Exhaust manifold options & Bore size recommendations

[supra90mkiii]

Hi guys, been awhile since I've posted on here. I'm slowly building another engine for my 90 5-speed. I have a couple questions though...

I have an unmodified/raped 7mgte block and head. I measured the cylinder bores, and they all look to be even, and original diameter. My question is how much should I bore over? (Planning on using Probe pistons) .020" or .040" I'm not expecting a performance increase (noticeable) between the two, which is making me lean toward going .020" instead, for reliability reasons. Opinions? suggestions?

Also, I am still using the stock exhaust manifold, I have the "new" sp61gtbb. Are there any other options for exhaust manifolds for my type of application(other than custom)? Or would it be most cost effective to just port the original manifold?

Thanks everyone.

 

[CyFi6]

If the bores are original size and have no out of round or taper, go with std size probes. They come .003" oversize from stock, so it gives your machinist room to fit them properly. Depending on your power goals you can figure out what piston to wall clearance you need. Check out this thread, pretty helpful.
http://www.supramania.com/forums/showthread.php?t=53469&highlight=probe+pistons

 

[WhtMa71]

You only want to bore as much as you need to to get a good surface. Same goes for decking the head or block. Make sure you have the timing belt tensioner torqued to the block if you do get it bored over.