How To Separate Transmission From Bellhousing
On any quiet evening you lot might hear it. That distinctive pull in Third gear as the engine accelerates and the listener waits for that lift of the throttle and a quick, clean manual trans gear change into Fourth. Just instead, you hear that instant rpm proceeds that telegraphs to all within earshot that the commuter missed the shift and the engine revs to the moon or bangs up confronting the rev limiter.
Universal blame is ordinarily reserved for the driver's lack of hand-pes coordination. But frequently nosotros tin can trace these missed shifts to a more mechanical source. Arraign then ordinarily shifts to the bellhousing but adept blueprinting technique will indicate out that information technology's ultimately more an result of misalignment that tin can come from a combination of problems, something the engineers telephone call tolerance stack-up.
Muncie four-speeds (foreground) use a ball begetting input shaft that can withstand a large amount of misalignment. Afterwards Tremec five-speeds like this 3550 (background) use a tapered roller bearing for the input that exercise non function properly when faced with misalignment well. This is why late model transmissions demand that the bellhousing be at or within the 0.005-inch spec
To perform this alignment test correctly requires a magnetic base and a dial indicator. Nosotros prefer to remove the spark plugs from the engine and plow information technology slowly past hand using a ratchet on the crankshaft bolt. Never attempt to spin the engine with the starter motor.
Information technology'south this stack-up that can cause the misalignment between the crank centerline and the heart of the transmission'southward input shaft. When this occurs, the input shaft is forced into a diverging line compared to the crankshaft centerline. Because the input shaft is supported by the airplane pilot bushing or bearing, this misalignment forces the input shaft into a demark. Think of engine rpm equally a force multiplier that aggravates an already bad situation.
In the past, this wasn't a huge concern. Early transmission transmissions like the GM Muncie four-speed, the Super T-x, Ford Top Loader, and Chrysler A-833 all employed a brawl begetting to support the input shaft. These brawl bearings will have a significant amount of misalignment. The overall spec for transmission transmission alignment is 0.005-inch. So if the input shaft was off by mayhap as much as 0.010 to 0.015-inch, these older transmissions could accommodate that mismatch and allowed the driver to complete the shift.
Earlier testing for parallelism, McCombs says its best to apply a sanding block or whetstone to run over the engine block and both ends of the bellhousing to remove any dirt, debris, or burrs that may cause a misalignment. Even a small burr can easily multiply over the distance to the trans mounting confront. It'south too a good idea to run a large straight border over the block mounting surface to make certain zippo protrudes that could affect bellhousing alignment. We noticed on LS engines that the top portion of the rear cover plate volition hit the inside radius of some steel pocket-size-block scattershields that volition forbid the bell from sitting flush with the block.
In rare cases, the block mounting flange may not be foursquare to the crankshaft. We fix up our punch indicator on this old 327 cake and found the block was high on one side past 0.011-inch.
The best way to measure parallelism is with a magnetic base placed on the crankshaft with a dial indicator positioned then that the unit is placed 90 degrees to the bellhousing transmission mounting face up. We prefer to aught the dial indicator at the 12 o'clock position and take readings at the iii, six, and 9 o'clock positions as viewed from the rear of the engine. If the reading is more than than 0.005-inch, this will require modifications to the bellhousing in gild to bring the bell into spec.
After measuring dozens of bellhousings, one situation defenseless our attention. The application was an iron-cake six.0L LS engine in which we had installed a bandage aluminum engine bandy oil pan. When we installed the pan, it concluded up slightly rearward on the engine block. In a small- or large-block Chevy for instance, pan placement would not be a problem. But LS engines use the oil pan's rear face as a structural member that is bolted to an automatic transmission bellhousing.
On LS engines, make sure the oil pan does not contact the bottom portion of the bellhousing and create a trouble in parallel. We filed our oil pan until we could measure a minimum of 0.005-inch clearance between the bottom of the bellhousing and the oil pan. That solved our parallel problem.
In our case, the oil pan moved the transmission manual bellhousing rearward over 0.020-inch, which is iv times the allowable spec. To rectify this situation we had a pick to either relocate the oil pan or file the thick oil pan flange. Nosotros decided to file the oil pan because we didn't want to risk an oil leak by moving information technology. We filed the pan until nosotros created a 0.008-inch clearance between the lower portion of the bellhousing and the LS oil pan. Once this was corrected, the bellhousing was well within spec with only 0.002-inch of vertical top to bottom.
If there'south still an upshot, the side by side procedure is to check to see if the block or the bellhousing is at fault. Borrowing some other bellhousing to check is one mode to decide where the issue lies. If a second bellhousing creates a similar fault, then you lot tin can assume that the block may be the source of the problem.
Another option is to bank check the original bellhousing on a different engine to see how it compares. This may be more hard based on access to another engine of the aforementioned family unit. If the bellhousing too measures out of spec on a different engine, so you lot tin can assume (yep, we're aware this is not an ideal check) that the bellhousing is at fault. A final check would be to use the punch indicator to check the cake bellhousing flange at multiple positions to mensurate its relative location. Nosotros checked an quondam 327 Chevy block and discovered it was angled relative to the crank by as much equally 0.012-inch.
If the bellhousing is at fault, it's possible to take information technology to a machine shop to face the transmission side to foursquare it upward relative to the engine. This also means that in one case the bellhousing is machined, information technology tin can only exist used on that specific engine as it will likely be way out of spec on a different engine.
Assuming the vertical check is within spec, nosotros tin now move on to measuring the concentricity for the input shaft. Again, we'll use the magnetic base of operations and dial indicator but now the indicator will measure off the within diameter of the input shaft opening in the bellhousing. Once more, nosotros prefer to start at the 12 o'clock position and zero the gauge and and so take measurements at the iii, 6, and 9 o'clock positions.
Let'south assume for this give-and-take that the bellhousing is good in parallel but is off center in concentricity with 0.015-inch downward at the 6 o'clock position. This will frequently also reveal that the 3 and 9 o'clock positions will show inward motion. What we're looking for is the largest amount of motion on the dial indicator so correct for that maximum offset. In this particular case, information technology is at the half-dozen o'clock position with 0.015-inch of run-out. Since the maximum allowed is 0.005-inch, an offset dowel pin that can relocate the bellhousing upward effectually 0.015-inch is needed. Proceed in mind that when measuring concentricity, the punch indicator is displaying total indicator movement – what the engineers call total indicator run-out (TIR).
Merely when determining the amount to correct the error, we will use an kickoff dowel pin rated at half that dimension. Outset dowel pins come in 0.007-, 0.014-, and 0.021-inch offsets. Each offset dowel will compensate for twice its outset. So a 0.014-inch dowel will relocate the bellhousing 0.028-inch. The same doubling consequence is true for the other two dowels. This means total bellhousing relocation tin exist as much equally 0.042-inch.
In our situation where the bellhousing checks 0.015-inch downward relative to crankshaft centerline, we can use a 0.007-inch offset dowel to move the bellhousing upward by double its value – 0.014-inch. Theoretically, this should place the bellhousing with 0.001-inch of start which is near perfect.
Lakewood, Moroso, and others offer kickoff dowel pins. Before installing these get-go dowel pins, we like to mark the face up of the dowel with a Sharpie to indicate the position of the maximum amount of offset. Then, the non-offset portion tin exist driven into the block while positioning the offset in the desired direction. In this particular example, we placed the bushings with the maximum offset at the 12 o'clock position. Information technology's of import to make sure that both dowels are both positioned exactly the same or the bellhousing will not fit over the dowels.
Information technology's e'er best to recheck your work to ensure that the dowels were placed correctly as it's easy to get them backwards. In our case, the re-test revealed our bellhousing now within 0.002-inch of perfect alignment.
With most engines you lot have a choice of either a pilot bushing or begetting. If you choose a bearing, this ane uses a seal located just inside the tapered entry. Always place the seal pointed toward the manual. This retains the lube in the bearing and prevents damage to the bearings from outside dirt and droppings.
Equally we mentioned earlier, the key to checking alignment is to start past ensuring the bellhousing is parallel. If it is non, this creates a hole that essentially becomes an oval. No corporeality of changes with offset bushings volition right this. The bellhousing must get-go be as parallel to the block and perpendicular to the crank inside the 0.005-inch spec.
Information technology'southward also worth mentioning that creating this proper alignment does wonders for creating much smoother clutch operation because now the input shaft is not placed in a demark relative to the crankshaft. Proper bellhousing alignment will besides extend the life of a airplane pilot bushing or bearing. We know mechanics that practise non like pilot bearings considering they sometimes seize on the input shaft. This is nearly often caused by a bellhousing misalignment. When all the components are in most perfect alignment, the clutch and transmission will terminal much longer and perform as intended.
Armed with this updated data, it should exist relatively easy to accurately make up one's mind if your bellhousing is within spec. Once that's accomplished, that will make your 21st Century manual trans shift as smooth equally silk. And so you lot tin be that commuter who executes that perfectly-timed high rpm 4th gear shift to the acknowledgement of all your friends.
Source: https://www.holley.com/blog/post/how_to_align_your_bellhousing_for_maximum_transmission_and_clutch_life/
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