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MG MGB GT V8 Factory Originals Technical - Lifter Adjustment
|I've finally completed the basic assembly of my engine and I thought I'd share my experience with setting tappet preload. There are several things in the build-up that affect tappet pre-load such as head or deck milling, depth of the valve seat, length of the valve, push rod or lifter, trueness of the rocker shaft mount pads and rocker pedestals, and condition of the rocker hardware. All of those parts in my engine were new, and my machine work was done by a very competent shop and I still had huge differences in tappet pre-load from lifter to lifter. When I initially test assembled the rocker gear, I had over .100" of preload on some lifters. A small difference in valve stem height is multiplied by 1.6 (rocker arm ratio) at the tappet. Similarly, shims installed under the rocker pedestals reflect a value 1.6 times greater at the lifter. I had at least one lifter on each bank that was quite far out of limits, so my solution was to use different pushrods of shorter length. The standard pushrod is 8.031". I needed a shorter pushrod because I was having to add roughly .060" of shim under the rocker pedestals. I found that the small block chevy pushrod in +.200 oversize is 7.996" long which is roughly .035" shorter than the stock Rover pushrod. I also used a couple of +.150" small block pushrods to fine tune tappets that were still unequal in height. I finally wound up with 14 +.200" chevy pushrods, 2 +.150" chevy pushrods and .012" of shim under the pedestals in order to achieve .030" of uniform lifter preload. The pushrods are 5/16" diameter, and it's a good idea to enlarge the hole in the cylinder head where the rods pass through, as even the smaller stock rods sometimes rub. The oversize chevy pushrods are commonly in stock at hot rod shops and large mail order Performance shops, and they are availabe from Federal Mogul through Checker Auto Parts and other FM retailers. I paid $80 for mine (Comp Cams) at a local Hot Rod shop, but they are available for less. I hope this information is of some help and I apologize for the length of the post.|
65 "B" 4.0
|Apologize!? Nonsense! This is exactly the kind of info. that saves the rest of us time and expence. We didn't even have to ask! Thanks Gordon.|
Thanks for the great free info. I will check on it tomorrow first job!
|werner van clapdurp|
|Gordon, how do I check if my 3.5 has a tappett preload problem...........would a compression test show it up??? |
If your engine is running well, it achieves the maximum RPM you desire within reason, it's smooth operating and there is no excessive noise coming from your valve train, I would not be concerned about the pre-load. If you do have valve train noise, as in tapping, you may have improper preload. Before I proceed, let me say that I wish I had installed adjustable pushrods in my engine. Without having adjustable rocker arms or pushrods, the only way I know of to verify lifter preload is to remove the intake manifold and gasket to gain access to the lifters. If they were merely improperly adjusted, then shimming the rocker stands or changing pushrods should solve your problem. However, If a lifter(s} is bad, I would also look carefully at the face of the lifter and cam lobe for evidence of abnormal wear. Some cam manufacturer web sites show examples of badly worn cams and lifters for reference. If there is no abnormal wear, but the lifter won't pump up, replace the lifter and check the preload on all. I personally would recommend replacing all the lifters at the same time, to avoid repeating the work a short time later. If you will do a search on "tappets", "Lifters" or "Preload" in the archives, you will find tons of excellent advice on the topic. The website for RPI in England also has technical info on the subject.
Crane cams sells adjustable pushrods if required. Although more than costly it could simplify the job as pushrods are sold in .050" increments and no shims are required.
Lifter preload can be checked without removing the intake manifold and gasket. Remove the rocker covers and position the lifter on the small dia. or heel of the cam. Set a dial indicator on the rockerarm and slowly release the rockerarm tension until just touching. This will be the preload. If you don't have a dial indicator, mark the pushrod when set with preload and with rockerarm released then measure the distance between the lines
This thread was discussed between 07/04/2002 and 15/04/2002
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