Topic: 750 main bearing clearance, a compendium of information. Re-run.

[Don R]

 Just so Me and Jerry stay on the same page, I was talking to him on the phone about main bearings.

[Don R]

   A full set of Yellow bearings are on the way from Yamiya. The price was favorable and shipping is quick considering the distance. If yellows were good enough for RC, they're good enough for me too.

  I've been thrashing on the 20 year sleeper gl1000, it has a hard to cure miss, but I need the drag bike engine off the bench and in one piece. It's hard to stick on one project at a time.

[Don R]

 The Yellow bearings are in, plastigauge says mostly .002 a couple bearings closer to .0015. (formerly the too tight ones)  After checking, I cleaned and oiled the bearings and torqued to 174 inch lbs. again. It rotates easily with no strange tightness. I'm assuming it was reassembled with too thick bearings at least on the two nearest the points end.
 Looking at some things I found, like chewed up o rings in the motor and one under the copper head gasket I'm going to assume it was bolted together the last time with little expectation of him racing it again. There were pegs bent forward, a missing gas tank and wheelie bars. It may have been wrecked or had a trailering accident. Now I can put it together, then back on the stand and attempt a startup.

 What is a good set of bore gauges for use with these bikes? The adjustable set you adjust to the bore and then check with a good caliper or micrometer?

[scottly]

Don, is this the stroker crank? Have you measured the journals yourself, or are you going on the factory markings?

[Don R]

 Plastigauge engineering. And no, I didn't measure the crank or bearings.  I took my old buddy's motor apart just to check it, clean it up and try to start it.
 Done properly, I'd need to throw most of it away and start over. Old stroker + aluminum rods + ARD + unknown number of runs. I have to assume it's on borrowed time. If it runs OK, I may make a pass or two down the drag strip but this is just in remembrance not as a reliable racer.  In reality, it's a cacklefest bike. But it will get a belly pan if I make a run or two on it.

I'm already collecting parts for a decent motor. I have an under 10,000 mile 77F My long range plan is to use the bottom end, built head, new steel rods and a 1,000cc cylinder block with forged pistons.

[scottly]

All bets are off with stock markings on a welded stroker crank; I would guess if you put a mic on the journals you would see the .0005" difference.

[Don R]

 Agreed.
  The original rolled rim that was removed from my sandcast bike was put on this bike. I actually bought the drag bike just to get that original rim back on the sandcast. I assumed the rim and hub could cost a grand to buy and the ARD mag was worth $800. The rest might possibly be worth nothing, it was missing everything, the carbs came out of the Labrea tar pit and what was there was painted spray bomb black. The engine wouldn't kick over but I could bump it by rolling the bike in gear on the concrete drive.

  The family was stuck on $1800 for the dragbike, since I bought the sandcast at the shop sale, they thought I was junk drunk and had lollypop and $$ signs printed on my forehead.  I didn't mind overpaying for the drag bike, it eased my conscience a little after what I got the sandy for, only two bidders after a thousand. The other guy claimed to know all about sandcasts yet here he was giving up way too early. 12 more bids and it was mine.

 And now I own a sandcast cb750 AND the "big 4 express"

[Don R]

Bump.

[Nose736]

Ran a CR750 replica for 9 years here in Canada , Ran yellow bearings Rods and crank , No problems and no brake in, Yellows will generally plastigauge about  .0015

[Don R]

 I had forgotten about this thread. The drag was caused by a folded oil ring and lack of compression was sunken sleeves, which I described in the autopsy report.
  The yellow mains showed contact on alternating case halves, #1 top, #2 bottom #3 top #4 bottom etc. It did not have case studs, just cylinder studs.

  I may have finally learned my lesson about making old junk run without doing an entire rebuild job.
  It would have saved me a lot of time and yours too. Hopefully the discussion brought good information to light, thanks for the advice guys. 

[Tracksnblades1]

So did you determine it to be case line bore? Green case warp ?crankshaft run out?

[Don R]

 The crank checks good with bearings only on the ends and a dial indicator on the other mains. The case was raced a lot, clearanced for a stroker with aluminum rods and welded in a couple places. I even had new yellow bearings.
  Something odd happened to the bike before I got it, pegs were bent forward and some other really odd damage like it was hit from behind, fell off a trailer or took a tumble. A points plate mount had been broken off possibly from behind and replaced by welding and an aluminum dowel, there is welding in that area to the bottom case so it may have thrown a rod.
 When people asked about the bike the PO wouldn't show it to them, maybe because he had parted it out a lot. The Fibermold tank, wheelie bars and weber carbs were gone. A lot of small parts had been robbed since he ran a small bike repair shop.