Need a 351w pro

[stewd8]

I'm rebuilding a 97 351w raptor motor for my 16' jet drive side winder....i'm looking for the correct piston ring gap. that will run at 5000rpm..this is the second motor, as the rings expanded and sized the motor, killing the pistons and block,,,,looking all over the net, i'm getting all sorts of conflicting info,,but i did get this,,,,

At operating temperature, the top ring end gap should be as small as possible. Precise machining of the cylinder bores is critical, and is the reason why rings should be fitted to the cylinder in which they are to be installed. A diameter variance from one cylinder to the next changes the end gap of the rings in that cylinder by a factor of pi (3.1416). For example, a cylinder .001? larger in diameter will increase the ring end gap by .001 x 3.1416 = .003?,
rounding off, which could increase cylinder leakage in that cylinder and decrease performance.

that sounds good

stock piston bore,,stock pistons,,lake water cooling
i read that the top ring gap is differant from the second, and the second is differant from the oil rings...what are the stock #s and how much over do i go for the added heat on them
any thoughts please, and thanks

 

[haulnazz15]

Re: Need a 351w pro

Take the engine to a performance machine shop and have them hone the cylinders. They can provide you with the correct ring size.

 

[alldodge]

Re: Need a 351w pro

Since your going to achieve the max performance and right against the edge of breakage you may want to ask this on offshoreonly, but here is what I have found.
http://www.manleyperformance.com/dl/tech/piston-rings.pdf
http://www.jepistons.com/PDFs/TechCorner/SCPDrawings/piston_instrc4032.pdf
Ring gaps - PistonHeads

 

[stewd8]

Re: Need a 351w pro

i took the motor to a perfornance rebuilder, had it washed, tanked, and machined,,that was 4 months ago, its not in there records, i did get a tag with the motor saying the bearing sizes to use, but nothing about the ring gap,,,all the info i can find is for cars, not boats, should i go with the circle track figures, as they are running full out,,,4-6000rpm round and round......0,0055 and 0.0057

 

[alldodge]

Re: Need a 351w pro

No expert here, but I would say running that close would put you back in the area of breaking again. Folks running that close are ones which want every spec they can get out of an engine and also have the bucks to rebuild it often if something goes wrong. The load on a boat engine is greater then then the street and doesn't ease up. I would suggest going to the 0.0045 range and let the extra 1/2 HP you might gain in your wallet.

 

[stewd8]

Re: Need a 351w pro

thanks for your thoughts, i'm not worried about 1 or 2 hp,,,,.005 is bigger gap then .004..... .005 is safer????
silly question???? is that .005 x bore = .005 x 4.00 = .02in ????

 

[stewd8]

Re: Need a 351w pro

hastings says 0.008 x bore, for street towing and circle track and marine

 

[Walt T]

Re: Need a 351w pro

Cast or Moly type of rings, forged or cast pistons, compression ratio and build type all figure into it. High compression engines develop much greater heat in the combustion area so the ring gaps MUST be larger to allow them to expand. The gaps shown are MINIMUM. Cast rings will have a much greater gap than moly, but moly rings require more work to get them to seat correctly. I've done gaps 25 and higher, my own stroker motor was 25 minimum but compression is 9.7. I intend to install closed chamber heads to get compression up to 10.5 so I need big ring gaps. I would personally not use .008 even in a stock engine but I'm not an engineer.

Your machine shop did you a disservice not advising you on ring gaps unless you didn't buy the pistons and rings from them. They have a vested interest in that engine too. Develop a relationship with a builder and ask about this information. There are so many variables in builds that an experienced builder can help you with the right gap for your assembly. The formulas are nice but they can be wrong for your specific build. I know how Jetboats love 5000 rpm all day long so you need to build for that. These websites are pretty helpful and you can see the variance is pretty big and can lead to more confusion.

 

[stewd8]

Re: Need a 351w pro

that makes good sense, its stock stock and stock, high rise cam edelbrock manifold and 600 carb,,probly woulnt even get to 5000rpm, but 4800 would be nice,,,,,,,stock car #s are .016 performance .018 blown racing engines are .024 - .028 ......so .023 seems huge
i think .020 sound good,,,,i'll research it more...thanks for your thoughts

 

[Walt T]

Re: Need a 351w pro

I agree it seems huge. I personally would not go smaller than .020 on a blown racing engine, but here again, It's all dependent on how the engine is built. It all has to work together. Blown gas flat drag boats with blown 454s were .030 and ran 7500 rpm. Of course we tore it down all the time also. Not really a good comparison but just saying.