RV275 pistons - pics, bowl size, compression #'s

Pics of the RV275 pistons shown below...

Specifics:
Ordered grade "A" pistons
1310 grams each (bare)
4.011" diameter skirt
2.411" diam bowl
0.419" down to first ring
0.669" down to second ring
1.314" down to oil ring
48 cc bowl
0.710" deep bowl (deepest portion)
0.112" ring gaps

The bowl is 3cc bigger than my HO pistons I'm replacing. It seems to be the same size bowl as a SO piston? However, when I put the first piston/rod assembly in the engine, the piston protrudes 0.035" at TDC from the block (block was only decked 0.005") Not sure what a stock piston protrudes, but it couldn't have been this much or my stock compression ratio would have to extremely high...

Using the following numbers: (feel free to comment on these)
bore = 4.02
stroke = 4.72
compressed headgasket thickness = 0.060
combustion chamber (cc's) = 5.65 (calculated from recession of valves 1.445 diameter space 0.050" deep for each valve)
piston dome (cc's) = 48
piston deck height = -0.035

C/R is 17.68 :badidea:

If I mill the pistons 0.040", it also reduces the bowl by 3cc's, so using a deck height of 0.005 and bowl of 45, C/R is 16.3, with a .020 over gasket it is 15.4.

If I mill 0.060", reduce bowl by 4.5cc, C/R is 15.68, with a 0.020 over gasket is 14.82.

My concern is that to reach my reduced compression of around 15:1 I had to reduce the bowl to a smaller than stock size. Originally I went with the RV275 pistons thinking one advantage was alot bigger bowl (more room for fuel/air). I didn't think it would protrude so much farther than stock. Adding space above the piston outside the bowl isn't really the best place for it...

Should I even worry about it?

Over my head Jesse but it is looking like you are getting everything done. My truck won't be ready for next week but the following pull it should be.

Sounds like .050 off the top would be about perfect, IMHO.

Milling 0.050" off would put me at 15.98:1 with a stock gasket and 15.09:1 w/ a 0.020" over gasket which would work. Do you have to run a 0.020" thinner injector washer to get the injector back where it should be like on a 12V when you put a bigger gasket in?

Just ran across an old thread that said a stock gasket was 0.070" thick? I was thinking it was 0.060".

Anyone know the compressed thickness of a stock gasket or a marine 0.10 over or 0.020 over?

I know HO motors have I think a full point higher compression than an SO motor.

HO roughly 17.1-17.3:1 stock
SO roughly 16.3:1 stock

Difference that I know of is in the piston bowl. 45cc HO, 48cc SO

Also, I've been told the 0.070" gasket thickness was uncompressed so I think the 0.060" number I used above is still good.

Ok. I figured you already knew but thought I'd throw it out there.

A used +.020 gasket measures .090" after it comes back out.

Those look different than the 275 RV pistons I have used...... What is the Cummins part number of those?

Isn't that the part number on the top of the piston?

definately NOT the pistons I used...the ones I used had some sort of anodized black ceramic coating on the top. They were grade A ISB 275/300 pistons straight from Cummins

These were a grade A RV 275 piston from cummins. Part number was 3800323 for the piston kit.

Doug do you have a part number for the ones you have used?

OK, thanks for the replies. So it turns out these are RV275 pistons, for RV/Emergency vehicles, however they are not the same as the ones others have used. Will have a correct one tomorrow. Thanks Snedge!

I'll take some measurements and cc the dish...

GOT-Torque said:

OK, thanks for the replies. So it turns out these are RV275 pistons, for RV/Emergency vehicles, however they are not the same as the ones others have used. Will have a correct one tomorrow. Thanks Snedge!

I'll take some measurements and cc the dish...

Click to expand...

:Cheer: Sure. My pleasure. Keep me posted on your build progress. You know where to find me.

Will do, I'll get some pics up soon. I've already got the new gear housing polished...

Not sure why I even bother to paint everything and polish parts. Looks nice on the stand, but won't be clean again after the first dirt road... :hehe: