Just a quick question. I am pulling apart a 400 from my 77 F250 SC to rebuild. Once I pulled the heads, I saw stamped in the top of the pistons .030. I am assuming this means they are .030 over pistons. Is there a way to find out what my compression ratio is? Will these be marked with a part number some where? Haven't pulled out the pistons yet, but had time to ask my questions. Or, is there a way to measure?

Thanks.

 

If the rebuild was just with "stock" parts, you won't see anything on the piston top except what overbore they are. If they're the stock "flat-top" with recessed valve reliefs, you probably have about 8:5:1 compression. If they're "domed" the CR will be higher.

 

I'm not sure that a 400 ever came with flat top pistons - if they did it would have been 1971. The VAST MAJORITY of 400 engines have dished pistons and you are lucky to get 8:1 to 8.5:1 compression.

 

Well, from you guys descriptions, I would say they are stock style. But with deck surfacing that should have happened when boring was done, and what i will do at machine shop, my compression will come up a little more. I am not looking for a drag motor, just a good running towing motor. I think a cam, 4 barrel and headers should help this out a bunch.

Will hopefully have the time to tear the motor down the rest of the way today. if I see any other markings i will post them with pictures.

 

-Calculating Engine Compression-

 

Yes, early on the 400 could have had flat tops from the factory. I've came across a few sets over the years.

 

Came across these on ebay. https://m.ebay.com/itm/FORD-400-400M-MODIFIED-FORGED-FLAT-TOP-HIGH-COMPRESSION-PISTONS-/253088573838?_trksid=p2385738.m4383.l4275.c10&_mwB anner=1

 

Hmmm, now that's a new speed part. By my calcs, that's a flat top with 3.5cc valve reliefs (going by the numbers they give on fleabay). Now I just wonder about there quality.