How To Make a Locking Dowel Pin?

[snub]

Sorry for the confusing title, I'm not sure what the proper terminology might be. Basically, most automotive engines have 2 dowel pins on the back of the block that register the bellhousing so that the transmission input shaft fits perfectly into the pilot bushing or bearing in the back of the crankshaft. In days gone by this wasn't super critical as the input shaft bearings were simply roller bearings. Todays modern transmissions use tapered roller bearings so it's necessary to "dial in" the bellhousing.

Long story short, several manufacturers offer "offset" dowel pins that you insert into the engine block ( they are slightly undersized) and then dial in the transmission register on the bellhousing, turning the dowel pin to where it needs to be, then locking in the dowel pin.  I want to make my own for 2 reasons. I think I can and I'm cheap. They want $30.00 to $50.00 U.S. for a pair, plus shipping, exchange etc. I was able to accomplish the machining of an offset pin, but I can't figure out how they get them to lock. The instructions from one of them says to " tighten the set screw until the pin is locked in place".

The part of the dowel that goes into the block is slotted so that it can expand, but I can't figure out how a set screw would accomplish this. Do they thread the pin on a  lathe with a taper? Is the set screw tapered? I tried using a pipe thread tap and a male pipe plug to accomplish this, but the sizes of pipe thread aren't quite right for the job and I don't think that is what they are using. Some pictures below of the commercialy available dowel pins.



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[Pete.]

I never heard of these off-set dowels before, but they would be easy to make. Drill your dowel the 2/3 depth and tap for a grub screw then drill the remainder for a pin and turn a shallow taper, say 8 degrees included angle. Use the compound to make both the tapered hole and the pin on the same setting. Make the pin the same diameter at the big end as your tapping size and the grub screw will never bottom out. Make sure you cut your expansion slits to the whole length of the tapered hole.

The one I drew here is for a 10mm dowel, 20mm long, 8 degree taper and drill/tap for a M6 grub screw. (I know the dims don't add up to 20mm but it's good enough).

[wgw]

I've never seen one either. I would just drill a clearance hole for the grub screw and a tapping size at the split end. Just use a taper-tap at the split end and the screw should expand it. Might have to experiment a bit with tapping depth.

[NeoTech]

I actually use RobbMC offset dowels between my Bigblock engine and the TH400..  These are used when tolerances is going bonkers because you have to much horsepower that the little play, or rubbing that can occur will make explosive or destructive effects on your whole setup.. Like me having 1100hp, with a stupid amount of torque in the beginning..

The drawing is quite accurate, just that the set screws used are long ones that are somewhat conical and needs alot of brute force to get in place..

[Lew_Merrick_PE]

Back in the day we used to use tapered pins to align things to insanely close tolerances.  The (.250 inch per foot) tapered reamers would allow you to "pull" a few ten-thousandths of an inch with moderate ease and establish good positioning and shear load capability.  It was an art required of apprenticeships back in the day...

Now, knowing nothing of the basic size or compensation required, my guess is that one portion of the OD cylinder is eccentric to the other portion of OD cylinder.  The question becomes how to "lock" the (installed far side) of the pin when you have it correctly adjusted, correct?  One of the tricks used in the munition industry is to use an inserted ball bearing or "barrel shaped" plug that is driven by the locking screw to expand the "fingers" of the "far side" to make the lock.  The thing that leaps out at me is that you will either need a very thin "wall" (with concomitantly reduced shear capability) or you will need a lot more slits to reduce the bending moment of inertia of the "fingers."

[snub]

Some very good info gentlemen, thank you. I found a picture of the Lakewood dowel and it appears as if they have just run a taper tap not quite to the end. It might be just that simple. In experimenting I made a dowel .001" undersize of the hole in the engine and using my pipe thread method it didn't take much to expand the dowel so that it was locked.

[snub]

Apparently it is....just that simple. I first cut 3 slots 120 degrees apart, then ran the tap through, not quite to the end. As I was tapping I could see the tap pushing the walls apart as it ran through. When I removed the tap the cylindrical section returned to its former dimension. When I ran a bolt through with the dowel pin in the block it tightened up securely. A thing of beauty!