drilling flex plate holes

ok i have a question I'm not sure about. Over the winter i bought a new Edge Racing torque convertor. Well it came with bolts that are bigger than factory. I'm running factory flex plate so the bolts wont fit. I have been told i can drill them out and then i have been told that is a big no no. I'm not sure what to believe. and if i need a new flex plate i need to find one with the right bolts holes. anyone run into this problem before and if so which route did you take?.....thanks Joe

I would NEVER drill out a flex plate, you are just asking for issues down the road with it cracking and spitting the bolts out into the back of the block. Happened to my D200 truck, I thought I had spun a bearing it was so loud! Found out two had backed out and a third was half way out and was hitting the back of the block, only because they were not properly torqued. But the flex plate you need...

thats what i thought too

Coan, Mancini, Summit all list plates.

Like Rhino I tried drilling one out on a 340 Dart. It lasted exactly 3 days and cracked into a million pieces.

How are the holes originally produced?

Me thinks if the bolts backed out it had nothing to do with the holes being drilled oversize...always use red Loctite. I've used a step drill to enlarge the holes and properly deburr them on "windowless" flexplates used on standard performance installations, never had a problem.

I am not saying to do it but I drilled one in the car, crooked cause the drill interfered with stuff and never had an issue. That was way back that one, I probably still have it somewhere.

Mopar made several different flex plates over the years, the original early one were for the 12 inch converter in all V8 that had 6 bolt cranks with 7/16 crank bolts and 5/16 converter bolts,
The Hemi motors had 8 bolt crankshafts and used a 11 inch diameter converter like the Max wedge cars came with and both used the larger 7/16 converter bolts like most aftermarket race converters use today.
I'm not sure of which year it was(1969 or 1970, when Mopar switch to the smaller 11 inch converter with the smaller bolt pattern on the converters. Bottom line is the smaller bolt pattern flex plates have material remove in between the center and the outer mounting converter holes which may or may not help a motor rev up a tiny faster
I have a sack of them on my shelfs in both sizes and some out of date SFI plates that have both converter bolt (12 and 11 inch with both 5/16 and 7/16 holes patterns
B&M, TCI and almost every other race tranny companies stock and sells the one you need for the 11 inch and smaller converters with the larger 7/16 bolt hole sizes.
I have drilled, enlarged several 11 inch small bolt patter flex plates to the larger 7/16 size holes with no bad results that I know of for drag cars
It's your call to drill or replace your flex plate

Drill them. Radius the hard edge. I've broken plenty of flex plates. But never once starting at bolt hole.
Doug

new one on order from mancing

being a machinist, i have seen hundreds [if not thousands] of mis-drilled holes in my lifetime.
this is with all types and thicknesses of materials.
i am not afraid of drilling any material. however, the proper machine must be used, as well as the proper drill [bit] for the job, plus having the work piece properly clamped and at 90 degrees from the drill spindle.
proper feeds and speeds [whether automatic or hand control] must be used so as to not work harden the item. this can be caused by too fast of a speed and too fast of a feed, or the inverse.
the best practice is to drill the hole[s] approximately .010-.020 [.015 preferred] undersized, then use a reamer of the size you want that fits the bolts.
however, the bolt holes need to be a little oversize to properly fit the intended bolt being used, and that oversize will vary depending on the bolt size.
for example, if your bolt is .005-.007 undersize, a 1/2" [.500 diameter] bolt measuring .493-.497 [which is common for that size bolt, fine or course thread alike] the "best" hole size for assembly purposes, will be .500, or 1/2", to an oversize up to .530" diameter. [approximately 17/32" which is .5312"]
as most hobbyist equipment have more runout than precision machines, it is not uncommon for the drill to cut the hole .005-.010 [or more] oversize, as well as egg-shape the hole. reamers will make the hole round again, thus the recommended drilling the hole undersize, then finishing with the reamer.
you must also deburr or chamfer the hole[s] when done to eliminate all burrs from the drilling/reaming process, and prevent stress risers from possibly occurring.
just my experience from drilling [possibly] millions of holes in my 45+ year machinist career in all types of materials.
your results will more than likely vary............

If it's SFI drilling it would negate the rating?

I think the flexplate steel makes it tough to drill. Personally if I had to do it I would be milling the hole larger with a carbide end mill on a cnc. My cnc spoils me..........

If hole is in same spot but to small drill it, back in the day I bought a B&M converter that had the 7/16” bolt, same spot as other converter. I would never drill a new hole unless plate was the solid type. Originally they were punched not drilled

my bridgeport does the same.

This is why I recommend the step drill, keeps the hole centered. The hole for the 5/16" bolt is 11/32", so increase the hole size by steps to 15/32" for the 7/16" bolt. The flexplate material isn't an exotic alloy, just a form of mild steel; so it drills easily with the correct speed.

correct John.
the difference being .031 in hole size vs bolt diameter allows for proper fit during assembly.
depending on a "super precise" item, it's possible to tighten up the hole size some, but you better be RIGHT on the money, within a few .0001 [that's ten thousandths of an inch] of the hole locations of both the [item with the] holes and [the item with] the threads to be able to assemble without the bolts either rubbing on the hole sides or the threads being deformed by the bolt not going in straight.
in the machinist world, you can't insert a precision 5/16" diameter plug in a precision 5/16" hole. [try it sometime. it won't fit without "pressing" it in.]

and a step drill works wonders for drilling ROUND holes, but i prefer to use it AFTER i drill the hole to a close undersize using a standard drill [bit]. the step drill will stay sharp longer doing it that way.
and one other thing about step drills. [that REALLY bugs me..... ] guys seem to want to smoke them through with as high a speed/rpm as their drill motor will go !
that will burn them up [the step drills], especially without using some kind of lube.
slow and steady is all that's needed to get the job done, and a touch of lube doesn't hurt either............
it's amazing what "tool abuse" [not THAT kind of tool you guys ! ] will do to both the tool and the operator [if he IS the TOOL you were thinking of. ] if something breaks or gets tangled up. i have seen people loose thumbs and fingers wearing gloves and have them [the gloves] get caught in a spinning drill press, and have seen a couple of guys get the shirt sleeve or entire shirt ripped clean off them, getting the shirt caught in a spinning drill [bit].
those guys that lost their shirt [maybe that's where the "saying" came from ? ] were the "lucky" ones, if you could call it that. it could have turned out a LOT worse for them.
BE CAREFUL AROUND MACHINE TOOLS GUYS !
just my 45+ years of being a machinist and shift supervisor, taking guys to the hospital to get "patched up" when they refused my idea of amputating with my rusty pocket knife, then patching the wound[s] with greasy shop rags.
[hopefully] your mileage will vary..........

And the worst gloves to be wearing are latex or nitile types.
That material just "sticks" to spinning shafts/bits/etc.

In this case if the hole is smooth the diameter is large enough, there should be no issue. The plate is held to the converter by clamp load. It's not being driven by contact between the bolt and the hole anyway. Clearance is cclearance. Like I stated earlier. B&M plates never fracrure at the hole. They fracture about 1/2" away. A B&M plate is a drive plate, not a flex plate. Any misalignment or clearance in the crank snout can cause them to fracture.
Doug