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-   -   L98 heads and dome pistons (http://www.hotrodders.com/forum/l98-heads-dome-pistons-225105.html)

ardmsrt 10-17-2012 01:23 PM

L98 heads and dome pistons
 
Hello Rod Buddies. Well, here is my first post. I want to see if this is possible on hot motor build sbc 4" bore. Plan on running 480 lift solid cam and 110 petro and 3600-4200 stahl. Would like to use L98 aluminum heads (10088113) on my sbc with Wiseco dome pistons. .035 mls gasket and 3" stroke. Setting the head on I see the dome pushes against the structual rib that's by the spark plug hole inside. Can I grind the rib down for clearance and not worry about structural damage to the head. I have cast heads also, 375 F.I. and also 882's that will probably fit fine. I think I know I'll have high compression motor but don't know exactly how high. I've never used an aluminum head and would like to try this set-up if it's possible. Any help would be greatly appreciated.
Thanks...T.J. ;)

cdminter59 10-17-2012 02:57 PM

L98 heads and dome pistons
 
Use a marker to shade the area on the piston that's hitting. Using the gasket that you will assemble the motor with, set the head in place with 3 to 4 bolts. Roll the motor over until piston contacts head. Pull the head off to see where it is hitting. You will have to feather back (grind) the dome in this area. If you do not feel comfortable doing this take it to a machine shop. If there is too much dome preventing you from bolting the head you probably will have to buy new pistons. The L98 heads were probably designed for use with flattop and dished pistons. They are 58cc chambered heads. You need to know piston volume (+12.8cc or +8.00cc) which with these heads will give you a compression ratio 12 - 14+:1. The rotating assembly will have to be balanced after grinding pistons.

BogiesAnnex1 10-17-2012 06:11 PM

Quote:

Originally Posted by ardmsrt (Post 1600072)
Hello Rod Buddies. Well, here is my first post. I want to see if this is possible on hot motor build sbc 4" bore. Plan on running 480 lift solid cam and 110 petro and 3600-4200 stahl. Would like to use L98 aluminum heads (10088113) on my sbc with Wiseco dome pistons. .035 mls gasket and 3" stroke. Setting the head on I see the dome pushes against the structual rib that's by the spark plug hole inside. Can I grind the rib down for clearance and not worry about structural damage to the head. I have cast heads also, 375 F.I. and also 882's that will probably fit fine. I think I know I'll have high compression motor but don't know exactly how high. I've never used an aluminum head and would like to try this set-up if it's possible. Any help would be greatly appreciated.
Thanks...T.J. ;)

These are a pretty small chamber head like 58 cc's the factory used a.053 inch thick gasket with them. You need to find out just how high this compression is. I gather this a 302 4 inch bore 3 inch stroke. Actaully getting to the compression ratio starts with the cam and running calculations that drive out a Dynamic Compression Ratio in the range of 8 to 9:1 which drives out the higher Static Compression Ratio (SCR) once you have the dimensions that make this happen you can select the parts that make the needed ratio.

When you say the dome contacts the structural rib by the spark plug, the only thing I could see being called a rib by the spark plug on this L98 head is the spark plug boss itself. This is an essential part of the combustion chamber design that is intended to catch the wet fuel flow entering on the long side of the port and remix it with the air entering the cylinder so there is a homogenoius and burnable mixture of fuel and air.

I'd like to know more about the cam from the specs on the timing card. .480 inch lift doesn't strike me as all that much so I'm wondering about the super high stall converter and whether the cam really warrants that much stall. Although I have to admitt to thinking that on a 302 that cam will react as if it's bigger than it would on a 350 or 383, so maybe!

Bogie

ardmsrt 10-17-2012 09:05 PM

Horse-
power
LifterIntake LiftExhaust LiftIntake DurationExhaust DurationRocker RatioPart NumberCasting


290 M .485 .485 346 346 1.50 3849346 3849347
CAM SPECS EXACT AS FACTORY, NOT 480 LIFT SORRY

THE CC ON 67-69 Z28 HEADS WERE 63-64, SO HOW WERE THEY INSTALLED FOR CLEARANCE ISSUES WITH FACTORY DOME PISTONS. THEY REFERRING TO THE FACTORY ALUMINUM HEADS FOR THAT TIME TOO? ANYONE KNOW THE EXACT STORY ON 69 302 MOTOR? I'M REALLY INTO THIS...THANKS T.J.

BogiesAnnex1 10-18-2012 03:28 PM

Quote:

Originally Posted by hcompton (Post 1600318)
I dont think he can pick a cam since the heads dont fit the block. He is going to end up with diff heads or pistons before its right. At the very least he will need the compression ratio. If it super high those small cams will be more trouble than the lack of hp is worth. If you get my meaning.

I guess I've been at this too long, I start with a general idea of how much power we want and the RPM range we need that happen in. I have at that time a general idea of pistons and heads which probably comes from doing this more than once but never-the-less the cam timing is in large measure going to drive the compression ratio which will drive the combustion chamber and piston crown sizes and shapes. So once I'm settled on that choice and know where the intake valve will close in crankshaft degrees of piston position in the bore, then I compute the DCR and SCR that the cam needs to be efficient and effective. Then the heads and pistons start to get narrowed down as to port, valve and combustion chamber size, and piston crown shape, deck clearance, and gasket thickness that are needed to dial in the compression with minimal squish/quench clearance along with adequate spark plug, valve and chamber contour clearances. The head selection will drive on the piston crown shape and volumes as to flat top, dome, or dish for best fits and clearances that minimize having to mill the piston to shape as most catalog pistons don't have enough head material, nor should they, to go change the crown shape. If you're getting into that then you need to order a custom semi-finished slugs and machine the crown to suit myself. This obviously is on high buck motors; certainly that kind of effort isn't done on street or claimer engines. I realize, especially with drag engines that very wild cams end up driving you into domes alternate fuels can drive you into large open chambers which also take you to domes, but I bust my hump to avoid domes as best I can because they donít play well with flame front travel and create a lot of surface to volume which makes the whole chamber run like itís a giant squish/quench deck, this drags you to other compromises in fuel mixture ratios and ignition advance that drive sufficient perhaps but certainly less than optimal tuning solutions.

I'm always amazed how guys just order a bunch of parts without thinking this through. For the hobbyist and tinkerer their best strategy is to copy one of the many magazine and soft cover book builds or cruise the web for similar information, this can save the average guy a lot of money and time while letting him/her build a very nice engine they can be quite proud of.

Bogie

ardmsrt 10-18-2012 04:21 PM

thinking it through...
 
Well the best reply I can stumble thru is this:
1969 Z28 302 SPECS
Z28 crank 1178
Z28 dome pistons
Z28 cam grind
Near Z28 Head specs 64cc, intake and carb.

How do you replicate a 69 Z28 302?

Best I recall, GM had a pretty solid engineering prowess
when they threw all that crap together...lol. :welcome:

cdminter59 10-18-2012 05:53 PM

L98 heads and dome pistons
 
Check out your pistons compared to these Wiseco PT042H3 pistons. They are the correct pistons for 67-69 Chevy DZ302. The compression height is probably different. Wiseco PT042H3 - Wiseco Pro Tru Forged Piston Kits - Overview - SummitRacing.com. Expensive too! The 30-30 solid lifter cam(PN 3849346) was the cam used in the DZ302.

cobalt327 10-20-2012 03:01 PM

The valve train is bound to come up, so here's a page giving info on the valve train.

On a short stroke engine like this, the valve train stability and strength is VERY important. It's really the key top making an engine like this capable of surviving the rpm required to make any real power. The bottom end is also important, but is much easier to spec out- forged crank, good rods, light pistons and careful attention to detail along w/careful assembly will go a long way towards making the bottom end survive just fine.


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