Editing TH350 rebuild tech (section)
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==T-350 Direct drum assembly== For racing and heavy towing use, you will want to machine the direct clutch piston down approximately 0.160" to allow for one extra friction and steel plate for a total of 5 frictions, and 5 steels in the directs. This can be accomplished on a brake lathe. Stock measurement is 0.835", machining the piston to 0.685" to 0.720" will allow the use of 5 frictions using 0.070" steels. Ideally you would pre-assemble, and check clearance using a 4 clutch pack. Measure the additional thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb for most auto transmission clutch pack clearance is 0.010" clearance per friction. So a 5 friction direct setup would work well with 0.050" to 0.070" clearance. It will work with less, but no less than 0.040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, failed lip seals, and unsatisfactory shift quality. If the piston is too low, the bottom flat steel plate can drop below the grooves in the drum and bind up the piston. You can leave out center cushion seal if dual feed is used. [[File:T-350_direct_piston.jpg|thumb|300px|left|T-350 direct clutch drum pistons, piston on left cut for 5 friction use]] <br style="clear:both"/> Heavy duty applications should also get a heat treated intermediate sprag race. The heat treatment makes the race less brittle. A heat treated intermediate sprag race is darker colored than a stock part. The T-350 intermediate sprag race and sprag (actually a roller clutch) take a brutal amount of force on a 1-2 shift at high rpm. Even more so with harsher shift calibrations. It is the single weakest part of a TH350. This is not to say a TH350 is not tough; they can reliably handle power in the 600 HP or lb/ft range if properly assembled using the right parts. [[file:TH350intspragraces.jpg]] Several companies offer an even heavier duty direct drum/sprag assembly. It uses a 36 element sprag (similar to what is used in the TH400) and a machined drum; it will withstand more torque than a stock unit or one with just a hardened race. These units can cost $300.00 or more. [[File:T-350_HD_drum_sprag.jpg|thumb|300px|left|T-350 direct clutch drum with 36 element sprag]] <br style="clear:both"/> It is recommended to always use a new intermediate roller clutch for HD builds. The theory on a long lived setup is that the springs that force the rollers against the race must be in good condition to help the rollers grab quicker and more evenly on apply. The parts go on the direct drum. [[File:T-350 drum sprag race.jpg|thumb|300px|left|T-350 direct clutch drum with sprag, race, retainer]] <br style="clear:both"/> Assembled direct drum sprag and race with retainer. [[File:T-350_drum_sprag_assembled.jpg|thumb|300px|left|T-350 direct clutch drum with assembled sprag, race, retainer]] <br style="clear:both"/> The T-350 direct drum should use a wider bushing even on stock rebuilds. Stock is 0.500" a wider 0.750" is available in some bushing kits. Most bushing kits include the wide one. There is a lot of load on this bushing, and it should ALWAYS be replaced on overhaul. [[File:TH350directbushings1.jpg|thumb|300px|left|The T-350 direct drum should use a wider bushing even on stock rebuilds]] <br style="clear:both"/> Also check the inside bore of the drum for wear from the sealing rings. A small amount of wear can be honed out. Visual shiny spots with no detectable wear are acceptable. [[File:T-350_direct_drum_bushing.jpg|thumb|300px|left|T-350 Direct drum with wide bushing, piston, springs and retainer installed]] <br style="clear:both"/> Next you will install the direct piston lip seals, and install the piston in the drum. Take note of the orientation of the seals, the lip will face towards the "oil" side of the piston, not the clutch side. Below is a high performance T-350 piston for a drum without a center seal that would separate the two halves of the direct piston. [[File:TH350directpistonseals1.jpg|thumb|300px|left|T-350 Direct piston with lip seals installed]] <br style="clear:both"/>
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