I've got what I'm fairly convinced is a 394 hi comp (red block) in my '54 olds Holiday 88 coupe, and I've got a question about the Pistons... I was reading in Egge that the 10.25:1 pistons have a (.040) dish to them, while the low comp (green engine) have a larger dish (I believe it was approx .2xx). I measured the dish on mine, and it comes out to about .040. Typically, I wouldn't associate hi-comp with a dish of any amount in a piston-any olds gurus able to confirm? Also, I noticed that the pistons have a very faint 00 stamped in them on the compression surface (top) of the piston. Pardon the FNG-this is my first Olds. I've read a lot of the Olds posts on here, but none I've found so far have mentioned much about pistons.
I *believe* the .040 dish pistons are for the 10.56 to high comp 394. Speedway Motors list pistons for the 394 Olds in their catalog. (www.speedwaymotors.com/371-394-Olds-Pistons-and-Rings,5377.html - item 9102084) I'm not sure if these are the Egge pistons or some other brand and the catalog listing doesn't specify what dish, (if any) they have. The catalog shows a pic of a flat-top piston, but that's maybe a generic pic and not actually specific to the actual 394 Olds piston they're selling. Maybe give them a call for a difinitive answer as to dish size and actual compression ratio. About your statement about not associating a dished piston with high-compression.The compression ratio of an engine is the ratio between displacement (swept volume) of the cylinder, relative to the space (static volume) above the piston at TDC. -ie - the combined volume of the combustion chamber, the head gasket volume and deck height - ie - the space remaining in the cylinder above the piston at TDC. A domed, or dished piston has the same effect as adding or subtracting volume from the combustion chamber and that's really what a dome or a dish does. In any particular engine with a given cylinder displacement, you can have anywhere from an extremely low- compression ratio to an extremely high ratio - or anything in between - with *either* a domed, dished or a flat-top piston, *entirely* depending on the total combined static volume built into the engine, compared to its swept volume. In your case, the Olds engineers designed the engine with a particular combination of cylinder head volume, gasket thickness and deck height that *required* a .040 dish in the top of the piston to limit the compression and keep it down to a a still high, (both then and now!), 10.25 to 1 ratio. Mart3406 ============================
Thanks for the info. I have seen that speedway ad, and am assuming that it's probably just a stock photo. The Egge catalog has a different image for the olds listing. I'm hoping to not have to order pistons, but the engine is pretty locked up (been soaking it in diesel for a week now...it's starting to show some promise...). Thanks again-still curious about the 00 on the tops of the pistons.
I was actually hoping that is the reason for it-since it would mean that I've got plenty of cylinder wall to work with should I need to have it bored.
Oldsmobile blocks are extra thick,overbores of .125 were not uncommon. I've got a 394 that's .060 over and I ain't worried. I've got the low compression pistons,but I might still shave a little off the deck and/or the heads to push it up around 9.00 to 1 which is where I like it for a street engine.
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