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-   -   sbc350 that eats cam lobes (http://www.hotrodders.com/forum/sbc350-eats-cam-lobes-2718.html)

eblyon 06-06-2002 10:39 PM

sbc350 that eats cam lobes
 
I have a sbc350 bored 30 over ,1.6 rockers,heads have screw in studs with guide plates and they have been ported & polished by a quality shop. My engine has chewed two cams[both times on #8 the worst but also on 7,6and5. I've used hydraulic lifter cams both times. Also ,I didn't have any problembs till I installed these heads. I think I may have found the problemb ,but I would value some other opinions. I noticed some of the rockers don't line up square on the vavle stems. They are ****ed off to the side a little. I'm wondering if this is causing the lifters not to rotate properly by putting a side load on them. Any help would be nice thanks

HQ Pete 06-07-2002 05:02 AM

need a little more info like type of heads ,cam, valve lift,valve springs,roller or stamped rockers?....i figure you no about the all important cam break-in procedure(1500-2000rpm for the first 15 min and heaps of prelube).You may be right with your theory,also check for coil bind in the spring depending on the lift you have and rocker ratio it maybe on the limit.are your guide plates lined up ok ?give us that little more info and we'll go from there .

eblyon 06-07-2002 11:35 AM

the heads are GM castings with 2.02intakes .They are 76cc. I don't have the casting #s handy but I pretty sure they start with 4. The valve springs are double wrap 1.47 diameter . I have checked the spring pressure with a frends moroso checker. It takes 110 lb to open valve off its seat and fuuly open it takes 300 lb . The cam it just ate was a Comp Cams Extreme energy . Advertised duration is 274 in./286 ex. and at .050 in. is 230/ex. is 236. Lift with 1.6 rockers .517 in./.520 ex.The cam I have now is one step down. advertised duration 268 in./280 ex. at .050 it is 224 in./230 ex. Lift is .507 in./.510 ex. with the 1.6 rockers. The rockers are the stamped steel kind that I got from P.A.W.

[ June 07, 2002: Message edited by: eblyon ]</p>

eblyon 06-07-2002 11:42 AM

I almost forgot to mention I did prelube the cam very well and followed the break in proceedure to a T. I have also checked for coil binding and that is ok. I used the lube that came with the cam. I don't claim to be an expert but I have built several engines. I never had this problemb before .

slomotion 06-07-2002 12:42 PM

Did you install new lifters? An old set of lifters that were running on another cam will pick up a "finger print" that has tiny little scratches that you can't even see that will act like a file on a new cam lobe. It may be coincedence that the cam went south at the same time you installed the heads. That's my "swag" (sophisticated wild a** guess) Good luck!

4 Jaw Chuck 06-07-2002 02:58 PM

Whats the clearances on your cam bearings? Having oversize bearings on one journal will cause the cam to deflect and cause the lobes in between the two good journals to wear unevenly and eventually wipe the lobes. This is a very uncommon condition and is usually caused by the cam bearings being replaced with the wrong size at one journal position. Other than that there is little else that would cause this, you could check for this by comparing lift at the valves across all the cylinders with a good cam. If you find a few cylinders with low lift (0.003" or more) you will find the journal that is oversize. This is a good reason to check the diameter of every cam journal bearing before installation. This problem is hard to find because it only gets bad at rpm, if you were racing it you would probably break cams in half.

eblyon 06-07-2002 08:57 PM

my engine has around 10,000 miles on it since it was bored and new cam bearings installed. Also I did use new lifters with both cams. I also inspected the cam bearings and they looked fine. Thanks for your replies. I was searching for more power with the cam & head swap.

HQ Pete 06-08-2002 09:45 AM

ok that cam sounds ok but maybe the rocker ratio is too much .507/.510 lift.....is it just the exhaust lobes that are gone or intake as well ?...i would check the valve lift without a spring on there ,if its not coil bind then maybe it is bottom of spring retainer hitting on valve guide .it might just be that extra .010" lift that puts the damageing presure on the lobes ....i have preasures for a 272-h10 and they are 105lb closed 265 open but theres alot more too it than that (installed height and all that stuffs)sounds like your pretty on to it with the breckin procedure.seems strange that its the back cylinders with most wear(#8,7,6,5).i would look at simulating the lift on inlet and speaclly the exhaust without a spring in there and see wots going on .i have a set of gm 041 heads with a .467 exhaust lift and the spring retainer only just clears the guides with valve stem seals,depending on your installed height and rocker ratio this could be a problem . these are just suggestions and hope some else can shed some more light on the subject ....good luck ,get back to us soon :)

HQ Pete 06-08-2002 09:56 AM

ps...if you have the right rpm for breck-in ,heaps of lube ,and no machanical bind of any sort there sould be no reason why its eating lobes ,and just something else i thought i'd throw at ya is wot about the cam chain gear or sprocket is it new ? if so make sure the cam is going the full distance back into the block so its running on the lifters correctly and it should spin freely in there. i'm sorry for the double post ,i just had to get it out tho .. :)

4 Jaw Chuck 06-08-2002 10:02 AM

Eblyon, what was the clearance on the cam journals? HQ Pete has some good advice on the clearance for the springs and valve train.

HQ Pete 06-08-2002 10:18 AM

ive learnt alot from 4 jaw chuck(that man is a machine) :) and he has good reason to suspect bearing clearances.might this be due to the rear lobes going fisrt maybe ?

78 monte 06-08-2002 12:17 PM

Do you have oil restrictors in this engine? That could be it if its killed 2 cams in 10,000mi.

JdOwNj 06-08-2002 08:09 PM

Hate to point out the obvious, but have you checked piston to valve clearance?

4 Jaw Chuck 06-09-2002 12:37 PM

I just thought of one more thing, did you find any slightly bent pushrods? This would indicate a binding/clearance problem.

eblyon 06-09-2002 09:26 PM

Thanks for all your replies I'll do some more checking on your suggestions .I've been bussy setting up my new garage with electricity,water and what not. I'll post another message on what I find. It may be a few days. Thanks again and if anyone else has a suggestion feel free to post a reply.


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