Chrome bore port timing

[Burke Kilgour]

What's the general consensus on cutting the piston tops to alter port timing on a chrome bore motor? There's no other way to raise the exhaust port without re-chrome. I know it alters the intake timing as well, but what are the options? Would a generous chamfer on the top of the piston be a option as well? That would help keep the compression up so the heads don't have to be over-cut to compensate.
I've also seen pistons with the exhaust port area chamfered to increase effective port height....... pros/cons?

[David Borg]

Good question ,


im not sure but I think that one of the ways it can be done by decking the block .050 and the same amount removed from the piston crown (just to give an example)

I would also like to know,

[Prof. O/B]

What's the general consensus on cutting the piston tops to alter port timing on a chrome bore motor? There's no other way to raise the exhaust port without re-chrome. I know it alters the intake timing as well, but what are the options? Would a generous chamfer on the top of the piston be a option as well? That would help keep the compression up so the heads don't have to be over-cut to compensate.
I've also seen pistons with the exhaust port area chamfered to increase effective port height....... pros/cons?

You can match-up the deck height and the piston crown height to adjust the port timing. I would not do any more than it took to achieve a 2.100" height from the top of the deck to the top of the "transfer" ports. And yes, I realize others have removed more. In any event, I would not deck the piston anymore than .030" because you are decreasing the crown thickness of the piston, and this would be all I would care to go. If you choose to deck the block insure it is done with a sharp cutter, or you are apt to chip the plating at the top of deck. I would not chamfer the pistons, you would then be changing the "squish band" thickness, which I would consider to be counter-productive..........................O/B:)

[Burke Kilgour]

OK, I see. What about the chamfer I've seen on some pistons just in the exhaust port area? What do you think about that? Seems like sort of a "cheat" ...... but does it work?

[Prof. O/B]

OK, I see. What about the chamfer I've seen on some pistons just in the exhaust port area? What do you think about that? Seems like sort of a "cheat" ...... but does it work?

The term Chamfer means to "bevel" an edge, if it done by the cutting of two surfaces that are perpendicular (90 Deg. ) to one another at the same time. If you did this with a say a 45 Deg. chamfer, you would be removing exactly the same amount of material from the top of the piston (the crown) as you would from the side of the piston. The poblem is, pistons aren't concentric, because they are "Cam Ground" (really cam machined). The top to bottom dimension (as it sets in the bore), on a 2.5 Ltr Merc. for example, has an approximate Dim. of 4.485" when measured at the bottom of the shirt. 90 Deg to that orientation is the "thrust side" of the piston, which is going to come in somehere around a 4.492" Dim.

Since the piston is not concentric, if you place in in the standard 3-Jaw self-centering chuck, the crown will be out of concentricity and it will "wobble" making any concentric cutting of the edge of the crown impossible. You can counter this, by using a 4-jaw non-centering chuck and "indicating" it in by the use of a dial indicator, if you care to do so.

But I'll tell you the truth, I have always placed the pistons in a self-centering chuck (and you will need either "soft" jaws or a soft material between the chuck jaws) to prevent the piston from being marred. In that position, I do "Radius" or round the edge of the piston. You can do so at relatively low RPM with a Mill Bastard or 2nd. Cut lathe file. But you MUST be extremely careful not to get the file into the chuck, and always, I will repeat ALWAYS, use a file handle, it would not be the 1st. time I have seen the Tang of a file drove through someone's hand or wrist. The purpose of doing this is, to round the sharp edge of a piston which is a potential detonation point on the piston, and there is no greater detonation point than the edge of the piston in the center of the exhaust port.....................O/B

[Burke Kilgour]

Well, what I was thinking was to put the piston in my indexing head and "bevel" the exhaust port area only, probably at about 30-35 degrees.
I've seen this done before and was just wondering about it.

[us1]

Keep in mind you are affecting the squish band unless you deck the piston and match the block by decking it the same amount.

[Burke Kilgour]

Here's what I've seen.................

[Burke Kilgour]

Very true John.............. hmmmmmmmmm
The thing is, my buddy has a motor he wants to rev up fairly high and the stock porting falls off in the higher RPM quite quickly. I was looking for a "cheat" to get around the chrome bore issue.

[kingsbiship]

Make sure there's enough meat on the ring land to be strong enough!

[Prof. O/B]

Make sure there's enough meat on the ring land to be strong enough!

Kingship and John are both giving you good advice. When you do a piston as shown in the "Pic", it becomes very sensitive to jetting. By that I mean, If won't be quite there, then you lean it down to make it run, Boom! The squish band is out of control.............O/B

[Burke Kilgour]

Hey, like I said, just looking for options.
So, from what I gather, decking the block and cutting the piston tops is my only option then..................

[us1]

Kingship and John are both giving you good advice. When you do a piston as shown in the "Pic", it becomes very sensitive to jetting. By that I mean, If won't be quite there, then you lean it down to make it run, Boom! The squish band is out of control.............O/B

Thats exactly what I was going to say. When I do 2 degree squish band heads the squish is tighter at the outside of the piston. This allows the use of lower octane and or more timing before a melt down. Cutting the piston at the edge so the squish has more volume at the exhaust port would mean you will be jetting, setting timing and choosing fuel for the large squish and not the rest of the piston. The only way I have seen it work OK was to deck the piston and deck the deck. As the Professor said you don't really want to cut it more than .030 off the piston so the timing advance would not be that great anyway. Time to get out that cool new 90 degree pencil grinder and go for a reNic. I love my grinder, thanks again for the tip.

[Burke Kilgour]

I'd be better off sleeving all 6 and just raising the ports the ole fashioned way then.
Thanks guys :D

[us1]

Thats why I go with the steel 2.5s.