I'd like to have a discussion with some others on their view with this topic. I do understand the standard is to go with 0.001" of oil clearance per inch of journal, but i've seen many successful engine builders mix things up and loosen the clearances or go tighter on the rods than mains vice versa. For me I have put together quite a few 4g63/4g64 engines and always used 0.0025"-0.0028" on the mains and 0.0020"-0.0023" on the big ends and used a thicker oil, based off what i've learned from others or on here.

Recently I had a discussion with a friend who owns a very reputable 4g63/evo shop in the states about clearances, and on all his 600-1200hp engines he always runs 0.0020" on mains and 0.0025" on the big ends. Now i can understand why he would go with looser rods on a high horsepower engine, the rods take MUCH more load. They take every downward beating as opposed to the crank that just spins and absorbs some of the motion. With tighter mains the rods will see less pressure loss and allow more clearance. I'm debating switching up the clearances I run in my 4g63 engines, and going slightly looser on the rods than the mains. I wanted to know what others think based off their their own knowledge or experiences!

 

Also remember that looser clearances will allow more oil to flow across the bearing face - keeping them cooler.
I think the 0.001" rule is pretty good, but you can go either side of that for whatever preference you might have.
Oil weight and operating temperature and probably the main ones.

 

not really on topic with your question, but my understanding of tolerances is that they have more to do with the intended application of the motor and how much thermal expansion needs to be accounted for. For example engines made for drag racing will see very different amounts of thermal expansion compared to a track car.

 

not just that but crank bending under load...

 

Asking engine builders about bearing clearance is like asking a chef to give up the recipe for one of his secret sauces...

 

It all depends on just about everything. Lol

Over 800hp is probably a drag application. saying he uses these clearances in everything has me worried. Definitely you would have different clearances between a 600 hp and 1200 hp engine, no doubt. The only time I could see something that tight being used for a 1200 hp application would be aluminum rods and a billet crank and dry sump system with exceptional oil cooling system. Even then it would be pushing it imo.

How rigid the crank is has a lot too do with the equation. A billet piece with flex less than a forged unit. Also is the block filled? That makes a difference too.

If your running different parts than he runs it makes a world of difference. Oil weights are paramount also.
going loose too a point won’t hurt you, going tight on the other hand will result in failures.

im not saying those tolerances won’t work, just saying make sure you have all the parts he recommends.

Really the only thing your going to gain from tighter clearance is you’ll be able too run a lighter oil. This will give some benifits as far as windage and friction, in the neighborhood of 15 horsepower gain for a 1000 hp engine.

imo you are better off playing it safe with more clearance in case of deto or preignition. If your looking for some free horsepower look into horizontal gas porting you’re pistons. I’m totally amazed at how many big power builds I see that don’t use gas porting. Trust me it’s worth waaaay more than most think, otherwise they would be doing it!

cheers, 2win

 

The application is:

2.0 long rod with turbo tuff i beams, wiseco pistons, and a new oem crank.
Both mains and rods are 0.0025" and ill be using 15w50 motul or 20w60 redline.
It has an FP Zero that ill be running at 35-38 psi or so and 8600rpm, should make around 650whp all out.

Did your build with 2.5 thou on mains and rods ever give you any problems?