Is it worth getting the block over bored? What if any advantages are there? I know sometimes a over bore is a must as the ridge at the top of the block is bad. But if the walls and everything is ok would there be any performance advantage?

 

anyone?

 

I believe there is no performance advantage from boring it. you will achieve slightly lower compression but slightly higher engine size, approx. 0.025L when doing 20 over bore.

 

If you overbore a lot it will help airflow as it unshrouds the valves and slightly increaes displacement. I had done a 2mm overbore and the engine was great but hg sealing was an issue on the 6 bolt 4g63. A good 6 bolt 87mm hg is available now so it should a non issue plus the evo head is more rigid helping the hg seal better as well. There is a factory metal hg for evo style 4G64s and they are 87mm so it is a non issue for evo uses. It does leave the block somewhat unrebuildable though unless you sleeve it. With the cost of an evo block though sleeving is cost effective.

 

Over boring does not lower compression. Compression is dictated by the choice of pistons you use.

Over boring is often necessary to clean up the bore on a rebuild, to make sure the cylinder is round and does not taper, or simply to obtain the proper piston to wall clearance for a new set of forged pistons, which are often set up to have higher piston to cylinder wall clearance because they expand differently than a cast piston, or because they are intended for higher performance use, and the wider clearances are necessary for that use. If the bore is worn at all, or needs to be honed to fit a new set of pistons and have the rings seat properly, it's hard to get the clearances where you want them with a stock size set of pistons. Thus, the next oversize is used.

The usual 1st overbore from stock is .020 inches, which is approximately 0.5mm. That does not give a huge increase in displacement. The real benefit is having the correct piston to cylinder wall clearance, which affects ring break-in, ring sealing (and therefore compression), etc.

 

On a 4G64 I would keep it 86.5 and have pistons made to fit. Do a fresh hone but keep the bore as small as possible to promote the best sealing you can get. Then buy a really good gasket.

 

all depends if you have high milage block , with time cylinder walls wear out making the wall not perfectly round , thats where 0.20 bore is beneficial