#307 Powertrain Demise update

[blackheart]

Well,  last summer as posted I noted metallic debris in the oil on this engine.   I did not ride this year as I could hear a slight knock on initial start.

Finally got time to tear it down before I dropped in the replacement Powertrain from Arizona.   To my utter amazement the cylinders look fantastic.  Near zero scoring and no scuffed piston.

Turns out the drive side (LH) main bearing spun and I was observing bearing debris in my oil filter.  Case is not torn up, crank journal looks good with a crocus cloth polish.  I'll have to measure case bore and new bearing shell and see if there is adequate interference fit after an assumed operating temp of 250F.

I don't know if I should just rebuild what I got and machine my own bearing with more interference and pin it, or just run on blind faith and drop in new Powertrain.   I could tear down new one and pin both mains, send the piston to Swain to have the crowns ceramic coated as well as combustion chambers and exhaust ports (reduce heat transfer).

As for bearing material I'd use a leaded  bronze (#660) if the shaft is hardened.  I don't know much about aluminum bearing materials and if they are available.  I'll do some basic research on suitability for the use of #660 bronze given the shaft speed in SFM.  The rod journal looks great, but has not been induction hardened, but the crank could be nitrided, although I seriously doubt it. 

I also plan to shoot the compensator, crank, rods, etc. for material analysis just to keep everyone informed.

Opinions?  Run the new 27 mile young Powertrain on blind faith?  Update the 27 no powertrain??  Rebuild it and teardown new Powertrain for proper preservation.

[wytfut]

Well... Rich...
if money is no object... the new motor is already out... I'd rebuild/beef up now... even if you don't use it, the turn around could be quite good...
as for the other one, its already out too... might as well go for it. Everything looks good from your comments..... 
Bruce

[blackheart]

Thanks Bruce.  When I pulled the crank from the right case half I noted the bronze thrust washer on the inside against the crank was torn up and smoked.  I've decided I will pull down the new engine and pin the mains leaving them in place.  Then at that time I can verify proper clearances.  I don't have a torque, or turn of nut, or bolt stretch amount for connecting rod bolts.  Anyone know what that is?

Another interesting observation.  #307 has the later compensator with the bolt to hold alternator rotor on, #204 has a nut and threaded projected stud as part of crank to hold on with a nut.  Anyone know if the removal process for early rotor is the same?  I'll have to make a hollow stud to pull it off the taper.

I'm also going to have to pay attention to cam chain timing marks as the service book is missing art.

[Jumper]

I think the Torque specs are in the TECH section?
Jumper

[blackheart]

I never have found the tightening specifications for the rod bolts,  I suspect they use a light preload torque,  then use turn of nut to tension properly (in this case turn of head).  I suspect that is why it's not listed in the published torques, or I might be overlooking it.  If Atlantic wont advise when I purchase new rod bearings from them,  I'll assume a 150ksi UTS fastener and calculate the required stretch to tension properly. In order to do so I'll have to spot the bolt ends to use a stretch gauge or mic with balls.  The other thought I had was to mark the existing and loosen and see how many degrees past snug they where tensioned to.

So I decided to keep the original cases for #307 and just rebuild it.  What looked ok in the beginning, was not upon closer inspection.  The left side main bearing bore was out of round and tapered.   So I bored it oversized and made a new bushing from a modified #64 bronze,  which is 12% lead.  I set the interference fit for 0.002/ inch and heated the case to 240, froze bushing in dry ice for a drop in installation. Then I bored to size.  I am not going to pin the bushing since the fit won't loosen as heat rises as does with the steel backed factory bearing.  I'm a bit concerned the bronze isn't as good for embedibility as the aluminum alloy faced stock bearings.

Yesterday I finish sized the bores and chamfered for the crank radius.  The right side which has the thrust washer, I made the thrust integral to the bushing as the old one was smoked. I was shooting for 0.002" clearance but overshot on the left side at 0.0035" or a little less.  I'm not sure I won't have to redo it. I'm afraid it's too much.  I  just wasn't used to my boring attachment and on the last cut changed the way I adjusted it. I think I should advance it, lock down via dovetail clamp screws and back off advance screw, I think it moved more after I spun it up to speed.

Rich

[Donkey Hotey]

Rich, I'd love to see pics of some of your machine work as you progress through this.

Just thinking out loud with you here: if those bolts were indeed designed to torque to a specific stretch, I would expect the ends to be machined flat and tightly controlled. They had to do this on an assembly line and it would have been a crappy engineer to leave something like that to juggling ball bearings and a micrometer to get the dimension. In fact: I'd expect them to be precision finished to length if that were the case.

I believe it would be either a basic torque value or a lower torque value and some amount of extra rotation, exactly as you suspect.