How to shorten Jabiru push rods- a treatment.

[RFguy]

@MarkDunn developed a good technique. We did all 12 rods yesterday, with technique optimized over the course. Helps having two people, runs like a production line after the first couple. This was to shorten the rods from 207 to 206 to permit clearance for valves to fully shut , for many engines out there with negligible clearance and or head recession.

 

First, I made up a measurement jig using an old C clamp , and M4 screw in one end .. this way we could quickly measure/judge the length, ... instead of  messing with a large vernier caliper.

 

The rod end #1 was put into a lathe chuck tightened around the push rod tube just ahead of #1 end.

 

A slotted U-piece was fabricated and attached to the screw drive traveller of the lathe.

 

The #2 end of the push rod could be dropped into the slot that is just wide enough to accept the tube, but allows the ball end to be pulled. (Mark we should have got some photos) .

 

The traveller is extended, pulling the ball end off the rod. Depending on what gives away first, either #1 or #2 end is removed.

 

The Ball ends appear to have been pushed onto the rod until the rod bottoms out inside the ball end 'female'. IE you cant push them on any more.

 

set measuring gauge (set gauge  screw length) or use something else.....to check length now the end of removed to make sure you dont miss something - IE maybe one of the balls wasnt actually driven home during manufacture.

 It's also worth looking at the  end that did not come off, it might have moved. So, this may also need to be pushed  back to home as below.  Ours didnt move- once one end moves, it keeps on going.

 

The rod is then  mounted in the lathe chuck for rotary work.

 

We took 1mm off the tube with a cutter, and then put a slight chamfer on the end of the tube to make mating into the ball end easier.  

 

Linishing paper was used to take a few microns off the tube diameter in the overlap area. This makes it MUCH easier to push on. Remember- these ends are under pressure- they are NEVER going to fall out, They could just sit there....

 

Heat the ball end on a hot plate-stove to maybe 180-200 C. We used gas but I am sure  heat from renewables would also work.

 

Put the tube tight in the lathe chuck. tight ! Clamp it close to the rod end that will be pushed on. Otherwise it will be a long column under compression
 

Of course if you crush the tube, you went too tight We crushed NO tubes, and infact didn't even mark them at all....

 

The next bit you need to achieve before the heat soaks...

 

Grab with end with pliers  off the hotplate. push onto the end of the tube a little. then grab a piece of soft young pine stud , put that between your 2 pound hammer and the the ball end and wack it home.  you'll see it stop

 

check into the measuring gauge for desired length.

 

voila. 12 shorter pushrods. money saved - $900.

 

 

Edited June 5, 2021 by RFguy

[facthunter]

On such small diameters 200 degrees won't give much dimensional change, but the principle's there. The important thing is to start the  "pressing on" with everything square. and a small chamfer. Progress eh!   THAT would HAVE to make some difference on how it will behave in service. Nev

[Yenn]

How about putting the ends in the freezer for a little more clearance?

[RFguy]

Freezer. yep. Could do that also . but they are on decent as they are. and you wouldnt get the difference temperature as readily.   a little diameter reduction with linishing paper  I think is very fair. These things are not going to fall apart.. as along as the oil doesnt get out...

 

Anyway, pretty foolproof method if people need to do such things

I am SURE those mechanically minded out there have already done same. 

[onetrack]

On such a small diameter as the pushrod, you won't get any appreciable amount of shrinkage just from a regular freezer.

Using liquid nitrogen or dry ice, you get more shrinkage - but on a small diameter item, the shrinkage is barely measureable.

On a component that's 50, 75 or 100mm in diameter, different story, the shrinkage is very measureable.

[facthunter]

There's practically No oil pressure in the push rods. The top is open. The little spring loaded ball holds all that in the follower. Pressing the ends on with the right interference fit is fairly universal though a lot of high volume stuff is resistance welded hollow ball in each end.  Failure is unlikely  if the fit is right. For a long time now Gudgeon pins have been press fitted into the conrods and turn in the piston bosses only. Failure of the fit to locate is almost unknown. Probably less than circlips.  Nev

[RFguy]

New push rods installed.

clearances all in desirable range....

Engine start. 

Runs well.

Makes slightly higher static RPM than before (engine is clean, so not unexpected) .

 

Might have slight oil leak on O ring of a push rod O ring seal on the block. Not sure yet. Now  pressure cleaned and will have a look next week after a little running.

 

[Thruster88]

Good luck and enjoy your flying Glen. It has been a pleasure watching you work and the thought you put into every aspect. You have the knowledge to write the SB addressing the marginal clearance engines. A simple check could save a lot of grief for owners of affected engines.

[RFguy]

Thanks Stuart. Your guidance was essential and the guys at LVA were very helpful.