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Old 05-14-2010, 06:41 PM
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383 Knock At Start Up??

i recently built a 400hp 383 using a 96 block, my problem is that when i first start it up it knocks till it warms up (bout 15 min). i had baught used crane roller rockers ($200 new ones), it sounds to be vavletrian related, when roller rocker go bad will they knock? it has great oil pressure so i dont think it has anything to do with bearings,my buddy said it may possiably piston slap.. it has speed pro hyper pistons, this motor has less then 1000 miles on it please let me know wat you think. thanks!!

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Old 05-14-2010, 10:49 PM
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Yes, roller tappets tend to tap if they get trash in them and GM roller tappets are kinda bad about this. Before you do your next oil change, pour in half a can of seafoam, start the engine and let it idle for about 15 minutes. Allow time to cool, then change the oil. However, what your hearing is also common fold for hyper pistons if one or more has slightly too much clearance. Today`s hyper pistons are tough pistons and they barely expand once heated. This is why now days especially the machinist measures each piston and matches it to the bore size. The clearances have to be around .0015 skirt to wall clearance measured with a feeler gauge. You may have one or more bores that came out slightly to the large side so it`ll knock until it warms up. Just make sure you keep the revs low during cold start and it won`t hurt anything. I`m using the same pistons in my truck and thus far they`ve served me well. Before I rebuilt it, it had over 260,000 miles on it with factory hyper pistons. When I would cold start it, it sounded like a diesel until it warmed up.
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