing for the second time in 25k miles. Haven't yanked the motor yet, but I'm trying to make a list and allocate funds for the build-up. I am looking at getting a new crank, bearings, and a cam. What is the best cam to run? I rock crawl, dunes, mud, pretty much everything. I also live in California and have to pass smog. Any thoughts? I have seen a part number XE..., but can't find it now. Also, any thoughts on what is causing this re-occuring problem? I am thinking low oil flow in the block. This is the first of many posts to come... I'll have pics soon.

Thanks

 

[dirtwarrior17]

could be... 25K is barely broke in.... have you checked the oil yet?


too late to look up cams right now but crane 4x4 cams is always a good choice. Also if you want a emission legal cam crane makes computer cams.

might be able to get away with the 4x4 cam if you run acetone for the sniffer... supposed to clean it up a bit.

 

[pismorat]

I run Mobil 1 15-50W oil, and I always keep it full, the motor doesn't burn it and I always change it within 3000 miles and 3 months. This being the second time for this exact thing to happen makes me wonder about the block.

 

[muscle]

Hmmm this being the second time.... after the first time was the block machined? Line bored etc? were the crank and rod bearings properly matched? be more specific as to what happened when the motor went. were you in the throttle hard?

 

[pismorat]

I had all the machine work done, rods centered, line bored, crank and bearings matched, all of it. The first time it went very suddenly, driving down the freeway and the bearing spun. This time it was gradual, started quiet then got gradually louder. I was just rock crawling, so very low rpms. Oil pressure was ok the whole time too. I need to find a diagram of how the oil flow works in the motor...the other bearings look a bit over-worn too.

 

[slimlynn1]

Did you assemble the engine yourself? What were the clearances on the #1 rod bearing? After the first time it spun did you change the rod bolts and have the rods resized? Also has the crank been turned and do you have the correct bearings? I would check all of those things and fix what needs to be fixed before the next round. You can get a Scat 9000 crank very cheap and new rods or rod bolts are not that expensive either. Just my 0.02.

 

[diesel4me]

I've seen this before too!

The #1 main and rod bearings seen to fail first on many small blocks for some reason..I think its because they are farthest from the oil pump,and those bearings starve slightly for oil compared to the others.(especially going uphill!)..my older brother who was a GM tech said many cars with A/C had the #1 main bearing fail due to the added load of the A/C compressor belt on it,according to a few service bulletins he read at work...I bet a loose cam bearing or other worn area like where the lifters go in the block could "bleed off" the oil pressure to certain other bearings,causing them to overheat and fail also...

Only other possibility is a clogged oil passage,or maybe the crank was "tapered" and it only rode on one spot on the bearing instead of across the whole surface like intended...or a rod is slightly bent.....what sucks is you may NEVER find out what did it! -

-I took apart a 307 motor I thought I spun a bearing in after racing the motor way too high in neutral,showing off one night at mcdonalds!---it was rapping like I blew a rod bearing!--Took it all apart,the only thing wrong I could see was a thick layer of carbon on # 5 piston,with a big chunk of the carbon missing!--probably would have quieted down and been fine,had I been brave enough to just keep driving it!--

 

[pismorat]

ttt

 

[sweetk30]

The #1 main and rod bearings seen to fail first on many small blocks for some reason..I think its because they are farthest from the oil pump,and those bearings starve slightly for oil compared to the others.(especially going uphill!)..my older brother who was a GM tech said many cars with A/C had the #1 main bearing fail due to the added load of the A/C compressor belt on it,according to a few service bulletins he read at work...I bet a loose cam bearing or other worn area like where the lifters go in the block could "bleed off" the oil pressure to certain other bearings,causing them to overheat and fail also...

Only other possibility is a clogged oil passage,or maybe the crank was "tapered" and it only rode on one spot on the bearing instead of across the whole surface like intended...or a rod is slightly bent.....what sucks is you may NEVER find out what did it! -

-I took apart a 307 motor I thought I spun a bearing in after racing the motor way too high in neutral,showing off one night at mcdonalds!---it was rapping like I blew a rod bearing!--Took it all apart,the only thing wrong I could see was a thick layer of carbon on # 5 piston,with a big chunk of the carbon missing!--probably would have quieted down and been fine,had I been brave enough to just keep driving it!--

i will second that stuff he said. i had a spun #1 bearing and it was from to high revs with over heated oil from missing fan belt. i wonder how that came off ? the frt is the last for oil supply. get a good high volume pump and make sure clearences are with in spec. and ask some local racers what thay do to stop this .