The Stovebolt Site Forums The Shop Area Engine Shop Distributor clamp help

My dad and I are having a heck of a time getting good access to the screw on the distributor clamp. A short screw driver doesn't give us the room to turn it, and longer ones don't get a good angle because of the pan draft tube. Do you have to remove the draft tube in order to get at the dist clamp screw? If so, how does it come off? Maybe there is an easier way? Also, the screw on our clamp looks like it has been worked over pretty good with a screw driver that was too small--the slot isn't very straight any more. Is there a good place to get a new replacement screw? Thanks in advance.

Greg, I just took mine off a few weeks ago and one off of a truck thats sat for 40 years. Loosen it up with some penetrating oil and use a long screw driver. I wedged the driver between the tube and the block and it aligned pretty good. There's not much handle to grab, but it can be done. I would think if you get it off you could match the screw at any supply store. Let's see what anyone else has to say.

gjberche,

It has been awhile since I have goofed around down in that area, so this might be bum info.

I thought the distributor was held into the block with a 5/16" bolt in a elongated hole to allow for timing adjustment. That bolt goes through the bracket you are speaking of, which is clamped around the housing with the funny screw.

The vacuum advance unit is also attached here, and maybe a octane selector if the rig is old enough.

That screw isn't made to loosen for distributor adjustment. It can be installed to the proper depth and tightened while the distributor is out...easy access on the bench with a screwdriver. I thought some of those had a small hex head as well as the screw slot.

Stuart

I guess it is possible I'm missing something, but I only see two places for adjustment on/near the distributor. I see the bolt which is the octane selector, and I see the screw that clamps the distributor shaft. This is a stock 235, and the shop manual refers to loosening the "distributor clamp", so I'm pretty sure this un-accessible screw is what I need to get at. Anyone else have any ideas on getting a good angle on this screw?

I had the same problem with mine. So I did away with the short 1/4-20 screw and used a longer bolt with a little collar. Now I can tighten the clamp with a 7/16" wrench.

I've got some pictures of the modification in My Webshots Album "making point setting Easy". Check it out and put this little inconvenient problem to bed for good!!!! Hope this helps.

Some draft tubes are bolted on with a bracket.My 216 just pulls off.Grab it and pull on it,it should come off.You are talking about the vacumn advance clamp,right ? Removing the draft tube will make it easier to tighten or loosen the vacumn advance clamp.

gjberche,

As I recall...if you were to dissect all the crap at the base of the distributor you would end up with; a octane selector (if equipped), a vacuum advance unit and a clamp assembly. These are all separate pieces that are assembled into one unit.

The distributor housing goes through a hole in the vacuum advance unit and inserts into the block. With no other pieces installed, you cannot clamp the distributor to anything. The clamp assembly must be fastened to the vacuum advance unit.

The clamp assembly holds the distributor to the vacuum advance unit and the vacuum advance unit allows for distributor movement for timing purposes AND holds the whole shebang to the block.

Any timing is done by loosening the bolt and rotating the distributor. The clamp holds the distributor to the vacuum advance unit so it can rotate when the vacuum can is acted on.

If you look at a naked vacuum advance unit, you will see the elongated slot for retention and adjustment, and you will see it has no way to hold the distributor to it.

After all this typing, I sure hope I remembered this correctly.

Stuart

That is a really nice modification, Fla54Chevy3100. I'll probably try something similar if I can get the clamp loose! I'm thinking the easiest solution is removing the draft tube--so do all the draft tubes just "pull off" after the securing bracket is disconnected? I can't imagine it screws in since the down tube wouldn't allow the whole assembly to rotate. Thanks for your help Tim.

gjberche,

Here is a picture that shows the setup as I remember it. The bolt that is pictured in the elongated slot is how you adjust all timing.

The screw is difficult to access because it was never intended to be accessed on the rig...it isn't a adjustment point, only a assembly point.

http://chevy.tocmp.com/1948_51truckshop/51ctsm0619.htm

Stuart

Tim...it is just a "press" fit....remove the clamp, if you still have one, and use a soft hammer to loosen it up. Once you have it out you can get a BF screwdriver on the 1/4-20 screw. You really don't have to weld the lower nut on the modification....a nut,lockwasher and nut, torqued down on a longer 1/4" bolt will do the same job. It sure simplifies making adjustments. Hope this helps.

I think we are talking the same stuff here, but I took the liberty of annotating the shop manual page you linked to so we can be clear. If you look here you will see that I've marked two things. The item marked "A" is what I am calling the octane selector. You can loosen this to do minor timing adjustments, but it is not meant to be used to set the initial timing. The initial timing should be set with the octane selector in the middle ("0" position) so that you would have some room to make adjustments later.

The item marked "B" is the nut-side of what I am calling the distributor clamp. I could see also calling it a "vacuum advance clamp" since it really does secure the distributor body to the vacuum advance bracket (which is secured to the block via the octane selector bolt). The other side of "B" is a regular flat-head screw that I am having trouble getting good access to.

The shop manual says (highlighted in green) to "loosen the distributor clamp and rotate the distributor body...". That is why I'm surprised that the screw is such a pain to access, especially if it is part of setting the timing. Perhaps the engineers figured that once the timing was set at the factory it would be "close enough" such that the octane selector would be the place where owners or repair garages would adjust the timing. That would be consistent with what you said about it being an "assembly point" and not an "adjustment point". What do you think?

Sorry for the long post--I just like to be as clear as possible. And thanks entertaining my questions. This is my first time working on a vehicle like this, and I'm the kind of guy that "needs the data".

gjberche,

Your are right on the money, and of course who could doubt the shop manual.

That screw is difficult to access. If the plate bolted to the block (octane selector) is centered in its slot and the engine is then timed to the ball or mark using the troublesome clamp, then any subsequent timing issues can be addressed with the bolt/slot in the vacuum advance plate.

I guess my thought was that the clamp screw was so difficult to get to...why use it! Use the bolt and slot in the hold down plate.

When the distributor is removed with all the hardware still attached, it allows you to reinstall close to the original position...the hole where the bolt goes lines everything up for you. This only applies if you locate #1 and reinstall accordingly...but we all to that!

The advent of the modern timing light makes things quite a bit easier.

Stuart

The good thing is that once I get the timing on the BB, I shouldn't have to mess with this silly clamp screw for a long time, as the octane selector appears to have a decent amount of play for small adjustments.

You'd think the octane selector would be called something more true to its usage, like "timing trim" or "minor timing adjuster". When you start talking about "octane" I think it just confuses things, especially since fuel octane is just one reason why you might need to adjust the timing.

Greg,

I just looked at my 235. I think it is a '58 version. The problematic clamp screw was flip flopped in the clamp and has a small hex head that is real easy to get to.

This engine has the same setup...a bolt clamps through a elongated hole to allow the timing to be adjusted. There are no BTDC/ATCD marks on it as was on the plate for the 'octane selector'.

The octane selector was a merely the timing adjuster (GM gimmick) in drag. I didn't realize a '54 still had this device on it. I think some were adjustable by hand, and I think all had timing marks stamped into the plate...does yours?

When we would time one of these engines, the bolt was loosened and the vacuum advance was a real convenient handle to grab to slightly turn the distributor either way...then the bolt was tightened down.

Stuart

Stuart,

Yep, the 235 in my '54 (which I believe to be original) does have the octane selector with timing marks stamped into the plate. The shop and owner's manual also show the timing marks stamped into the plate.

Your description of using the vacuum advance as a "real convenient handle" is *exactly* what my dad and I did over the weekend!

Sort of switching gears a bit: when using a timing light and the octane selector set in the middle, the BB on the flywheel was just outside (above) the little window next to the starter. Is it correct to say that if the BB is appearing above the timing window that the timing is late/retarded? I know the radiator fan turns in a clockwise fashion if you are looking at it from the front of the truck, which I'm assuming means the flywheel also moves clockwise.

Thanks for your help--these days I have more time to think and read about truck stuff than I actually have time to go out there and work on it!

Greg,

Your right about the flywheel rotation. If one were to rotate the flywheel so the BB and little pointer lined up perfectly, this would be TDC...where #1 plug fires.

Any flash of the timing light BEFORE this point would be ADVANCE. Any flash AFTER that point would be RETARDED.

All engines run with some amount of advance. What I have never know is if the Chevrolet engineers built some advance into the pointer/BB relationship...maybe someone knows.

The pointer/BB alignment happens twice during the two rotations of the crank. One alignment is correct, and one isn't!

Stuart

Acording to the 1954 Shop Manual, setting timing to the ball on the flywheel produces an ignition timing of 2 degrees ATDC (After Top Dead Center) on a 235 engine and 5 degrees BTDC (Before Top Dead Center) on a 261 engine.

Tim,

Interesting info. I guess I stand corrected on my statements about never timing a engine ATDC.

Stuart

Stuart,

I was a little suprised, too. I knew the 261 was 5 degress BTDC but I thought the 235 was the same.

Tim

I did the bolt mod on mine a while ago, too. It allows me to use a wobble extension. The slot is just a pain to get to, especially once it starts to wear out.

Can someone explain to me why setting timing to the ball on the flywheel produces an ignition timing of 2 degrees ATDC (After Top Dead Center) on a 235 engine? I thought you'd always want your baseline timing to be few degrees BTDC so that you maximize the pressure, force, and fuel economy during the compression stroke.

Is there something particular about the 235 engine that would make you want to have the baseline timing retarded a bit? Or maybe the engineers at the time were worried about detonation and the availability of the correct octane fuel? I know the flywheel ball is just a starting point, but I wouldn't have guessed that timing to it would produce 2 degrees ATDC. Hmm..

Greg,

I was surprised too. My guess would be ease of starting. As soon as the rig is started and brought off idle, the vacuum advance does bring the timing BTDC, and don't forget that the centrifugal system is just hanging out waiting to get into the fray too.

Although initial timing might be retarded, for all intensive purposes, the engine runs most of the time advanced.

I think any shade tree mechanic would dial in some initial advance anyway...to hell with the shop manual, we want power!

Stuart

I hadn't considered the ease of starting issue, but that makes sense. And I think you are right about the shade tree mechanic approach--especially since a lot of people opt for 12 volt starters anyway.

I'll admit that I haven't disassembled my distributor yet, but I wasn't aware that the original distributors had mechanical (centrifugal) advance systems built in to it. I guess I'll find out when I take mine apart!

Thanks again for your help.