What Pushrod To Use?

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scamp340

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Sudbury, Ontario, Canada
I just bought a Comp Cams roller rocker set 1322-16 from my 1970 "340" with a Mopar Performance "Purple Shaft" cam. I am replacing my old "273" adjustable rocker set up. The old set up used the Crane Cams ball and cup style pushrod. I now need a ball and ball style set up. I have four lengths to chose from in the Comp Cams catalog. The lengths are 7.450", 7.500" (stock length, according to Summit), 7.513 which the Comp Cams catalog says is for non adjustable rockers, and 7.550". I've read that the stock length for a 1970 "340" is 7.534". As well I plan on using lash caps from Comp Cams.
 
Really the right way to do it is to use a pushrod length checker. Smith Bros and Manton sell them. If you need a ball/ball pushrod, that's the easiest to check.

Measure them right and get the correct set the first time would be my suggestion.
 
I guess what I hoping to read was that since I don't have extensive machining done to my motor, the most likely length would be ... I am on a very limited time table here, and was hoping to go racing this weekend. I only have two places to try my car at the track each summer and this weekend is one of them.
 
Buy all of the sets, find the right one, return the rest...

Machining variances from the factory, cup location in rocker, camshaft base circle... all come into play when determining pushrod length.
 
Smith Brothers makes some really good adjustable pushrods too...you could order an adjustable set and then fine tune each one... Just another option, but you'll still have to give them some initial direction on the desired length in order for them to build them.

Ray
 
I ended up buying a pushrod length checking tool, and checking the contact pattern on the lash cap by marking them with magic marker. My correct pushrod length turned out to be 7.450", a full .063" shorter than stock. This is with one thread of the adjuster screw showing out of the bottom. I have the correct pushrods on order and should have them soon!
 
I would call Damon at Diamondback Racing engines about those rockers . They were knocked out of an engine challenge useing these because the cup on the adjuster was too soft and kept brakeing. Maybe the problem has been fixed by now but I haven't read it anywhere.
 
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