Roostabunny's Long overdue Top End Rebuild GS850GX

**roostabunny** · 02-20-2016, 02:33 PM

My account was deleted due to inactivity, I think, I'm assuming my one-attachment limit is due to my being busted back to Forum Newbie. Sorry about the multiple posts. I'll try to figure out how to make Google Photos links work.

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**eil** · 02-22-2016, 11:40 AM

There should be a stationary mark in the upper part of that window. You might need a really good flashlight to see it if your shop lighting is poor. Check out page 3-10 of the service manual (page 30 in the PDF).

**roostabunny** · 02-23-2016, 09:36 AM

Thanks, Eil! Yeah, that's exactly what I was looking for. I think I might have figured it out, though - I think that stationary mark is on a thin stamped metal plate that's just not there. Possible it got lost when I replaced the ignition advance years ago after it was damaged in an accident - in fact, looking at this old pic I took is what clued me in.

P1070050.jpg

So, I used this pic to index just to the left of the screw hole and verified with a dowel in the spark plug hole that the piston was actually at the top of its throw.

Here's where I'm at now - and concerned that either my cheap digital caliper is off, I'm looking at the wrong specs, or the cam lobe heights are below the service limits. Did that in a hurry, and need to remeasure and recheck the specs. I'd be shocked if the cam lobes were worn out, but I'm the deepest into an engine I've ever been, so it's new territory and I'm learning as I go. Edit: One example - I started marking the caps for location, cause that seemed important to get them back in the same locations... turns out it's so important it's molded into the head AND the part, A..B...C...D. Duh.

**eil** · 02-23-2016, 03:01 PM

The 850s are legendary for being bullet proof so I'd be surprised if the cams were shot unless the engine was run dry of oil or something. Not a fan of cheap digital calipers, see if you can lay hands on a more spendy pair or even better a micrometer. If you're lacking money for tools at the moment, swing by a machine shop with the cams and a six pack in tow.

**BassCliff** · 02-23-2016, 03:29 PM

Hi Mr. roostabunny,

You're an inspiration! I'm gonna get my bike back together soon. All I need are a couple of free weekends. Even my daughter has been bugging me to go riding again. Wish me luck!

I'll be looking forward to your tutorial!

Thank you for your indulgence,

BassCliff

**roostabunny** · 02-24-2016, 09:31 AM

LOL, guess I had to give up your coat tails one day, Cliff! Edit: And yes, good luck to us both!

**roostabunny** · 02-24-2016, 09:44 AM

Not looking good on my camshafts - here's the numbers...

Camshaft lobe measurements (mm).
Cyl 1 2 3 4
IN - 36.41, 36.26, 35.93, 35.94
EX - 35.66, 35.41, 35.21, 35.05

Meantime, this from the manual...

GS850 Camshaft Lobe Specs.JPG

Thoughts on this? I guess I'm just surprised to see them obviously out of spec. Would you question my cheap HFT digital caliper or measuring technique, or would you start looking for replacements? Would this possibly explain the fact that the last two years of valve adjustments (when I was riding almost every day) had almost no shim changes?

Edit in reply to Eil: I'm friends with the guy at work who runs the mechanical engineering lab (he builds stuff) who also happens to be a serious gear head. I'll bring the cams in to work and I'm sure he'll have both an accurate tool and comments on visual inspection if he notices anything.

**Nessism** · 02-24-2016, 10:14 AM

I wouldn't worry too much about the cams. Even if the lobes are worn a little it won't affect the engine performance enough to matter.

Of greater concern is the condition of your bores, valve guides, valves proper, and pistons. I'd measure those features carefully.

**roostabunny** · 02-24-2016, 10:28 AM

I'm glad to hear you say that, Ed, thanks. Will double-check my measurements when I have a chance, but continue teardown to the more interesting parts in the meantime.

**londonboards** · 02-24-2016, 11:47 AM

Perhaps more importantly than the lobes are the journals. Like Nessism says, wear on the lobes is perhaps not criticial unless you can see gouges and lumps of metal missing like this:

see how much of the lobe is missing on that!

More critical would be if you had any appreciable wear on the journals. This is the part of the cam that rests in the journals of the head and the cam caps. Measure at these points to see if they are in spec. Once those start to go out of spec they can wear quickly. See the ends of the cams in the above pic. Also inspect the head and the cam caps:

If you are worried I'm sure you can pick up a set of cams on eBay for not too many dollars.

**roostabunny** · 02-24-2016, 01:33 PM

Oh yikes - thankfully I've got nothing that looks like that, but I think those pics are more of a cautionary tale about what happens when you run it out of spec on those journal/camshaft/cap interfaces.

I'll have to look in the shop manual again, but I don't remember seeing a measurement method that didn't involve a "plastigage". A little reading has me less mystified by this tool than I was an hour ago, but is that the only correct way to check that? Is it the best way? Sounds like Plastigages are available enough, and the method's not difficult, just curious.

My parts look really good in comparison to these rough examples you guys have posted.

**roostabunny** · 03-02-2016, 05:00 PM

Not much of an update, except that I've ordered a plastigage for the cam journals, but planning to forge ahead in the meantime - those areas look good on visual, and on the off chance there's too much clearance I'll correct during re-assembly.

**roostabunny** · 03-06-2016, 06:17 PM

Quick update and a request/inquiry...

Finally got the head on the bench.

Innards look grimy, but I think that's pretty normal.

Cylinders are off, too. Bores look smooth and undamaged but shiny, which is also no surprise.

Valves look... well, they look still installed, and all pretty much like this. Going to borrow a friend's tool next day or two. He's also offered to profile them for me, which is very handy. He's got a machine for the purpose, and lots of experience, just said I needed to verify the spec and let him know the angle(s), which I'll double check in my handy dandy FSM PDF from the Library of Basscliff.

My request/inquiry is related to later re-assembly. Anyone have the plate that I'm missing from behind my ignition? It's the thin one with the TDC mark stamped on it. I'd like to buy or borrow one, but Ebay's not turning up just the plate, only plate + ignition advancer. Anyway, just curious if anyone's got one hanging around that I can paypal either shipping (and then return to you) or shipping plus purchase price? I've got a picture here of the damaged one I had once but can no longer find.

Thanks for any tips!

Josh

**roostabunny** · 03-14-2016, 02:12 PM

OK, may have a line on that ignition crank alignment plate thanks to Mr Raygun's GS850 part-out - we'll see if he's got a match.

Meantime, I've got the valves out with some help from my youngest.

Hard for a photo of a valve to do it justice, and I'm not sure what I'm looking at here, but everything looks in order at this point - some pitting but not burnt from what I can see. I mentioned I've got a local friend who said he'd profile them on his machine for me, and he'll also be willing to take a look and tell me if there's any trouble before I clean them. Looks like I've got some margin left - still need to clean, check the specs, and measure.

Ginormous borrowed spring compressor and home-brew pressure ring bit -

My organizer and the little dollar store micro containers that make my geeky brain really happy by being exactly the right size for all the spring bits.

Here's an exhaust valve -

And an intake -

Roostabunny's Long overdue Top End Rebuild GS850GX

Roostabunny's Long overdue Top End Rebuild GS850GX

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