Crank rebuilders - old DKW Hobby 2-stroke?

[malcolm2010]

Can anyone recommend an outfit to rebuild a small 2-stroke crank? I'm in the process of restoring a 1957 DKW Hobby scooter - it's a 75cc 2-stroke.

The issue I can't get my head around is how the bottom-end roller bearing will be renewed. Looking through the lubrication slots in the big-end, I can see that the rollers are packed in solid, without gaps or a cage. I have a parts list but unfortunately the pages covering the crank are missing.

I assume the big-end bearing surface would be ground out, the crank pin ground down a little (?) and new rollers obtained from somewhere/somehow?

Can anyone explain this process or point me towards a company who do this sort of one-off rebuild? I've found a few companies on-line but they only seem to do modern rebuilds where they use a new pin and press in a whole new bearing.

Any help would be greatly appreciated.

cheers

Malcolm

[gammaguy]

Can anyone recommend an outfit to rebuild a small 2-stroke crank? I'm in the process of restoring a 1957 DKW Hobby scooter - it's a 75cc 2-stroke.

The issue I can't get my head around is how the bottom-end roller bearing will be renewed. Looking through the lubrication slots in the big-end, I can see that the rollers are packed in solid, without gaps or a cage. I have a parts list but unfortunately the pages covering the crank are missing.

I assume the big-end bearing surface would be ground out, the crank pin ground down a little (?) and new rollers obtained from somewhere/somehow?

Can anyone explain this process or point me towards a company who do this sort of one-off rebuild? I've found a few companies on-line but they only seem to do modern rebuilds where they use a new pin and press in a whole new bearing.

Any help would be greatly appreciated.

cheers

Malcolm


HPE engineering in Christchurch

03 3794166 i think is the number

[Grumph]

HPE don't have the precision grinding facilities as far as I know..
Gav Hill will probably recommend Barry Lynch, Beaumont Engineering ChCh who would certainly be able to do it.
More local to you, try Colliers in Levin.

The OE bearing will be what's called a crowded roller - no cage.
If someone can source a modern bearing - caged - this would be the way to go.
TKRJ have an online catalogue quoting bigend bearing dimensions....

[malcolm2010]

Thank you very much Gammaguy and Grumph - I'll do some calling around. But from your comments it sounds like the crank can definitely be rebuild, even if OEM parts are unavailable.

I have yet to order the over-sized piston from Germany, as I'm waiting for the cylinder to come back successfully from the cast iron welder (I cracked the flange getting the old piston unstuck - I still feel bad about that).

Gammaguy, so from your comment I take it that the bottom end is ground out to form the new surface for the rollers. And presumably the pin is also ground down to get it round again. I'm a bit confused by all this as I was under the impression that things like crank pins and bearing sruface were case hardened so you can't grind them away much?

Thanks again for your help.

cheers

Malcolm

[Grumph]

There's a lot can be done in this area. From the relatively simple cleanup grind of the pin and hone of the rod eye to suit oversize rollers - good if you can source oversize rollers....
To opening out the pin hole in the crankwebs to accept a more modern crankpin and rod assembly.
The second option is more common on 4 strokes as rod length is not critical to the motor running...ie,you can shorten or shim a barrel. But on a 2 stroke you're largely stuck with the same length rod and piston or your port timing becomes a mystery.

Pins can usually be cleaned up to a depth of say .010in without going through the hardening. Small continental crankpins in my experience are usually very good quality and have good depth of hardening. You can see the pin ends so can measure the pin OD.Length is an easy measure.Bearing OD inside the rod can be worked out pretty close without stripping. Given these dimensions particularly roller dia (Bearing OD less pin OD divided by two) you can start looking round.

[malcolm2010]

Thanks alot for explaining that, Grump. Today I've learnt a few things.

I guess I'm guilty of thinking about the engine's guts as something magical that is created in a big factory and only repairable by getting new parts from said factory. But as you point out, with the right gear and skills, something can be made or modified to work.

cheers

Malcolm

[malcolm2010]

Small continental crankpins in my experience are usually very good quality and have good depth of hardening.

Yeah, although the DKW Hobby is just a scooter, the quality of the engine and transmission is very evident when you strip it down. Compared with the little Honda engines I've worked on in the past, it has very nice materials (and workmanship) - quality screws (no stripped heads and no impact driver/drilling required), nice solid alloy which hasn't corroded and welded to everything, no sintered gears, grease nipples on the belt-o-matic transmission, the splines and bushes of which look and feel brand-new, despite having 30k miles on the clock and a long-ago broken speedo cable. The little OHC Hondas are marvels and well ahead of their time of course, but if only they could have built them with quality materials like the Germans.