This is my first engine build so all advice () is welcome. I am using aftermarket heads based off of the 906 casting. One of the members here gave me some very useful info, mainly that i would need to enlarge the spring seats to accomidate a cam large enough to produce the the horsepower I am looking for. It will cost about 100 dollars to buy the cutters and guides to perform the alterations. However i am a novice so is this something that I should leave to a machinist or is it safe for me to attempt? Also I have read about using a half inch drill bit on the push rod holes. I dont understand what the purpose is but at this point there is a lot i don't understand.
Thanks.

 

do you mean the spring seats?

it's popular to cut the valve guides to put normal size springs in. but i'd personally leave it up to a shop IMO.

 

yea that too

 

i recall reading that the drill bit mod is to clearance the pushrods for 1.6 rocker arms.

while it's apart at the shop it wouldn't be a bad idea to ask for screw in studs.

 

thank you sir

 

The cutters for the spring pockets aren't hard to use; but if you're using brand-new heads, as I seem to recall, then you could order them from the mfr with that work already done, basically for free. Every head has to have those cuts made; it's just a question of which cutter they chuck up in the mill.

Another alternative, if you already have the heads and now it's not free to set them up right any more, is the "beehive" springs, along with their matching hardware. They give more clearance, better control of the valve motion, and less weight; but they're a good bit more $$$ than others. OTOH they may cost less than the tools.

THe push rod hole thing is a separate matter.... THINK about what "rocker ratio" is for just a minute. It's the distance from the rocker stud to the valve stem, divided by the distance from the rocker stud to the push rod seat in the rocker, right? Well, how would you go about changing that ratio? You can't change the first distance, can you? that is, bolting on a rocker doesn't move the stud or the valve, right? So what choice do you have? Right: you change the distance from the push rod seat to the rocker pivot at the stud. To increase the ratio, you make that distance smaller, while leaving the other as is (since you can't change it anyway). That sometimes moves the push rod far enough inward that it binds against the hole in the head. That's one reason for drilling them out.

THe other is, there must always be one feature, and ONE ONLY, locating the rocker to the valve stem. There are 3 possible systems: the hole in the head fits tight to the push rod; guide plates, that fit tight to the push rod; and "self-aligning" rockers, whose tips have little ridges to hold them aligned to the valve stems themselves. One AND ONLY ONE must be in effect at all times. THerefore, if you have the narrow slots in the heads, you cannot use guide plates or SA rockers; if you have guide plates you must drill the holes and not use SA rockers; and if you use SA rockers you must drill the heads and not use guide plates.

The vast majority of aftermarket heads are already drilled and set up for guide plates. You probably don't have to worry about that. You can ask the head mfr to be sure.