1980 GS850 top end rebuild

[JHook]

I need to do a top end rebuild on my GS850 that has 48K miles on it. A friend who formerly ran a shop and has experience rebuilding both KZ and GS bikes diagnosed the top end after I tore it down, and he showed me where a few of my valves had started to burn and become warped. He recommended I replace all the valves (springs, retainers, seats,oil seals also) including valve guides. The cylinders just need honed and I know I'll need piston rings. The valve seats are in decent shape with just some light "freckles" on a couple. Will I need a shop to cut the valve seats for new valves and have them install guides also? Adding up the cost, this rebuild is pushing the budget and involving a machine shop might double it. Recommendations?

 

[eil]

If your friend has experience building GS engines, then follow his advice. Alternately, you can show us some pictures and the gurus here will try to help you out.

Why do you think you need to hone the cylinders and re-ring? You only need new rings if yours measure out of spec and you only need to hone if you have new rings. (Some folks advocate new rings and honing the cylinders every time an engine is taken apart but I don't subscribe to that... if there was nothing wrong the the cylinders or rings when you took it apart, then you're just accelerating wear and decreasing the amount of time it will take until you have to bore out the cylinders and go oversize on the pistons. Which is more work and money than you absolutely have to spend now and later.)

As to the valve seats, again, measure before making a decision.

Do you have a copy of the factory service manual that tells you how to do all this stuff? Check out the mega-welcome link in my signature below for your copy, along with tutorials and reference material of all kinds.

Welcome!

 

[Nessism]

It's always best to measure first and then make a decision on what to do based on facts. To measure is to know!

Typically with a GS of that mileage all you have to do is bottle brush hone the cylinder and install new rings on your old pistons. Regarding the head, the service manual details a "wobble test" to judge the condition of the valve guides. The service limit is ridiculously loose which I've never understood, but realistically, as long as the valves have less than .005" of wobble you are okay with leaving the guides alone. Again, typically on a GS with 48K miles the guides should be fine.

If you let anyone cut the valve seats make sure to have the valve stems shortened some or the valve shim range will shift down to the low end of the range. You should NOT have to replace things like the spring retainers, locks, and spring seats. Those parts are very understressed and will last through the life of the engine easily.

BTW, I have a brand new OEM valve spring set if you choose to replace those. Send me a PM if you want to discuss obtaining them.

 

[JHook]

Thanks for the advice so far, guys! It looks like I'll need to get an inside caliper gauge to get in where I can measure the ID of the valve guide and some prussian blue to get the dye ring to properly measure the valve seat. If a few of the valves themselves burned slightly and are out of spec, wouldn't I need to use new valves to get a correct dye ring to measure? I do have the shop manual downloaded, by the way, ( thank you BikeCliff!) and I've been referencing it along with my Clymer manual. The threads on GS resources have also been a huge help with all the wrenching I've done so far!

 

[Nessism]

Do the wobble test. Details in your service manual. Much easier and it will give you a better understanding of the fit between the parts than trying to measure the ID of the guide.

The valves can be ground if they are not burned too much.

The seat width is going to be pretty wide. Doesn't seem to hurt anything though. Lap the valves in and then measure the seat width based on the polished width.

Good luck

 

[JHook]

. Ok the guides appear to be in spec. I plan to replace the valves themselves as my friend recommended. The seats are where I'm trying to decide. I'm not sure if they will just need lapped or if I should be concerned with the black spots. Can I lap in new valves with my seats as is? If I have to have seats cut for new valves I may as well send it in to a shop to have a valve job done. Should I order new valves and try to lap in or just have a shop do the valve job?

 

[JHook]

The pic was of # 2 exhaust valve seat..the others were looking better.

 

[Grimly]

Might just need the lightest touch with a seat stone - only way to find out is to do it. If it's actually burnt too deep, then a re-cut is in order. Hell, maybe all it really needs is just a bit of coarse and fine lapping paste - only you have it to hand to tell.

 

[Nessism]

I'd have the seats touched up. After cleaning the seat it will be wider than the service limit allows. The valves may be fine, let the machinist decide. I've got a brand new set of OEM valve springs if you want to spluge. PM me if you want to discuss.

 

[DimitriT]

Valve lapping. If you get a clear unbroken grey circle after a light lapping on both surfaces you are probably fine.

 

[JHook]

I took my cylinder head to S&K racing in Dayton to have the valve job done. The shop took AWHILE to finish the job but their rates were reasonable and they were happy to share some rebuild knowledge. The rest of the rebuild I did in my toolshed. After getting the head back on, I realized I forgot to put the dowel pins in on either side that fit between block and head! I know the pins are there for proper alignment, so, I've already torqued the cylinder head nuts down..if I pull the head again to put them in, won't I have to replace head gasket again ($$) or will I be ok to leave the pins out? Alignment looks good as it is. First rebuild, beginners mistake, I guess!

 

[DimitriT]

That happened to me the first time I tore down the top end.

I don't think the guides provide any structural support. There are very few lateral forces applied to the head.

When it happened to me I tore it down again and replaced both gaskets. You can try running it. I suspect if anything moves around at worst you will get some oil seeping somewhere.