Lifter bore - Team Camaro Tech
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post #1 of 4 (permalink) Old Nov 24th, 16, 05:42 PM Thread Starter
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Steve
 
Join Date: Apr 2002
Location: Southeast Pa.
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Lifter bore

What are the pro and cons of a .843 and a .904 lifter bore choice?

67RSSS......owned since 69. A basket case for most of it.
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post #2 of 4 (permalink) Old Nov 24th, 16, 06:10 PM
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Dave
 
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Re: Lifter bore

The larger the diameter of the bore the less contact it makes with the cam allowing the tappet to ride on top of the ramp without digging in to the face of the cam with a more aggressive cam profile.

Think of the planet Earth as being flat compared to a sphere. We know it is a globe but with a large diameter and a tiny person making contact with only their feet it acts as though it were flat. Change the scale back to parts that fit back in an engine and you can see where a larger lifter has less chance of flattening a flat tappet cam.

In essence you are regaining the some of the advantage a roller cam has. Rhodes sells a mushroom tappet that has a large flat disk on the bottom of the lifter to accomplish a similar function. They are illegal according to all sanctioning bodies (NHRA, NASCAR, USAC, etc.) as they offered an unfair advantage because a stock ground cam with mushroom tappets acts as though the duration is actually longer and by lofting the lifter over the nose of the cam you can gain additional lift.

You gain that duration advantage (not the lofting part), while a bigger diameter lifter acts like a mushroom tappet, and they are legal in COMP classes just not stock.

I bush my big blocks tappet gallery for Chrysler roller tappets to gain an even bigger foot print and I am also moving the lifter a bit off center as well as straightening out any mess up in machining (not as common today with CNC machining as it was in the old days with humans machining the blocks). I run half inch push rods and triple wound springs with 0.800+ inches of valve lift to compensate for the tiny 2.30 inch intake and 1.98 inch exhaust valves in my head. The valves may sound big but consider the valve area per cube for a 302 with a 2.02 compared to a small 632 cube BBC with little 2.30 inch valves. You can easily see why I go to extremes with valves and port work.

Big Dave
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post #3 of 4 (permalink) Old Nov 25th, 16, 04:39 AM Thread Starter
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Steve
 
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Location: Southeast Pa.
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Re: Lifter bore

Dave, any side effects from the mix and match components? I see aftermarket blocks with a choice in lifter bores and started wondering about how to put a valve train together. I havent gotten anywhere trying to look up lifter combos on cam manufacturer sites. Just curious how pandoras box works out in this case.

67RSSS......owned since 69. A basket case for most of it.
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post #4 of 4 (permalink) Old Nov 25th, 16, 09:54 AM
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Dave
 
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Re: Lifter bore

No there is no difference in going with the larger bore since you should be buying your pushrods to fit your valve train anyway (changing to a roller rocker changes the contact patch location so you should buy a custom length push rod to match it.

For those of you too young to know the BBC shipped with 7/16 inch push rods back in 1965 and only dropped down to 3/8th inch push rods to save money on the peanut port light truck engine built after 1974. Unless you like bending push rods I suggest going back to that size in a chrome molly push rod with adjustable push rod guide plates to match the size of the push rod. The staggered offset angle valves of BBC heads puts a lot of side loading on the smaller diameter rods that will fail at higher RPM or with a stiffer high performance spring. The larger diameter rod is stiffer to resist flexing.

You won't need the half inch Manton push rods that I run with stock heads, and only street/strip spring pressures (however I recommend that you always buy the "premium" valve spring upgrade offered by your cam grinder to match your cam), as it will save you money in the long run.

I would also buy an on head valve spring pressure checker and check your pressure readings at every oil change. Might surprise you how soon you need to replace your valve springs as Big Blocks eat them for breakfast. I consider valve springs to be a consumable and add it to the cost of racing.

Big Dave
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