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318LA Rocker Shaft Bolt Specs

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Hi, rebuilding my 318LA and 2 of the the rocker shaft bolts broke when we removed them (had been replaced with grade 5)
Does anyone know the proper width, thread and length? These appear to be 3/8 16 1-7/8" but I am reading different sources some saying they should be 5/16 - 16?

Also can not find a definitive source on length. Tried to order a new set of bolts and hold downs but they are all on back order

Thanks!
 

Rich G.

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The 340 I have laying around has 5/16-18 , 1-3/4 long . I’d guess they would be the same .
 

tonysrt

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I just checked one from a J head and they are 5/16 x 18 and 1 3/4 long.
 

Chryco Psycho

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3/8" is for big blocks
you can use grade 5 as original or grade 8 either will work fine
Just make sure you do NOT overtorque them , you can oval the shafts & cause extensive damage to the whole engine depending what rockers you are using , IIRC it is 18 ft lbs
 

volunteer

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3/8" is for big blocks
you can use grade 5 as original or grade 8 either will work fine
Just make sure you do NOT overtorque them , you can oval the shafts & cause extensive damage to the whole engine depending what rockers you are using , IIRC it is 18 ft lbs

I have a 2 lb. can of used (OEM) bolts and they seem to average out at 1.85" (straight section) or 2.10" overall. These are all grade 5 and I replace every one of them with grade 8's., which are gold-color and two threads longer. Suggest do not re-use old ones or stay with gr.5 as this is weakest 'link' on entire valve-train. Big block versions were 3/8" but still only grade 5 and torqued to 25 ft-lbs. Even grade 8's at 35 lbs. no problem with shaft 'integrity' so, the extra torque (25 lbs.) with gr.8 small-block bolts just 'cheap insurance'.
Consider the stresses with higher strength valve springs - but retaining (OEM) quality bolts. Bad engineering. Compare to S.B. GM engines which had one 3/8" stud (bolt) for each rocker arm - - not 5 'skinny ones' for all 8.
 

Chryco Psycho

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I would disagree with "bad engineering" I have not seen a shaft bolt fail unless it was overtorqued & the rocker shaft is far more stable & has no stress twisting or rolling away from the pressure like a ball stud system does + at least the shaft is threaded down not just pressed in like the Chev they might be 3/8 "but I have seen many of them pull out of the heads with just stock spring pressures , they have to be threaded or pinned to stop them pulling out , JUNK IMO
 
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