camshaft lobe base circle

[lewtot184]

i believe i have a tappet problem in the stock engine in my 69 r/t. the car sits a couple of weeks and the lifters collapse. real noisy nuisance. so ive been thinking the only cure is to replace cam and lifters. i got a few cams out and just started looking at things and decided to measure the base circles on the lobes because i'm using a stock valve train. i got numbers anywhere from 1.242"-1.325" on five different cams. my numbers probably aren't "perfect" but they're more than close enough to throw up a flag. four of the cams were between 1.242" and 1.289". that's a .047" spread and could vary preload by .0235", and might negative effect preload on the tappet. the fifth cam had .083" difference between the smallest and that could be as much as .0415" difference in preload and i think a deal breaker.

what i'm getting at here is check the base circles when doing a cam change on a non-adjustable valve train. it may save some parts breakage, time, and money.

 

[Don Frelier]

You could try disassembling, cleaning and reinstalling the lifters. I did it many years ago on a 360 and only one lifter ticked afterwards.

 

[lewtot184]

You could try disassembling, cleaning and reinstalling the lifters. I did it many years ago on a 360 and only one lifter ticked afterwards.

I've thought about that and it would be easier than a cam change, but from experience I think the tappets are messed up. They were never quiet cold.

 

[R/T Boy 67]

The higher the lift, the smaller the base circle.

 

[R413]

Take apart a few lifters and see what's inside. Keep them in order.

Old cams can look funky on the lobes and be nice and quiet.

 

[lewtot184]

The higher the lift, the smaller the base circle.

The cams I measured were between .447 lift and .535. The largest base circle was with the .447 lift cam. Big rollers will have a reduced circle, but these cams were street hydraulics. I'm very certain my engine is very clean inside. No high mileage and very **** on oil changes. Oil drain back on filters is a real problem. I've used many brands and eventually the problem is always there; some worse than others.

 

[gtxrt]

if worried about preload with different base circles on the same cam you can get different length push rods to make up some of the difference. plus lifters have a travel distance of 140 thousand up to 200 thousand. check your lifter travel. guys talk about preload at 25-40 thousand no need for that. at least 60 and up to 100 thousand preload is fine. with that I am sure you can make your camshafts work.

 

[lewtot184]

if worried about preload with different base circles on the same cam you can get different length push rods to make up some of the difference. plus lifters have a travel distance of 140 thousand up to 200 thousand. check your lifter travel. guys talk about preload at 25-40 thousand no need for that. at least 60 and up to 100 thousand preload is fine. with that I am sure you can make your camshafts work.

I know and have done all that. There's people out that don't know and don't want to know. They just want to slap stuff together and go. I'm just trying to get folks to look before they leap. It was an eye opener for me.

 

[gtxrt]

I know and have done all that. There's people out that don't know and don't want to know. They just want to slap stuff together and go. I'm just trying to get folks to look before they leap. It was an eye opener for me.

and i was trying to tell them after they look how to leap. lol

 

[Krooser]

The higher the lift, the smaller the base circle.

Not really....the base circle has zero to do with lift.

You can take a small base circle cam like a sbc and still have a .350 lobe lift. Mopars are larger than Chevys but can have a much smaller lobe lift.

 

[R/T Boy 67]

The base circle has everything to do with lift. That is where the extra lift comes from. The lobes can't be made much taller because the cam would not be able to be installed.

 

[Geoff 2]

If you want to use a cam with more lobe lift [ smaller base circle ], you could use lash caps on the valve tips to take up the difference.