I'm having my 12valve bored .040" over and putting in Mahle Marine Pistons. How far over the piston size should I bore the cylinder? One guy told me piston size plus .005", but not sure if thats correct for a performance application??????

I need to let the machine shop know as soon as possible.

 

The machine shop should know. But either way, cast pistons run a tighter tolerance than a forged piston since forged pistons expand more. If your looking for all around performance, have them bore the block, then use a torque plate and hone the cylinders for .005" clearance if they are cast. If they are forged, you may want to go a little looser. Just remember, the looser you go know, the faster its going to loosen up and wear the rings

 

whats stock for a marine application? Whatever the stock or recommended gap is, thats fine.

 

if the machine shop doesn't know, you had better find a different one

 

depends on piston material, skirt design and overall bore. you might want to look into a different machine shop or asking a shop who has used the same pistons many times to get the best answer.

 

If the machine shop doesn't know you should contact the piston manufacturer. They can give you the proper info. It should have been in the literature with the pistons.

 

I think your right, there is a paper in the box with the pistons. I bet thats it. Good ole guys not reading directions........

Talked to Scheid also, best customer service in the world!, and they recommend .018 over for performance applications. So that what I will go with.

 

If it is .040, the block should be finished to .040 over standard. The piston manufacturers make the piston to fit that size, whatever clearance is required is built into the piston. You can call Mahle to be on the safe side but that is probably what they will tell you. Have bored quite a few of these blocks without a problem. I would rather have one on the loose side of the tolerance than on the tight side.

 

Ditto.

 

Picked up the block from the shop yesterday and began assembly. I ended up going with .006 over, so its slightly on the loose side of stock spec. Just couldnt bring myself to go as far as .018. So far its looking good and should run like a monster!

 

Give us a report back on how it works out.

 

I sure will. There will be a write up with pics along the way of the build. So far I only have the block painted, new piston cooling jets, crank and bearings in, and put the new pistons on the rods. Having never torn one of these motors down before, the 12v is built like Fort Knox! Seven large main bearings and connecting rods that weigh about 5lbs each. And the block itself has reinforcement ribs all over the place. No wonder a stock bottom end holds 1,000whp like its nothing.

Now with the marine pistons I will be able to go wilder with injectors and 155* spray pattern, without crazy haze due to spraying outside the bowl.

By the end of this build I should have between $15-18K total invested into the 91', but it will be one of the most powerful and fastest VE Pump cummins first gen, new paint, short bed conversion, frame off resto, basicaly new/rebuilt from the ground up. With some custom touches along the way like dropping it down level, smooth out the body, 18" wheels, bucket seats, ect. In all its going to be 10x better than when it came from the factory new, and last for many years to come. Better than spending $40k on a new truck plus $10k in mods to have it run 11's.

 

Thats great. I wish i had the money to go hog wild on my truck. Are you going to keep 145* injectors and be able to add eveb more timing with the 155* pistons or are you getting 155* injectors too?

 

Did you get them to qualify what the mean by performance? .018" seems really loose. I could see this for a competition motor, but not a hot street engine.

 

I will likely stick with the 145* spray pattern so I can run lots of timing and still be in the bowl. I'm not too worried about cost or price, just want to build it for my goal. I'm by no means rich, which is why this project is slowly progressing. I do all my wrenching myself, planning, specing, building. There was a shop I used for awhile, but decided to part ways and go unsponsored.

For the 0.018" overbore, thats what Scheid does for performance build like drag race, sled pull, ect. Not a daily driver or street/strip truck. Which is why i went with a loose but acceptable .006" overbore. Its enough to cut down on friction, but still be long lasting and streetable.

My ultimate goals are:

1. To have a very nice custom first gen that can be driven daily and achieve 25+mpg.

2. Be the first to run a VE Pumped 5 speed into the 12.5 or better range. Maybe even go for an 11.99.

3. Once I feel I have done all I can with the 5 speed, I'll swap in an auto and try for the first VE Pumped cummins to run 10's.

Yes I know most will say its not possible, but they are all wrong. haha. I will be making around 600whp, maybe close to 700 with injectables. In a very light first gen that has been lightened even more, these goals are very reasonable. With the 5 speed, 3.07 gears, and 28x14.50x16 slicks I will be able to do a boosted launch in 3rd gear. Then only 2 shifts, so boost loss will be minimul and respool instant.

I have the fuel cell, battery relocation, and several other things to aid in traction and weight loss. Dont want to give away all my secrets just yet, but its going to be one of a kind, and like nothing else on the road.