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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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IMHO you need to open up the valve radius on the piston or use heads with smaller valves, swapping cams wont help if you end up kissing a piston with a valve, besides you don't want to just have an engine running BUT could've been a lot better, that would bug the heck out of me and actually why I had a complete different engine built
 

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I did think of this, then I thought would it be better to just down size the cam?
I think you would have to go way smaller to get the clearance. Let me see if I can get the guys at my warehouse to take a picture of the underside of one of the pistons. Might be later on today. I have a feeling that is a solid dome piston and probably has more than enough metal there to do some grinding.
 

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Kudos for doing a great job measuring.

Look how tight/non-existent the clearance is now - just imagine when everything is moving/wiggling around at 6000!

Provided there is enough meat in the piston, I'd use the piston relief cutting tool as mentioned. I've used it several times and it works excellent. Doing it by hand I think will result in inconsistent cuts and very time consuming.
 

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I heard of people fly cutting 581 pistons for larger valves. But never seen or done it my self. I would try contact sealed power and see if they have a tech with them that knows if it can be Done.
 
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Discussion Starter · #25 ·
Hmmm lots of food for thought here

Problem with the fly cutting tools is I'm in Australia and getting those tools are expensive and will take weeks to get here

From what I can tell I would only need to take off probably 20 to 30 thou where it's currently at to reach a safe 50 thou, and give the valve is right at the top of the pocket when it touches wouldn't be too hard do at all, again will be time consuming having to mask every cylinder up and do each piston individually but would solve my problems.

Can anyone see any dramas in taking off 30 thou off the below area with a small dremel and sanding drum provided its safe to do on the pistons ?

Damn about the cam, I'm pretty keen to use these heads as the flow numbers are good and will do most of the work compared to the cam doing the work so stepping down to a 224/230 wouldn't lose much
But if I'm not gonna gain any clearance then I won't bother

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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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I've used a Dremmel tool on Harley forged dome pistons without any problems and over quite a few thousand miles on the last one before spinning a bearing race and doing a complete rebuild including new pistons, just do your best to remove the same amount from each piston and I'd go all the way around the Eyebrow not just the Red area you posted the pic of, I'm by No means an engine builder just know what has worked for me
 

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I've used a Dremmel tool on Harley forged dome pistons without any problems and over quite a few thousand miles on the last one before spinning a bearing race and doing a complete rebuild including new pistons, just do your best to remove the same amount from each piston and I'd go all the way around the Eyebrow not just the Red area you posted the pic of, I'm by No means an engine builder just know what has worked for me
Rick, I'm just curious here: When you spun a bearing on that Harley engine awhile after you cut the pistons by hand, did you see score marks in that bearing that spun, and/or in the other bearing as well? I'm just wondering if you may have had aluminum fragments running through the oil which may have scored the bearings. I know that aluminum is considerably softer than iron, but I also know that main and rod bearing shells are pretty soft too. Of course my question is based off the assumption that you modified the pistons with them still in the engine.
 

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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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Rick, I'm just curious here: When you spun a bearing on that Harley engine awhile after you cut the pistons by hand, did you see score marks in that bearing that spun, and/or in the other bearing as well? I'm just wondering if you may have had aluminum fragments running through the oil which may have scored the bearings. I know that aluminum is considerably softer than iron, but I also know that main and rod bearing shells are pretty soft too. Of course my question is based off the assumption that you modified the pistons with them still in the engine.
The pistons I cut on the engine that I spun the bearing race (alum case steel race) the pistons were hanging on the rods but the cylinders were off and I had the engine covered with stuffed rags in the holes in the cases where the rods went to flywheels and plastic wrapped around the cases, I' m pretty sure nothing got in there, when I spun the bearing in Delcron cases I was racing 2 friends heading South on Hwy 41 around 2-3 am one of my friends was leaving for a cpl yrs so we were having some Extra fun that night, cost me a new set of S&S cases (or pin new races with 3 allen set screws drilled and tapped...Nope) so figured might as well get rid of the .070 OS pistons and cylinders and change it back to 98 ci S&S parts from 102 ci Harley over bored cylds/over worked old Chevy parts in the old heads
 

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Discussion Starter · #30 ·
Alright guys bit of an update


Did some thinking and decided I would look into the cam more.
I compared the cam specs and the smaller cam would open the intake valve 3.5 degrees later, which should create more clearance as the valve would be closer to the head as opposed to the piston throughout the stroke. (That's the theory in my head anyway)

So I called a local parts shop and picked up the new cam. The staff who I spoke to is a retired engine builder and agreed and estimated that I should pickup about 20 thou clearance.
He also said that using checking springs shows reduced clearance as actual vakve springs would provide resistance on the valve. (Can anyone confirm this???)

Anyway I got the cam, installed the new cam and degreed at 106. (Correct for cam)
I then clayed the cam and could tell that I'd picked up some clearance but still wasn't enough to keep me happy.
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To me it looked to be about 35 to 40 thou on the closest edge

Anyway I decided to retard the cam 2 degrees, I set it at 108 ICL which I believe will shift the power band up about 100 rpm so nothing too major at 2 degrees.

Again went through the clay process.

I checked in two locations
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Now the clearance is definitely at the 50 thou mark and I'm pretty comfortable with that and the exhaust valve still has loads of clearance
 

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1968 Malibu sport coupe, 489 ci. 590 hp 600 tq, RV T-400 Freakshow 3200 stall, 3.73 12 bolt posi
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All you need is a rev 5-5500, kiss a valve and push it up and the keepers come off then Drop a valve into the cylinder, reason I cut into some pistons in the past, Harleys are a lot easier to do but still I wouldn't want to take the chance, these pics were a HUGE PITA to do but took my worries away, and this was only a HFT 226/226 dur. @ .050, .533/.533 lift 108 lsa cam
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Take the pistons out and cut them. All it might take is one over rev and valve float and you are done. Like said above. Piston hits the valve, pushes it up,l keepers fall off, valve falls, piston comes back up jamming the valve. Hole in the head, hole in the block, rods twisted and or broke, crank gouged. Cut the pistons or change them. Put the other cam back in.
 

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Discussion Starter · #34 ·
Take the pistons out and cut them. All it might take is one over rev and valve float and you are done. Like said above. Piston hits the valve, pushes it up,l keepers fall off, valve falls, piston comes back up jamming the valve. Hole in the head, hole in the block, rods twisted and or broke, crank gouged. Cut the pistons or change them. Put the other cam back in.
Hi mate I've got the desired amount of clearance now, my only issue was radial clearance which I've now got the correct amount of clearance with checking springs so with the correct springs will have even more clearance.

Cam was borderline too large for my applicstion anyway and was most likely going to make me change to rear end ratio, so slightly smaller is probably a blessing in disguise anyway.
 
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