TH350 rebuild tech
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Then install the 3 tang thrust. | Then install the 3 tang thrust. | ||
| − | + | =Direct drum assembly= | |
For racing and heavy towing use, you will want to machine the direct clutch piston down approximately .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. I try to keep a few machined on hand, and machine several at a time when I do so. If you need one email me and if you can supply a core, I MAY be able to sell you a piston. | For racing and heavy towing use, you will want to machine the direct clutch piston down approximately .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. I try to keep a few machined on hand, and machine several at a time when I do so. If you need one email me and if you can supply a core, I MAY be able to sell you a piston. | ||
Stock measurement is ~.835, I cut them to .675". Ideally you would pre-assemble, and check clearance using a 4 clutch pack, meaure the additonal thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb on most auto trans friction clearances is ~.010" clearance per friction. So a 5 friction direct setup would work great with .050" clearance. It will work with less, I wouldn't go less than .040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, busted lip seals, and unsatisfactory shift quality. | Stock measurement is ~.835, I cut them to .675". Ideally you would pre-assemble, and check clearance using a 4 clutch pack, meaure the additonal thickness of the extra friction and steel, then cut the exact amount needed to leave you the exact clearance you desire. Rule of thumb on most auto trans friction clearances is ~.010" clearance per friction. So a 5 friction direct setup would work great with .050" clearance. It will work with less, I wouldn't go less than .040", and will work with quite a bit more, but excessive clearance can result in delayed engagement, busted lip seals, and unsatisfactory shift quality. | ||
| − | Stock 4 piston direct piston vs. machined for 5. | + | Stock 4 piston direct piston vs. machined for 5. Check the thickness of the piston, it should be NO less than .700 thick at the widest point. If the piston is too low, the bottom flat steel plate can drop below the grooves in the drum and bind up the piston. |
[[file:TH350directpistons.jpg]] | [[file:TH350directpistons.jpg]] | ||
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| − | HD applications should also get a hardened intermediate sprag race. It is darker colored than a stock piece. | + | The HD applications should also get a hardened intermediate sprag race. It is darker colored than a stock piece. |
The intermediate sprag race and intermediate sprag (actually 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 weakest single part of a TH350. | The intermediate sprag race and intermediate sprag (actually 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 weakest single part of a TH350. | ||
This is not to say a TH350 is not tough, they are under-rated and can reliably handle in the 600HP or lb/ft range based on my experience. | This is not to say a TH350 is not tough, they are under-rated and can reliably handle in the 600HP or lb/ft range based on my experience. | ||
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Take note of the orientation of the seals, the lip will face towards the "oil" side of the piston, not the clutch side. | Take note of the orientation of the seals, the lip will face towards the "oil" side of the piston, not the clutch side. | ||
Also take note, | Also take note, | ||
| − | this is a hi- | + | this is a hi-performance TH350 and I did NOT install a lip seal in the drum that would separate the two halves of the direct piston. |
| − | This is part of the process of "dual-feeding" the direct clutches. On a stock rebuild you would | + | This is part of the process of "dual-feeding" the direct clutches. On a stock rebuild you would normally install this lip seal. |
You can do this on any rebuild but you must also block a passage in the case near the pump to prevent loss of pressure to the reverse circuit. | You can do this on any rebuild but you must also block a passage in the case near the pump to prevent loss of pressure to the reverse circuit. | ||
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Also of note, | Also of note, | ||
| − | if you are using almost any aftermarket kit, these kits use a feed plate between the support plate in front of the valve body and the | + | if you are using almost any aftermarket kit, these kits use a feed plate between the support plate in front of the valve body and the separator plate. This feed plate dual feeds the directs and plugging this hole is unnecessary. Also most manual valve bodies, trans-brakes, etc will have you omit the direct drum center seal and the sealing ring on the stator but plugging of this hole is also unnecessary due to the design of the separator plate. |
Don't assume you need to plug this hole. Only plug this hole if you plan on building a unit yourself. | Don't assume you need to plug this hole. Only plug this hole if you plan on building a unit yourself. | ||
You can omit the center seal, plug this passage, enlarge the 2nd and 3rd feed holes in the stock separator plate and you will have greatly increased the torque capacity of a stock TH350 as well as gained a firmer shift. | You can omit the center seal, plug this passage, enlarge the 2nd and 3rd feed holes in the stock separator plate and you will have greatly increased the torque capacity of a stock TH350 as well as gained a firmer shift. | ||
| − | Even firmer shifts can be accomplished with | + | Even firmer shifts can be accomplished with check ball removal and modifications to the accumulators. I recommend NOT modifying the 1-2 accumulator. Retain the cushion spring and do not block the circuit. The 2-3 accumulator can be blocked by several methods. |
Removing the 2-3 accumulator spring and using a spacer (nut, washers, cut down section of tubing) between the piston and the e-clip is the "shade tree' method. You can also block the feed in the valve body to the 2-3 accumulator. | Removing the 2-3 accumulator spring and using a spacer (nut, washers, cut down section of tubing) between the piston and the e-clip is the "shade tree' method. You can also block the feed in the valve body to the 2-3 accumulator. | ||
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You can buy or make a tool similar to the one pictured or use 2-3 5" or larger c-clamps (much more of a PITA). | You can buy or make a tool similar to the one pictured or use 2-3 5" or larger c-clamps (much more of a PITA). | ||
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Forward drum procedures are almost identical. No center seal to worry about on a TH350, it typically already has a 5 clutch pack, except light duty applications. | Forward drum procedures are almost identical. No center seal to worry about on a TH350, it typically already has a 5 clutch pack, except light duty applications. | ||
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These regulate the amount of fluid, or how fast, it can apply the clutches. Drilling them bigger allows a faster apply. | These regulate the amount of fluid, or how fast, it can apply the clutches. Drilling them bigger allows a faster apply. | ||
| − | However, | + | However,bigger is not always better. |
| − | bigger is not always better. | + | |
Remember that as the load increases on the trans, so does the pressure. The increase in pressure will cause the fluid to move faster and shifts to be firmer. | Remember that as the load increases on the trans, so does the pressure. The increase in pressure will cause the fluid to move faster and shifts to be firmer. | ||
| − | If you drill the holes too big the shifts become too harsh at light throttle openings. This causes undue wear on the internal transmission parts, rear differential and axles, | + | If you drill the holes too big the shifts become too harsh at light throttle openings. This causes undue wear on the internal transmission parts, rear differential and axles, drive line and u-joints. |
The ideal situation is a shift that gets progressively harder with greater throttle openings. | The ideal situation is a shift that gets progressively harder with greater throttle openings. | ||
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[[file:TH350feedholes.jpg]] | [[file:TH350feedholes.jpg]] | ||
| − | Also not the 2-3 accumulator hole marked in the upper portion. This hole can be blocked but it is easier to block the passage in the VB or the 2-3 accumulator piston as previously mentioned | + | Also not the 2-3 accumulator hole marked in the upper portion. This hole can be blocked but it is easier to block the passage in the VB or the 2-3 accumulator piston as previously mentioned. |
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