Reverse taper in cylinder

[John B]

While finish honeing my cylinders for my new pistons I notice that one cylinder has a taper of .0015" that is bigger at the bottom than the top! The bore is already at the max clearance for the piston skirt at the widest point which is the bottom of the cylinder. I don't dare try to straighten the bore for fear of going over. What harm will reverse taper do?

[mock_glh]

.0015 isn't much. Since it's oversize you won't have to worry about seizing so leave it alone.

[Frank]

Just check compression when done. I think it will be fine... maybe a slight hint of piston slap????????

[mech1nxh]

must comment here as i have been biten by this demon!!!!!
(fried piston canape') in my case .002 at bottom of #2 cyl
seems trivial ....or so i thought.......ah but the rest of the story..
the phenomenom is called "core shift'" i.e@ when the internal and external
casting molds move during the block casting process....seems trivial right?
what happens is the crank centerline/cylinder centerline/ and sometimes the deck horizontal centerline are not at right angles .
machining can correct the angles , but the cylinder internal wall thickness usually is not uniform due to the same shift.
in my case the block integrity was compromised when hot......16psi boost
8.2/1 comp ......
as you are concerned about .0015 (i would be to) find access to a machine
shop with a cleaning oven and heat the block to 125 deg F.
remeasure the bores for ...oval ... and taper....i hope they are within .001
if so good to go.
if not my humble advice is to find another block to build.
apologies here, not trying to be the prophet of gloom and doom
and for the long post
MECH1NXH

[22dodge]

when I build my race engines, I intentionlly reverse taper the cylinders .0015". I bore it .003 undersize then finish hone the bottom to exact spec while keeping the top to .0015 under... On the dyno, this technique increased hp by reducing blowby to vertually zero.

As for core shift... I had a really bad common block. I bored the engine the usual .030" over.. (this was an n/a daily driver) well 150 miles later the bottom of cylinder #4 cracked and dumped coolant into the oil. when I tore down the engine and sonic tested it... the bottom of all the cylinders had a core shift of over ¼".. one side was over .350" while the other side was under .050" thick... Bad Chrysler!!!

[mech1nxh]

GAAAWD........
that must be some kind of record somewhere.........
anyhow next boost monster build I checked into owning my very own
sonic testing device .....ack ack ack ack.....yours today for the
small sum of $3123 american.... so $174 check out at speed shop
did not seem so bad
have since saved myself 5 potential heartaches by sonic tests and
hot measurements.........BAD CHRYSLER INDEED

[John B]

I have never been able to leave well enough alone so I threw caution to the wind and spot honed the taper out and got away with it. The cylinder is straight and still in limits. I wish I had known about the reverse taper HP increase before though! I could have set it up that way easily. Next time. Thanks for the tip!