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Old 08-27-2012, 09:52 PM
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broke new motor need help

hey guys,
okay so this will sound weird but its true im not a dummy.
i have a gen 5 454 91-95.. I rebuilt the motor with all good parts. was trying to make about 550 horse. kb dome pistons big solid roller lunati cam. alum heads, the works. anyway got it running and it ran awesome. had unbelievable power and was a blast. however about 2 hours run time on this all new motor my timing chain broke at like 3000 rpm not even getting into it. well it bent 4 valves cracked the head ect! NO FUN! ... anyway never could figure out what would have caused that to happen.. my question is i put a gen 5 timing chain on it a comp cams.. my block casting number says its a gen 5 91to95 but the front of my block is drilled for a gen 6 cover which only has 6 bolts. so im assuming that it was made as a gen 5 but some how got switched at the factory and got drilled for a gen 6 cover.... the gen 5 and 6 timing chains are different part numbers but i cannot see what the differnce would be. what timing chain should i get?. Do you think thats what cause it to break??? anyway im stumped on what to do but i got new heads and really dont want that to happen again .. please let me know what you think..
thanks guys

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Old 08-27-2012, 10:28 PM
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Couple questions...

Quote:
Originally Posted by 1fast62 View Post
hey guys,
okay so this will sound weird but its true im not a dummy.
i have a gen 5 454 91-95.. I rebuilt the motor with all good parts. was trying to make about 550 horse. kb dome pistons big solid roller lunati cam. alum heads, the works. anyway got it running and it ran awesome. had unbelievable power and was a blast. however about 2 hours run time on this all new motor my timing chain broke at like 3000 rpm not even getting into it. well it bent 4 valves cracked the head ect! NO FUN! ... anyway never could figure out what would have caused that to happen.. my question is i put a gen 5 timing chain on it a comp cams.. my block casting number says its a gen 5 91to95 but the front of my block is drilled for a gen 6 cover which only has 6 bolts. so im assuming that it was made as a gen 5 but some how got switched at the factory and got drilled for a gen 6 cover.... the gen 5 and 6 timing chains are different part numbers but i cannot see what the differnce would be. what timing chain should i get?. Do you think thats what cause it to break??? anyway im stumped on what to do but i got new heads and really dont want that to happen again .. please let me know what you think..
thanks guys
Are you using a cam that uses the original retainer plate?

Was the timing chain that broke a single or double roller?

Is there a "Gen 6" cast into the timing chain area? Every BBC I've seen w/a cast 6-bolt timing cover has been a Gen 6.

What is the casting DATE of the block?
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Old 09-01-2012, 08:21 PM
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sorry for the delay i was out of town..
ok so to answer those questions
i am not using a cam the uses the retainer plate. i have a a cam that fits all the years from 65 to 95. the gen 6 are the only ones that had a retainer plate..

the chain that broke was a double roller.

gen 6 is not stamped in the front of the block. however it is drilled for a gen 6 cover..
this is what i found for the casting number.
12550313...454.......91-up...4-bolt, Gen.V crate motor, Gen. VI 4.25" bare block, fuel pump boss

Last edited by 1fast62; 09-01-2012 at 08:26 PM.
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Old 09-02-2012, 01:23 AM
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The only way I can see the chain snapping is if the (back of) the cam gear caught on the block.

Cam walk, cam gear to block clearance?
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Old 09-02-2012, 03:23 AM
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Quote:
Originally Posted by 1fast62 View Post
sorry for the delay i was out of town..
ok so to answer those questions
i am not using a cam the uses the retainer plate. i have a a cam that fits all the years from 65 to 95. the gen 6 are the only ones that had a retainer plate..

the chain that broke was a double roller.

gen 6 is not stamped in the front of the block. however it is drilled for a gen 6 cover..
this is what i found for the casting number.
12550313...454.......91-up...4-bolt, Gen.V crate motor, Gen. VI 4.25" bare block, fuel pump boss
Unless the chain was defective to begin with, I'd suspect the cam button was adjusted too loose and that allowed the cam to walk forward enough to let the cam hit. If this happened, there will be tell-tail evidence of this on the cam and block where they contacted.

If you are using a Mk. 4 type cam and a thrust button, use a Mk. 4 type timing set. If the block has been align bored you might need a shorter timing set.

BTW, some Mark 4, and Gen 5 and 6 have retainer plate bolt holes. The Gen 6 uses a smaller diameter bolt pattern on the hydraulic roller cam gear to differentiate it from the earlier set up.
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