FYI > Determining Pushrod Length during cam change .. How-To
04-02-2017, 07:36 AM
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FYI > Determining Pushrod Length during cam change .. How-To
Just something I typed out this morning for lack of anything else to do 
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Determining Pushrod Length
As with any engine that uses hydraulic valve lifters, the lifter requires a certain amount of 'preload' to operate correctly. On some engines, lifter preload is adjustable, however, on the LSx engines, it is not adjustable and can only be changed by varying the pushrod length.
On almost all aftermarket cams that have a greater valve lift and valve open duration than the stock cam, the aftermarket cam's Base Circle will be smaller than the OEM cam. And because the Base Circle is smaller, then the cam's Centerline to Heal distance will also be smaller (see photo below).
Depending on how much smaller the Base Circle on the aftermarket cam is (which results in a smaller Centerline to Heal distance), longer pushrods may be needed in order to achieve the correct lifter preload.
The easiest way to determine what pushrod length you need is to first measure the Base Circle of one of the lobes on your stock cam with a digital caliper. The last LSx engine cam change I did was on an LS1. The OEM cam had a base circle of 1.552". To figure the Centerline to Heal distance, you simply need to divide the Base Circle distance in half ... So, the Centerline to Heal distance was 0.776".
The Comp Cam that we had purchased had a Base Circle of 1.502". That gave use a Centerline to Heal distance of 0.751"
Subtracting the Centerline to Heal distance between the stock cam and the aftermarket cam told us that the aftermarket cam's Centerline to Heal distance was .025" less than the stock cam (0.776" - 0.751" = 0.025")
So, in order to get the correct lifter preload, we need to replace our stock 7.400" pushrods with a set that are 7.425" in length.
Stock LSx Base Circle: . _ . 1.552”
Centerline to Heal: . . . . . . 0.776”
Comp Cams Base Circle: . . 1.502”
Centerline to Heal: . . . . . . 0.751” . . (.025” shorter than stock)
________________________________
Determining Pushrod Length
As with any engine that uses hydraulic valve lifters, the lifter requires a certain amount of 'preload' to operate correctly. On some engines, lifter preload is adjustable, however, on the LSx engines, it is not adjustable and can only be changed by varying the pushrod length.
On almost all aftermarket cams that have a greater valve lift and valve open duration than the stock cam, the aftermarket cam's Base Circle will be smaller than the OEM cam. And because the Base Circle is smaller, then the cam's Centerline to Heal distance will also be smaller (see photo below).
Depending on how much smaller the Base Circle on the aftermarket cam is (which results in a smaller Centerline to Heal distance), longer pushrods may be needed in order to achieve the correct lifter preload.
The easiest way to determine what pushrod length you need is to first measure the Base Circle of one of the lobes on your stock cam with a digital caliper. The last LSx engine cam change I did was on an LS1. The OEM cam had a base circle of 1.552". To figure the Centerline to Heal distance, you simply need to divide the Base Circle distance in half ... So, the Centerline to Heal distance was 0.776".
The Comp Cam that we had purchased had a Base Circle of 1.502". That gave use a Centerline to Heal distance of 0.751"
Subtracting the Centerline to Heal distance between the stock cam and the aftermarket cam told us that the aftermarket cam's Centerline to Heal distance was .025" less than the stock cam (0.776" - 0.751" = 0.025")
So, in order to get the correct lifter preload, we need to replace our stock 7.400" pushrods with a set that are 7.425" in length.
Stock LSx Base Circle: . _ . 1.552”
Centerline to Heal: . . . . . . 0.776”
Comp Cams Base Circle: . . 1.502”
Centerline to Heal: . . . . . . 0.751” . . (.025” shorter than stock)
04-03-2017, 09:23 AM
#2
Drifting
Thank you Turbo6TA for posting this. I find the technical nuggets here interesting and informative. I'll probably never swap a cam or even change exhaust but it's still cool to read about.
