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Author Topic: Engine rebuilding. Main bearings, Crank journals, Grinding?  (Read 2969 times)
Matt Story
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« on: July 14, 2010, 05:29:00 AM »
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My pre-filter 850T has been a daily rider until last year when I took it apart for a new clutch & restoration.  The trans has been apart and is now refreshed with parts, re-shimmed, new return spring, seals etc.  The bike has about 35K miles.  After much deliberation I decided to do the oil filter mods.  Now I have the motor apart and I have been able to do a thorough inspection.  My current dilemma is what to do about the crank & bearings.

Here is what I have found:

All bearing surfaces are relatively smooth.  With a light polishing they should be good if it weren't for the dimensional issues that follow.

The front crank journal is a little less than a .001" out of round at 1.4950/1.4940" with the min diameter .0004" smaller than allowed by spec.
If I buy a new standard size bearing (will be needed for the oil filter upgrade anyway) and it comes in at the small side of tolerance (is this a safe assumption with Guzzi's?) my max clearance will be .0021" (the crank isn't quite round) and within the max allowed by the specs (.0010/.0022")

The rear crank journal measures 2.1242/2.1240 within the spec (2.1248/2.1233)
The rear main bearing 2.1270/2.1274 at .0003-.0007" over the spec (2.1260/2.1267)
The resulting clearance using my old bearing would be a little sloppy at .0028/.0034 compared to the spec (.0011/.0027)
If I buy a new standard bearing and it comes in on the low side of tolerance, the resulting clearance would be back within spec at .0018/.0020.

My rod journals are nearly perfectly round and smooth at 1.7319 and 1.7322.
As far as the spec go here I am a little confused.  The factory workshop manual gives a tolerance of 1.730/1.732 which mine are at the high side.  Haynes gives an A & B class of which I am smaller than spec on the smaller A class 1.7325/1.7328

If your still with me and you have experience in these matters, please consider the following:

1. How significant is the out of roundness of my front main journal?  I can get back to within the clearance spec with a new bearing that comes in on the low side.  I need to buy this bearing anyway for the oil filter upgrade.

2. Should I replace my rear main bearing to get back within spec?  i.e. is the the .0007" extra clearance a killer?  This would be about $140 cost.

3. If I go with a new rear bearing too, grinding the crank begins to make more sense.

4. What is up with the rod bearing specs?

5. How much does a crank grind & polish cost? and Who are some recommended sources?

6. Do replacement bearings come in on the low side of the diameter tolerance?

7. You are very patient if you've read this far and I owe you beer.


Thanks for you help

Matt
 
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'75 850T - First & only owner
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mtiberio
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« Reply #1 on: July 14, 2010, 07:14:04 AM »
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I don't have my books in front of me, and Guzzi manuals are wrought with errors. Obviously what's important is the clearance between these surfaces, not the ultimate ID or OD. The thickness of the rod shells comes into play as well (crank mains have build in babbit). I have had crank rod journals ground for about $30 (18 years ago). The radius where the ground surface meets the rest of the crank is critical to prevent cracking. It was .100" for the rod journals IIFR. Guzzis cranks are nitrided, and grinding removes this nitriding. In a hydridynamic bearing (plain) like guzzis have this should not be a problem. That said, if you had no problems before you took it apart, I'd replace the front bearing, throw in some new rod shells, and be done with it...

The whole A/B/C thing was Guzzis was of blueprinting, or in otherwords compensating for manufacturing variances. They did not represent oversized or undersized dimensions that you would resort to if you had to grind your crank to first undersize, or bore out your cylinder to first oversize. They represented mini variations within a given size range. It allowed Guzzi to match the slightly undersized rod journal with the slightly oversized rods that were the natural outcome of their non-cnc manufacturing process of the day. Sort of like what Caddilac did back in the 20's and 30's. Made for a fine long lasting and tight motor for sure...
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Greg Field
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« Reply #2 on: July 14, 2010, 09:14:31 AM »
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Unless you measure things for a living, find someone who does and have them confirm the measurements. Then, if theirs match yours, I'd grind anything that's that close to being outside spec, especially anything out of round.
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jcctx
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« Reply #3 on: July 14, 2010, 11:18:16 AM »
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I would put fresh bearings in and ride the pee out of it!!
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Matt Story
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« Reply #4 on: July 14, 2010, 12:05:22 PM »
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Thanks for your feedback.  Don't know who suggested it, but I had the help of a jouneyman machinist and cycle restorer to take the measurements.  We've obviously talked about the possible routes.  He suggested talking to some Guzzi folks since his knowledge is primarily with old brit bikes.

Matt
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'75 850T - First & only owner
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« Reply #5 on: July 14, 2010, 01:01:49 PM »
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The whole A/B/C thing was Guzzis was of blueprinting, or in otherwords compensating for manufacturing variances. They did not represent oversized or undersized dimensions that you would resort to if you had to grind your crank to first undersize, or bore out your cylinder to first oversize. They represented mini variations within a given size range. It allowed Guzzi to match the slightly undersized rod journal with the slightly oversized rods that were the natural outcome of their non-cnc manufacturing process of the day. Sort of like what Caddilac did back in the 20's and 30's. Made for a fine long lasting and tight motor for sure...

I haven't worked on Japanese stuff for a decade or more, but when I did they were selective fit too.  Rods were color coded and journals were letter coded (or vice versa) and you ordered bearing from a table to match the specific rod and journal.

As a kid I seized a Guzzi V-twin due to installing new rod bearings with one cap installed backwards.  Not one of my brighter moves, but there you go.  I cleaned up the mess of melted bearing material, polished with crocus cloth, and put it together with a new bearing.  Then I ran it another 50K miles without issue - I still have the bike.  Guzzis are pretty forgiving.

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« Reply #6 on: July 14, 2010, 03:01:28 PM »
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I picked up my 1978  850T3 in May 1979 it had 8,000 miles on it, in summer 1995 the engine was stripped down for the first time @ 185,000 miles, there was still a few thousand miles left in it but as we had it in bits we replaced the crank, bearing, pistons etc,  your 850T with 35,000 miles is just about run in, 
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Matt Story
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« Reply #7 on: July 23, 2010, 05:35:23 AM »
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My plan is to buy all new bearings and have the crank ground.

I found a guy locally who grinds crankshafts from everything from small engines to large diesels (that's all his shop does).  From the look of his shop, he does 100's or 1000's a year and I believe he does the work himself.  He quoted me $75 to grind and polish all three journals with a 2 week turn around.  He just wants to know the specs to get started.

The mains are easy.  I have the specs, and the pbearings are on order.  I'll mic them when they come in and instruct him accordingly.

I still don't get the rod journal specs.  There is the discrepancy in the specs between the Guzzi shop manual and Haynes.  I understand the factory used selective matching of the cranks and rods.  A and B and such.  I may not need to do any work to the rod journal, its round and smooth.  It is apparently under size depending on which spec you go by.  How can I assess if new stock bearing shells will put me into spec or not without first buying them?

Thanks

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'75 850T - First & only owner
'77 Honda CB750 Supersport

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Roundbarrel
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« Reply #8 on: July 23, 2010, 06:10:59 AM »
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I think where you're getting the spec discrepancy is that you are using the Guzzi 850T book, the Ts did not have graded cranks and rods. The A & B graded specs in the Haynes book are from the later SP1000/G5 manual. Stick with one book. I'd think if your rods and crank measure within spec by the 850T manual you'll have no problems with standard bearing shells.
I'm pretty sure the graded crank came in with the oil filter sump so probably applies to very late T's only.

Miles.
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Matt Story
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« Reply #9 on: July 23, 2010, 06:28:23 AM »
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Thanks Miles.  My factory reference is "workshop manual for v7 sport - 750s - 850t"  I am within the rod journal spec there.

Matt
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