TECHNICAL  BULLETIN

SUBJECT: Ford 4.0 cylinder heads - Part # 20059

DATE : 7/20/01

We have had questions about the exhaust port size of our late model,

1995-98 Cylinder Head. There were two heads introduced by Ford during the

years our part covers. The first heads bear the casting # 95TM / 97TM and

have a large exhaust port measuring 1.70" across. The later heads have a

casting # of 98TM and have a smaller exhaust port of 1.40" across.

4.0L    014ZK20059

The reason for this is that the smaller port size increased the velocity of the

exhaust gasses which carried more heat down to the catalytic converter.

This helped the converter "light off" sooner, so it did a better job reducing

emissions during the start-up and drive away portion of the EPA emissions

test.

Our heads have the smaller 1.40" exhaust ports, and can be used in place

of the larger 1.70" versions without any issue as to fittment, performance, or

emissions issues. The early large port heads are quite rare and you may

never see one in the field, but this information will help answer any questions

that may come up related to this issue.