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I'm in the process of replacing the vacuum brake booster in my '81 242DL B21F. The shaft that sticks out of the front of the booster and goes into the back of the master cylinder is suppose to be adjustable in length. So far I've been unable to turn the little nut on the end of the shaft to set the length to specification.
The instructions say there should be 0.020" - 0.030" clearance between the end of the booster shaft and the back of the master cylinder piston. This is to allow the springs in the master cylinder to completely retract the master cylinder piston when your foot is off the brake peddle.
IPD has this video about it: https://www.youtube.com/watch?v=Tde6xd8ezr4
Any advice please.
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Hi,
About that 14 point lock nut you really need to order special tool
Search for "brake booster wrench" in eBay
Look for Kent Moore tool
Contact the seller if this tool applies to Volvo booster. Volvo booster usually from Bosch, DBA (disc brake australia) or Bendix
Regards,
Amarin.
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Bought it. Thanks very much.
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For the benefit of others later finding this thread, the Kent Moore tool is J-37767 for that 14-point lock nut, variously called a Brake Booster Push Rod Wrench, Brake Booster Adjustment Tool, or Brake Booster Adjuster. Correct? Some online tool places are selling those for ridiculously high prices, with legitimate private sellers more in the $30-$50 range on eBay.
Being cheap, I'm trying to think of ways I might make a home made wrench that wouldn't risk damaging the nut or shaft, yet provide a really good grip. That soft aluminum clamp block in that old Brickboard posting is one idea. As was also mentioned back then, slip joint or other pliers with a curved section of coarse teeth that closely match would be worth a try as long as you don't slip or strip the nut. Now that I've seen what they look like, I'd probably try to fashion something like that Kent Moore tool out of flat bar, drilling an undersized hole and using a Dremmel diamond bit to carve for the 14 teeth, possibly with a very slight taper for a tighter press fit, maybe even try hardening it afterwards.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Hi Dave,
I’m with you on your post as well.
Too much $ for a wrench to turn something that really shouldn’t be all that tight to begin with.
I have known of machinist and mechanics alike, that think they need every special tool or gauge made to do a certain job.
That whats having inexperienced engineers paint themselves into a corner to cause some nothingness tooling to be so expensive.
I’m looking into breaking down a M47 and so far I’m finding work around and got the fifth gear assembly apart.
But then, I’m not using a shop hour time clock. 🤫
There is never going to be any torque applied to the rod as it’s in a compression situation besides.
I wouldn’t grind away the radius ball end to get even with anything or anybody!
Yes, this is an interference fit thread caused by the straight knurling process to bring the walls of the shaft or the tubing into play.
The example shown in the photos are just the opposite of rolling in some external threads into smaller shafting.
The metal is displaced upwards to the size of the standard thread and therefore saving weight and expense of a larger bolt behind the threads.
This way there isn’t an actual cut thread in the rod, or at least, a full depth of 75% anyway.
That is what machine taps leave in a hole.
No such thing as the a theoretical 100% thread being possible.
There can be Enough torque is at 75% to break taps, especially, if they get slightly dull.
If you open up the bore of a hole a few thousandths the percentage of thread drop dramatically but easier to tap.
My Rule of the thumbs is @ 75% thread, the head or the body of the threaded member will break first, before the threads pull out of most materials.
Holes threaded to a depth of 1 and one-half times the diameter of the thread is only needed.
Any longer than that, the laughing starts with us machinists, as this being an adjustment screw HOLE! 👀🤔😀😊🙃🫢😎 screws yes, but not the holes?
Rolling threads is very common practice and they can actually be stronger than cut threads. The body of the thread peaks are denser. It might be considered a cold forging process.
There’s no need to get over zealous with that thread in the rod. It never gets any torque from anything during its operations.
Just grip the rod to prevent slippage with a small curved vise grip pliers and turn the acorn end.
.163 is less than than the amount of thread showing.
If you had more than that it would really worry me that some is a miss and what compatibility issue there might be with M.C.
I’m surprised you have to move it that far to get the rod away from the master cylinder piston.
If it’s way too long the cylinder cannot get back far enough to get the brake fluid on a return stroke.
It won’t pump up correctly as in this case must be happening but not noticed as of yet.😬
If it’s too short the foot pedal goes down too far to get a full pedal.
A little distance change on the short end of the foot pedal make a big difference on the long end your foot uses.
The .020-.030 space specification is probably the best of both worlds but no more than that is needed.
Oh, He will get the adjustment made or else!
How have you come out or your brake pads and rotors dealings?
The Brickboard was down to me for several days this last week.
Phil
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Hi Phil,
The Volvo green manuals and TSBs are full of special tools. Many are Kent-Moore and were only available through dealer service. All were much more expensive than you'd want to pay. I've always wondered how many of these special tools the dealer techs had access to or bothered to use in their shops. Once you get past a crank and camshaft pulley hold tool, home made or otherwise, I can't think of many Volvo special tools I've ever really needed where I couldn't figure a decent alternative. Only special tool I wanted that I couldn't justify was a Volvo-only belt tensioning gauge for the DOHC 16-valve B234F timing and balance shaft belts with the early manually adjusted belt tensioner. A timing belt failure was bye-bye valves (interference engine), so something you wanted to get right. A balance shaft belt failure would often take out the timing belt. The gauge and the spec were in unspecified mystery units that didn't correlate with any units I could derive off a standard belt tensioning gauge. I made do with other methods for setting the tension as in my FAQ writeup, probably much like any indie would do.
Re. my pulsating newish front rotors, my latest post was about the very odd coincidence of the pad sized thin areas being at the rotor balance edge milling on both rotors. Weird. A Bosch manufacturing problem?? As I mentioned there, I'll make one attempt to dress the rotors back to full health, but I rather expect it won't last long unless I change to different pads better matched to my driving conditions and rotors better matched to those new pads, basically stepping up a whole level in pad and rotor quality. All those years on my earlier Volvos and no such problem as this with good, basic quality rotors, leastwise so early in rotor life, but then I've got double the miles on these cars than before. Got two 940s going with the exact same front Bosch pads & rotors and same drivers. I rather suspect the second one is starting to suffer from the same uneven friction surfaces in the rotors, so now on my inspection list. I've already got better rotors and pads standing by for when my experiment fails. Failing that, I'll start packing a ship's anchor on a rope to throw out the window when coming to a stop on the highway, or else reverse thrusters buried in the grill. There's always a way around it, just like special tools.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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Looked at the link you gave, great pictures for everyone to see what we're talking about, thank you.
You all are not going to believe this! I just had a major brain flash. That grab nut fits a 12 or 6 point box wrench perfectly. So it must be 12 points not 14. That grab nut is also the same size as the adjusting hex on the threaded rod so the box wrench fits right over it.
I suspect the grab-nut looks different than an normal nut with flats because it had to be formed onto the end of the push rod which has only a thin wall.
By the way, on my booster these are 7mm wrench size.
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Please ignore my previous speculation. It does not work.
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Yes there is the "grab nut" on the protruding end of the booster push rod. The adjusting nut(I've been calling it) looks similar to an acorn nut, is really a hex on the end of the threaded rod which screws into the chromed push rod.
Since there are no flats on the "grab nut", just the "teeth"-14?, I can only hold it with locking pliers. The "grab nut" appears to be formed on the end of the push rod.
So, the assembly order looks like: chrome push rod - grab nut - threaded rod - hex
I'm going to look into spacers and shims before I just grind the thing off!
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Have you managed to check the clearance? If you think it is wrong, what is it? A drop of Loctite might have been used on the nut. Doesn't seem a bad idea. Loctite will loosen with a bit of heat.
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0.163" interference. Rod is too long. Measured with dial calipers. Lots of videos and descriptions online for how to do it. If there is locking compound on the threads is must be the permanent type. I have not tried heat yet, thanks.
There is also mention of using spacers or shims but I have not researched it yet and would the right thickness be available or must it be custom made?
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