Drilling Orifices the Smart Way

Orifice Feed Hole Guidelines

Do not drill feed holes too large.

A small change in diameter can make a big change in the area of the hole the oil flows through.
Use this chart to determine drill diameter for any increase in orifice area that you think best suits the vehicle you are working on.

Original Orifice Dia.	Enlarged Dia.
Original Orifice Dia.	+ 10% area	+ 20% area	+ 30% area	+ 40% area	+ 50% area
0.040	0.042	0.044	0.046	0.047	0.049
0.045	0.047	0.049	0.051	0.053	0.055
0.050	0.052	0.055	0.057	0.059	0.061
0.055	0.058	0.060	0.063	0.065	0.067
0.060	0.063	0.066	0.068	0.071	0.073
0.065	0.068	0.071	0.074	0.077	0.080
0.070	0.073	0.077	0.080	0.083	0.086
0.075	0.079	0.082	0.086	0.089	0.092
0.080	0.084	0.088	0.091	0.095	0.098
0.085	0.089	0.093	0.097	0.101	0.104
0.090	0.094	0.099	0.103	0.106	0.110
0.095	0.100	0.104	0.108	0.112	0.116
0.100	0.105	0.110	0.114	0.118	0.122
0.105	0.110	0.115	0.120	0.124	0.129
0.110	0.115	0.120	0.125	0.130	0.135
0.115	0.121	0.126	0.131	0.136	0.141
0.120	0.126	0.131	0.137	0.142	0.147
0.125	0.131	0.137	0.143	0.148	0.153
0.130	0.136	0.142	0.148	0.154	0.159
0.135	0.142	0.148	0.154	0.160	0.165
0.140	0.147	0.153	0.160	0.166	0.171
0.145	0.152	0.159	0.165	0.172	0.178
0.150	0.157	0.164	0.171	0.177	0.184
Common Fractions and Decimal Equivalents
1/16	=	0.063	1/8	=	0.125
5/64	=	0.078	9/64	=	0.140
3/32	=	0.094	5/32	=	0.156
7/64	=	0.109	11/64	=	0.172

Too Much of a Good Thing Causes Problems

Drilling clutch feed orifices is a proven way to reduce clutch engagement time and firm-up shift feel. This helps with otherwise lazy-shifting transmissions, hardworking commercial applications or units modified for performance use. But many times when a feed orifice is enlarged, the hole is drilled too large and the change in shift feel is far greater than intended. This results in harsh shifts, which many customers do not like and view as a nuisance.

If you wish to enlarge feed orifices, these guidelines will help you determine what drill bit size corresponds with a specific percentage increase in orifice area so you are less likely to cause problems when making these common modifications. Experience has shown a 20% increase yields a subtle yet noticeable change in shift feel without causing harsh shifts that generate customer complaints.

Orifice Feed Holes

200-4R

TH350

4L60 (700-R4)

4L60-E, 4L65-E, 4L70-E

4L80-E, 4L85-E

4R70W, 4R75W, AODE

1-2a is the feed orifice, 1-2b should always be larger than 1-2a. 2-3a (some models) is the low-speed 2-3 feed, 2-3b is the high-speed 2-3 feed. 3-4 does not have a conventional feed orifice.

4T65-E

TH400

There are three different plates for the TH400: the '89-later seven-ball plate and two versions of the six-ball plate. The difference between the six-ball plates is the location of the 2-3 hole.