Ask Away! with Jeff Smith: Choosing and Installing a Manual Transmision on a GEN IV 454

I hope you can help me. I have a GEN IV 454 engine and I would like to install a manual transmission behind it. The late model overdrive transmissions like the T56 are way too expensive, so I’m wondering if I can use any other budget tranny like from an early model?  – G.A.

…

Jeff Smith: The simple answer is—yes!

From a manual transmission standpoint, swapping in an earlier manual trans like a Muncie or Super T-10 four-speed is very easy. There are a couple of small points to consider.

The variables are the proper flywheel for a GEN IV engine and choosing a clutch disc that will match the input splines on the transmission you choose.

All big block Chevy crankshafts use the same 3.50-inch flywheel bolt pattern, but they are not all the same. The MKIV big-block 396, 402, and 427 engines employed a two-piece rear main seal and the rotating assembly was internally balanced, meaning that the flywheel or balancer did not use an offset weight. With the MKIV 454, it still used a two-piece rear main seal but required a different, externally balanced flywheel. The later GEN V and VI big-block 454 engines (your engine) used a one-piece rear main seal and required the offset weight of 42.5 ounce-inch external balance spec.

Since all big blocks use the same bolt pattern, you must use an externally-balanced flywheel. These can be identified by drilled holes near the outside edge of the flywheel.

There’s a GM factory flywheel that incorporates the externally-balanced weight and is somewhat affordable. It’s a nodular iron flywheel which is not as strong as a steel unit and is not SFI-spec’d. You can find it here.

Some flywheels may be fitted for the later model metric-style pressure plate with pins—rather than centered on the pressure plate bolts. It is very important to know which pressure plate the flywheel is designed to accept. The bolt pattern is the same for both pressure plate styles. The difference is that the older pattern used a countersunk bolt hole intended for use with a pressure plate bolt with a locating diameter near the bolt head.

More modern styles locate the pressure plate with a separate pin and use non-countersunk metric bolts. You cannot use countersunk bolts in the newer flywheel or vice-versa. We’ve seen people drill out the metric bolt holes on newer pressure plates for lager SAE bolts, which is a mistake because the pressure plate is no longer properly centered. You might get away with it, but it’s not correct.

The next question has to address which style of transmission you will choose.

Most Muncie four-speeds used a coarse, 13-spline input shaft that will require this style of clutch. However, there were some later Muncies that included the 26-spline input shaft that is common with both Super T-10 four-speeds and the later five-speed manual transmissions like the Tremec TKO transmissions. Whichever trans you choose, you will need to make sure the clutch spline count matches the input shaft on the transmission. You will also need to determine whether you want an 11-inch or 12-inch clutch assembly. Since this sounds like a rather mild 454, the 11-inch would work just fine.

This swap will also require a bellhousing. The most popular bellhousing for the Muncie transmissions were intended for small-block engines using a 10.5-inch pressure plate that used the small diameter flywheel. Those bellhousings will not clear the larger big block flywheel. This will require the larger bellhousing intended for either big blocks or high performance small block engines. These are hard to find these days and when you do run across one, they will be pricey.

If you are not going to be racing this at the drag strip, you probably won’t require a scattershield. However, they are easy to find. We’ve tested the accuracy of the QuickTime bellhousings and found them to be easily within 0.002-inch of dead on center for aligning the input shaft to the crankshaft centerline. Other scattershields are less accurate.

Another bellhousing option would be to use the Chevy truck factory aluminum bellhousings. These are designed for the larger flywheels and are distinctive in shape with a notched lower area as opposed to the car bellhousings that are more rounded on the bottom. These bellhousings are plentiful and inexpensive—but there’s a catch. The input hole that centers the transmission to the engine is larger in diameter than standard passenger car bellhousings. This does not register the transmission properly to the crankshaft centerline which means the clutch won’t release properly. We know, because we learned this lesson the hard way.

This will operate poorly and destroy the pilot bushing in a matter of 100 miles or less.

However, we discovered that QuickTime makes a spacer ring that will properly register the transmission to the bellhousing. This ring is not in their catalog and we don’t have a part number so you will have to call them to buy one, but it would allow you to use the older truck bellhousing on a big block Chevy engine.

We actually checked the bellhousing’s input runout with the ring in place and in our case, did not require re-alignment as it was inside the 0.005-inch total indicator runout spec. Contact QuickTime by starting with Holley’s tech line—866-464-6553. They should be able to put you in touch with the QuickTime people.

We would definitely recommend checking the bellhousing alignment with a dial indicator.

As mentioned, the spec is 0.005 to 0.007-inch of total indicated runout between the centerline of the crankshaft and the diameter of the input on the bellhousing. If the bellhousing you’re using is off, Lakewood and Moroso make offset bushings that will move the bellhousing back into alignment as long as the misalignment is not more than about 0.040-inch.