Hey guys,,,I need some inspiration from the hot rod machinery mod fellas . These pistons are for a 330 DeSoto,,,,but I’m using them in a 291 . The bore is the same,,,but these are .031 thousandths taller by the compression distance compared to the original pistons . I was gonna gain a little C.R. help with these,,,,,since these old engines are fairly anemic with compression. Since my block work had already been done before,,,,I had assumed the original pistons were slightly in the hole,,,,,good reasoning I thought ? I had .020 decked from the block,,,,I figured only twenty wouldn’t get too tight . As it turned out,,,wrong ! These pistons are above deck about .055,,, and my gaskets are about.045 not compressed. So,,,,,the problem arises . Since these are Hemi pistons,,,,,they will only contact around the perimeter. The factory chamfer is slight,,,,I’m wanting to just lower the chamfer edge about.050 and come in on a 45 or 30 degree angle and that should clear the head alright. Also,,,,,,,,since I haven’t worked the heads yet and had them surfaced,,,,that will close my chamber perimeter slightly. I don’t know if anyone makes custom gaskets,,,,,so I figured the best bet is to relieve the piston crown radius ? Any opinions on my dilemma,,,,,,I’m wanting to contact a machinist,,,,just wanting to find an old hot rodder that knows about this stuff . Any ideas about an alternative suggestion?,,,,,it’s either relieve the tops or gaskets I figure . Tommy
We'd have to see the pistons to promise it's OK, but from here, looking at the photo, there should be no problem skimming .050" off the edge. Where we do need to clarify is the bevel. Consider taking the .050" parallel to the top, with the same slight bevel on the perimeter and the blend to the vertical and again from the vertical to the horizontal. Similar to this. There should be at least .035" piston-to-head clearance with a compressed gasket. jack vines
I'd look at the angle of the chamber where it meets the mating surface and make you chamfer match it and leave some clearance.
Tommy, the oem Chrysler pistons have a huge bevel, around 090", so the only issue is whether or not 'that' particular piston has adequate material...looks like it does. Be sure to check valve clearance.
Those pistons look like they’ve already got a valve relief so there should be plenty of meat. Skimming pistons is quite a normal thing. The manufacturer will be able to tell you how much is safe.
Yeah,,,,there’s plenty of meat in there to give clearance . And my valve clearance should be plenty ,,,,,,the valve reliefs in these are deeper than the stock pistons . My cam is only .440-.450 lift,,,it does have a fair amount more duration,,,,222 at .050,,,,but I’m still gonna check the clearance when putting on the heads . Tommy
Cut them and don't look back. There is plenty left above the top ring groove. Don't cut anymore than necessary because you are trying to build compression. PS. I case you are unaware, .050" clearance is what I would recommend on a steel rod engine if you're going to rev it. .035" if you are going to never go above 4 grand and .090" for any aluminum rod engine. If you can, try taking 2 head gaskets on the block and clay everything and see where it's close. That can give you a starting point to know what's close and what's not.
@Desoto291Hemi You are only tapering the crown ,not thinning it! I would do that in a heartbeat The only issue is the rings being further up the bore at TDC and if there is the slightest ridge it will break the rings. If the engine is being rebored with 330 pistons then it won't be a problem. I've done a similar piston mod using 289 Rods in a 302 Ford [289 rods are 0.065" longer] and the whole crown was thinned across the top
Yes,,,,,I’m only cutting the chamfer or bevel on top,,,,not the whole crown . This only needs added clearance to clear the head chamber,,,,,nothing more . Yes,,,,,,these are .020 over , and the block has already been finished,,,,no risk of any damage . As Gary ,(73RR), stated earlier,,,,Mopar usually had quite a large bevel from the factory. These being aftermarket I’m sure they weren’t concerned about my problems,,,,lol . If these were Mopar originals,,,,I would have room to spare . Tommy
Thicker gaskets will only defeat what you are trying to obtain in CR. I like the idea of cutting down those shoulders, sounds like cheap HP.
Did you consider trimming the edge of the combustion chamber. Wouldn't take much and you could use the gasket to scribe the line. Just a thought. .bjb
Speaking from my experience as a diesel combustion development engineer from when dinosaurs roamed the earth, chamfering the edge like that will get you what you want for clearance, but it’s going to hurt combustion efficiency. We used to do that (plus more) with what we called a cut back top land. It was strictly to allow gas pressure to get to the top and back of the rings for long term oil control and reduction in carbon buildup, but there were significant performance compromises since you end up with poor air utilization in what essentially becomes dead space in the combustion chamber. I don’t have any better ideas, though.
No !! [well yes the pistons were custom. ............. But customized by us using a lathe] Approx 15 years ago a friend and me built a "Grenade" race engine as a throwaway for club racing from a U-Pull engine that we hooked for NZD $400 [USD $240] This was fitted into a NZV8 Falcon Touring car. Because cams an compression are perfect bed partners we needed to up the compression as much as possible so we could use a longer duration cam. So we installed a used set of 289 rods that already had ARP bolts [from a previous project] On the stock block the piston were 0.016" down the hole , so we cut 0.035" off the top of the pistons netting us 0.014" above the deck at TDC. [this removed most of the small centre dish in the pistons] The bore was honed just in case there was ridge near the top and we used Felpro 8548 head gaskets which were 0.047" compressed thickness [normal are 0.039" compressed thickness] This netted a quench of 0.033" which was liveable but the extra thickness of the gaskets and also allowed us to cut 0.020" off the heads without manifold alignment and pushrod issues. The only head mods were back cut valves, a competition valve seat job , 0.020" milling, and valve springs. We used the intake / carb / road racing oilpan /headers from our other engine. This engine would stand it's ground against the $30k controlled engines [we had a much bigger cam] But a lot of this performance could be attributed to “lack of mechanical sympathy” [Driver enthusiasm] After the season we sold the engine to use in a street car [minus car specific peripherals] he test drove it in the race car. Sometimes you can get crazy amazing results without over thinking/spending everything [throw away those car magazines] I am not recommending you do what we did
That’s the hot rodder spirit ! A lot of people think unless you buy it as a custom race part that it can’t possibly work . I don’t understand why more don’t try to have some imagination and modify what they have,,,,,it might just run better if you try ! Tommy
Don't know anything about Hemi valve geometry myself, but wondering how the crown of the piston being that much higher and the deck lower than stock might affect piston to valve clearance.....And have you checked the actual compression ratio you will end up with? On a 500 Caddy using the same piston, switching from a 120cc head to the 76cc head takes the compression ratio from 9.2 to 13.1. Thats a larger engine displacement, and changing 44cc changes things almost 4 points. If you use a taller piston your displacement will be less than 291 but your compression will be higher as well as the piston top being higher at TDC. Need to do some actual measuring and calculating before proceeding. One other question. You said the comp height of the pistons was .031 taller. Did the 330 have shorter rods than the 291?
The differences in piston comp.distance is by the factory spec . The 330 pistons were just a little taller from the factory ,,,,like Gary said,,,,,the rods were different,,but that made up for the taller deck . The 330 also had slightly higher compression when compared to the 291 . Maybe since the cranks were different as well,,,,,they moved the piston up slightly to clear the counterweights,,,,? Since the compression went up from the difference in swept volume,,,,maybe they added a few more thousandths to get the compression where they were happy too ? Tommy
I always thought displacement was from the bore diameter and the stroke . After I bevel the piston tops,,,,,,they will look like they have little baby domes on them . Now,,,,,since the pistons in my 426 Hemi have a .500 dome,,,,and are well above the deck,,,,,does that mean it doesn’t produce 426 cu.in. ? Just kidding a little ! Tommy
displacement is just bore area times stroke, the piston shape has nothing to do with it...since it's the same shape all the time
I would say only to be careful how you hold the piston for metal removal. Chucking it in a lathe may distort the skirts, so be judicious on how you grab it... or ...grab it with a pin fixture and mill accordingly. We used 352 V8 FE pistons in a 300 six after removing .050" off the tops to get to zero deck. Ran great
Agree. We have the appropriate jaws to grab the piston in the ring groove. Chucking by the skirt is bad science. jack vines
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