I am putting a mechanical tach back in the race car. Taking out the electric and getting more "vintagy" . I plan to go back to the original TI distributor tach that was in the car. I have heard that "back in the day" racers would machine the tach to accept a roller bearing for the shaft. Does anyone have details on how to modify the tach and what bearing should be used or does one of you do this modification? Also since I will now be driving a tach cable what are your ideas on how to make the tach drive gear last. I used to have problems with the tach drive gear getting chewed up. My memory is a little foggy here but it seems like we took out the nylon button and put in a bronze button. Finally what is the recommendation on the gear drive. It is driving a high volume wet sump gear pump in a racing application so the loads are fairly high. Is a bronze gear accepatle and who supplies a good one.
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Re: Distributor Modification Roller Bearing
Jerry, Sun Tachs were used in the race cars in my area for the TI cars. Now this was in the 60's. The guys didn't mess with the distr/cable tachs for racing. We used a lot of Stewart Warner tachs also.
Maybe Clem will chirp in to verify my memory. JD -
Re: Distributor Modification Roller Bearing
Hi John.. I appreciate what your saying. I suspect that people just got tired of fooling with the mechanical tach drives...however I am hoping to get into Monterey Historics road race and the old mechanical tach has a nice vintage look to it so that's what I'm trying to do. I have the old Jones mechanical tach that was in the car in the 60's and I've just had it restored and calibrated so I'm on a path here. As you know restoration is not always best available technology, oh well.Comment
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Re: Distributor Modification Roller Bearing
Jerry-----
The only "roller bearing" set-up for the tach driven gear that I know of is the one that was originally sold by High Tech Innovations. They went out of business quite a few years ago but I think the roller bearing set-up they "pioneered" is still available from most of the Corvette parts vendors. Basically, what it is is a special tach drive fitting which has been modified with the installation of 2 sets of small roller bearings. The special driven gear then turns on roller bearings instead of just the fitting, itself, as in the stock set-up. I don't know how much good these things do, but they can't hurt and they're not really all that expensive.
As far as driven gear end thrust is concerned, the nylon button actually works pretty well. However, if you wish, you can change it to the bronze button. The important thing is to keep the driven gear from grinding a hole into the end of the gear cavity. When the gear does this, it eventually becomes mis-aligned and both the drive gear (on distributor mainshaft) and the driven gear get "wiped out". This is probably the most common reason for tach drive gear failure.
The other reason which I believe is a cause for tach drive gear failure is too little end play on the mainshaft. I know that all sorts of folks and even some earlier editions of the Chevrolet Service Manual recommend using shims to get a very tight end play of only about 0.005" maximum. However, every NOS GM tach drive distributor I have has an end-play of right at 0.060". Curiously, this is right in the middle of the range of GM manufacturing specs for the end play. You can do as you wish but all of my distributors will always be set within the GM manufacturing specs for end play.
As far as the distributor drive gear goes, if you are not using a steel core, roller cam of the type with a non-pressed on, cast iron cam gear, then you don't need a bronze gear. The stock gear will be fine and is what I recommend.
If you are using a steel core, roller cam without pressed-on, cast iron cam gear, then you have a few options:
1) You can use a bronze drive gear. Any of those available from the major cam manufacturers like Comp Cams, Crower, etc will be fine. Keep in mind, though, that bronze gears are HIGH WEAR pieces. I would recommend checking it after each race and replacing it after 2 or 3 races. I don't recommend a bronze gear, at all, for the street;
2) Use a Comp Cams composite distributor drive gear. This gear is the "21st Century state of the art". It's not cheap, though.In Appreciation of John HinckleyComment
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Re: Distributor Modification Roller Bearing
WOW. This is exactly what I was looking for. You are a wealth of knowledge. Thank you. JerryComment
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Re: Distributor Modification Roller Bearing
Jerry Hi Tech was located in Florida, several vendors, including me sold their products. I am sure that somewhere that someone has one of their distributors. I would go over to the HAMB (Hokey Ass Message Board) and go to the classifieds to look for or do a want ad for one.Dick WhittingtonComment
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Re: Distributor Modification Roller Bearing
use moly kote on the tach drive gears and on the gear shaft and housing and you will have no problems. make sure the tach drive cable is as straight as possible and you can even use a 90 degree adapter on the dist to do this. the straighter the cable the less stress on the gears and bushingComment
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Re: Distributor Modification Roller Bearing
Jerry------
The fellow, now deceased, that used to be the owner of Hi Tech Innovations was a great guy and I used to make a point of talking with him at the various Corvette shows. He had a lot of good information. However, one thing he used to tout was the stainless steel version of their fitting assembly. He said the stainless steel provided a much better "bearing surface". I don't think there was any profit motive influencing his opinion, though, since both the bronze and stainless versions were the same price.
I'm a guy that loves stainless steel anything so one would think I would just agree with him 100% on this. But, as it turns out, I did not. In the sort of assembly they offered with the roller bearings pressed into the fitting, the fitting has absolutely no bearing surface functionality, at all; the roller bearings provide the bearing surface. So, it really does not matter whether the fitting, itself, is bronze or stainless steel. Since it doesn't matter functionally and since the external end of the fitting can be rather easily seen, why not just go with the bronze for the sake of originality? Once installed, no one will be able to discern the difference between a stock fitting and the roller bearing-equipped fitting. If one uses the stainless steel version, they will.In Appreciation of John HinckleyComment
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Re: Distributor Modification Roller Bearing
Dick, The father died X number of years ago. I used to see him at the BG shows at Springfield, IL (my favorite BG show) and he was a really fun guy. Partied the whole time.
Then his son I believe took over but sold the business. I think someone in ILL bought him out. I will find out. JDComment
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