I've recently procured a 75 HighBoy. Its a two barrel FE motor which I had presumed a 360. I noticed that the two barrel intake is cleaner than the heads its bolted to so its obvious its been off more recent than the heads. Its got a newer 2 barrel carb so I had originally thought nobody is doing top end engine work and putting a 2 barrel on it if unless its a stock 360.

Well for giggles I pulled a plug out and marked an aluminum rod at the top and bottom. I got two different measurements. First time was real close to 4 inches. At TDC I could see that there is a considerable dish in the piston so that dish may have goofed my measurement a bit so I measured it again. I got closer to 3.75 inches the 2nd time. I'm doing this by myself in near darkness so there is a bit of fudge factor but its obvious that its more than a 3.5" stroke.

Woo Hoo! Not a 360! I think I figured out whats going on. Its probably a 390 that somebody had a nice 4barrel intake and carb on. When time came to sell the truck, the intake and carb were pulled and the 2 barrel gear installed in place of it. That explains the clean 2 barrel intake but surprising amount of stroke.

The pistons are quite obviously dished just from what I can see through the spark plug hole. Were 390s built with dished pistons or is it possibly something even bigger than a 390?

 

measure the stroke in daylight and you will have your answer. i dont know the answer to your question but there is someone on here that knows

 

Yes they can have dished pistons.

 

dished pistons means lower compression, I have 390 trw forged flat tops .040 in my 390, recheck in daylight to confirm stroke

 

So I pulled out all the plugs to make it easier to roll it over by hand. It didn't. Figured it would be good to blast them all in the cabinet and get em good and clean so I have a baseline when I get to tuning the thing. Re-measured the stroke and came up closest to 3.75" with the tiny amount of error on the long side. 390 without a doubt. I'm really happy about that but now I have to wonder about the pedigree. Figuring it was a stock 360 at least I thought I knew what I had.

 

Please tell me your not thinking of sandblasting your spark plugs. If your trying to find out if the engine is originally a 390 just look on the VIN tag located on your door. You'll have your VIN and other number/letter codes, it will tell you a lot about the way your truck was set up from the factory.

 

No 390s in 4x4, so if it's a 390, it's a rebuild or transplant.

As for the dish in the car 390 2v regular fuel motor, (9.5 CR) it is only about .147" deep:

Silv-O-Lite - Better Pistons Since 1922

391 pistons (God help you) have a .328 deep dish:

Silv-O-Lite - Better Pistons Since 1922

 

Yeah, I plan to bead blast the plugs.

From what I deciphered on the VIN tag it should be a 360. Now that I know its a 390 I have to wonder what year, whats been done to it. Has it had a timing set replaced, original cam or not, replacement oil pump, any head work done, ect.

The headers on the thing aren't truck headers. The #1 pipe was cut near the collector to allow it to be routed around the front of the engine mount stand. Its then reattached with an ordinary exhaust clamp. One of many exhaust leaks.