General Category > Bike Builds, Rebuilds And Restorations Only
1975 850T Survivor Project
Antietam Classic Cycle:
"Looks" are one thing, the micrometer tells the tale...
cmice:
Ok boys and girls, always remember to clean out your crank.
A sort of rubberized slug of chrome dust and who knows what else came out - almost in one piece, but it broke apart into lots of chunks when I tried to extract it.
The bung plug was in only finger-tight, despite being "staked" and came out with only a faintest hint of thread damage (just at the very top where the stakes are). So he's going back in, but sealed and sent home properly this time.
Antietam Classic Cycle:
This is from a '71 Ambassador with 98k miles on it.
cmice:
While it seemed I've gone quiet, I've been doing quite a bit of work behind the scenes. With the motor completely apart to assess the bearings, it was time to start building the shopping list...and that covered a lot of ground....
First-up was the re-plate vs. buy-new decision on the cylinders since my chrome has definitely failed and bare aluminum was showing, plus the bearing damage as a result. I followed Charlie's math to look at re-plate vs. new, and indeed, buying new pencils out to the best eventual outcome. I checked with the two usual places for re-plating, and one would come in about $100 cheaper than buying new (still no pistons or rings), the other would come in at $20 more expensive than buying new (with no pistons or rings). Plus shipping back and forth, when you add up all the nickels, buying new sets gets you the most bang for the buck. In the end, I flipped for 83mm Gilardonis which included pistons, pins, piston rings, and head gaskets. Existing con-rods will be re-used; question as to whether the small-end bushings need to be mic'ed and reamed to size for the new pins, or close-enough and just run it? Do the rings need to be gapped or do they come pre-gapped? (I'll check them regardless)
Speaking of conrods, that leads us to the great bearing replacement. The chrome seemed to have done a number on the conrod big-end bearings and the front main bearing. The rear main looks fine, and the journals all mic out to standard size and look great. The rocker pins also look fine, as did the cam jounrals. The cam bearings looked not great, so a *very* gentle touch-up with 1000 grit cleaned them up - called it close enough, we'll leave them alone. Can these ever be replaced? Seems like if you could find plain bearings the right size, a line boring machine would make this not too bad a job.
Ok, back to the update - given we're replacing the front main, that leads us to the topic of adding an oil filter. The newer-style (with filter) front main bearing is cheaper and more available than the non-filter one; plus by adding a filter, we should be protecting all these expensive parts so this won't have to be done again by me or the next owner. So new-style front main, plus a filter-in-pan off of Ebay, all that's left is the machining. I didn't take a lot of photos, but there's a great writeup of the process here for those considering the change:
https://www.advrider.com/f/threads/moto-guzzi-early-big-twin-oil-filter-conversion.257513/
Just finished the machining today. Hardest part, as others have found, is 1) having a 8mm drill long enough to do the deep oil passage (200mm will do it), and 2) the fixturing to hold the housing square with all the studs still in, and 3) having a machine tool with enough throat to hold this massive stack of parts plus extra-long drill (hint: the Bridgeport wouldn't accommodate just the housing with studs, let alone a Jacobs chuck and long drill).
Thankfully a friend has a habit of collecting old machine tools, including a 3-phase Rockwell gear-drive drill press - plenty of throat, nice and square and solid so nothing wiggles around while the quill spans a massive distance. Little spooky working on that thing because if it snags on anything it will take your arm off. All that's left is to fab the blanking plate for the old pickup, and then we can start putting the motor back together.
Lastly, earlier I'd posted a photo of the starter ring-gear and asked about how this could happen and whether to run it or not. While I was browsing on eBay for a filter-in-pan sump, I found a listing for an "EV" flywheel and cover for cheap. This alleged EV flywheel looked like a single-plate unit (which is junk) but figured what the hell - grab the assembly and if all I keep is the cover and ring-gear then great. It showed up - and to my delight, it is completely unused and is not a EV flywheel at all, but a good-old multi-plate flywheel. As an added bonus, the scale says the flywheel weighs in at 7.2lbs instead of the stock 9.2lbs(!) - so I guess I scored a lightweight LeMans flywheel by accident! Gonna run that mofo for sure. :)
Next update should be about putting the motor back together - plus a fun side-quest in 3d-printing.
Antietam Classic Cycle:
--- Quote from: cmice on December 11, 2025, 06:24:07 PM ---Existing con-rods will be re-used; question as to whether the small-end bushings need to be mic'ed and reamed to size for the new pins, or close-enough and just run it? Do the rings need to be gapped or do they come pre-gapped? (I'll check them regardless)
--- End quote ---
Yes, do check small-end bushing i.d. and compare that with the specs in the factory manual. The new pins will be the same o.d. as the originals.
The rings are supposed to be "pre-gapped" but it doesn't hurt to check them.
There have been reports of a few of this latest batch of Gilardonis not fitting into the crankcase - the "spigot" of the cylinder wasn't properly machined and is too large to fit. Check this before you begin assembly.
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