Doug Nash OverDrive

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Corvette 4 + 3 Overdrives

We have been rebuilding Corvette 4 +3 Overdrives since 1984. This transmission incorporates a Super T10 4 speed with an automatic overdrive. The overdrive can come on in second, third, and fourth gears.... thus 4 + 3! It is only used in Corvettes made in 1984 to 1988 and is not recommended to be swapped into street rod applications.

The 4+3 Overdrive was plagued with many design problems in 1984 to late 1985 cars. The main problems were an underdesigned bearing in the carrier assembly, a direct clutch thrust washer breakage problem, and a pump sealing problem. The carrier bearing failed because of poor bearing design, bad preload, and poor lube to the bearing. This was resolved with GM part number 14105940. It incorporated a heavy duty bearing, and feed oil to the bearing. It was up to the technician to preload it correctly. When a carrier goes bad you will hear bearing noise in direct drive and it will go away in overdrive. Sometimes it can sound like a tire whirring noise. The Direct Clutch thrust washer would break because it could spin in the case. Once broken it would jam in between the overdrive piston and case. This would cause the overdrive to get stuck in two modes and burn up. The new piston GM# 14091496 resolves this problem by putting anti-spin tangs on it. It was redesigned by us for GM again in 1997 to utilize better heat treating capabilites. The pump assembly on 1984 and early 1985 cars used a rear pump cover with no seal between the output shaft and cover. The new cover uses a teflon sealing ring to prevent pressure loss and pump whine. The GM number for the new spool is 14091349.

Below is a list of some a the parts and services we offer the Corvettes with 4+3 Overdrives. Late 1986 to 1988 Corvettes fail because of lack of service to the overdrive and 4 speed. It is recommended that oil and filters be changed in both 4 speed and overdrive every 10,000 miles. If you overheat your car always change all lube in the engine and gear box.

We have sold our hard parts and rebuilding service to SK speed at SKSPEED.COM

We have been rebuilding these units as we said since 1984. We also have built them for the Corvette Challenge cars in 1988 as well as upgraded many of the Callaway twin turbo Corvettes built in 1987 and 1988. Call 631-957-9427 for tech questions. If you have any technical questions please E-Mail US to get questions answered via Email.

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