I am determined to build a stout street motor for my brother's 74 Duster, and he really wants a hydraulic cam to avoid periodic valve adjustments. I know valve adjustments are not a big deal, but that is what he wants.

I have heard that most people don't like hydraulic cams, mainly due to the non-adjustable rocker arms and the extra effort required to set them up properly to retain correct geometry after modification.

I plan on milling the heard to obtain a "better" dynamic compression ratio and on regrinding the cam to a "better" profile. Here is my question- will I retain proper valve train geometry and avoid having to purchase custom made pushrods if I reduce the thickness of the base circle of the cam lobes by the same amount of material as is removed from the head? in other words, say I remove .050 from the head (a totally arbitrary number picked for illustrative purposes only), then I would have to reduce the thickness of the radius of the cam lobe base circle by .050 as well, correct? The lobe of the cam will be custom ground to whatever profile I settle on, but, so long as the amount milled off the head and the amount removed from the radius of the cam lobe base circle are the same, then the pushrod length should also be able to remain the same, right?


Thanks for helping me puzzle this through.

Reed,

I would mill the block and head whatever amount is needed to achieve the compression ratio appropriate for your cam choice. Then buy custom length pushrods to achieve proper lifter preload. As long as the valve stem heights are close enough to one-another you should only need one length of pushrod. The critical valve train geometry (the relationship between the rocker arm and valve stems) does not change with block or head milling. You cannot take enough off the cam base circle to compensate for the milling required to achieve a decent compression ratio. I had to take .060" off a head to get 8.5:1 and that means .090" more valve lift.

_________________
Joshua

Curses! Foiled again. Okay, thanks. Since this is an extremely low-budget project, I will see if I can't convince my brother to go with a reground mechanical cam. I have extra mechanical pushrods and rocker arms/shafts so all it would require would be a reground cam and lifters.

Make sure your brother knows he only has to run the valve lash every 20-25,000 miles, unless it gets noisy or he abuses the crap out of it and t needs to be checked... It's not like he's going to need to do it every oil change at 3K...

-D.Idiot

I know, he is just not the most mechanically inclined. Realistically, I will be the one who would be adjusting the valves anyway...

The stock hydraulic cam has .378 lift according to the specs I have.If you add .090 to that it would be .468 lift.But if the cam is being reground some will come off of high spot of lobe as well.So lift will be less.The question is "What is the smallest base circle a cam can be ground to use the stock size hydraulic lifter?".I am pondering this same question.Reed do you know the deck height of the engine?Mine is .150".My head has 60cc before milling.I will have to go at least .030" over on bore.If I take .066 off the head the static compression ratio comes to 8.97 to 1.

Nope, I haven't gotten down to measuring the specific such as cylinder head volume and deck height.