Burning oil piston photos

[Guest]

Stripped down the top end of my GS750 for a second time to do the 850 piston/cylinder upgrade. This is partly for the extra CC's but more for excessive oil consumption - over 4 litres between oil changes (3000km).

I changed valve seals last year whan fixing leaking head this made no difference to oil consumption. That was about 5000kms ago.

Looking at my pistons and cylinders they actually don't look too bad (too my untrained eye). Pistons look like new and cylinder scoring less than I'd remembered. I havn't measured anything yet.

So looking at this photo can anyone "read" what the problem has been?

Piston domes are black & wet whilst underside of head is dry.

 

[Guest]

And another photo

 

[tkent02]

It looks like the rings were sealing well, as there is no carbon down the side of the pistons, above the rings.
The other likely path in for the oil is the valve stems/guides. Is this a high mileage engine? You replaced the seals, but how loose were the valve stems in the guides? I have reassembled some car engines with loose guides, out of spec. They ran fine, but burnt some oil.

 

[Guest]

Yeah that's what i was afraid of. 50,000 kms by the clock.

The valves could be rocked slightly, but I didn't measure them.

 

[Guest]

Measured rings & piston of cylinder that fouled plug the most often. They're right at service limit for piston diameter & ring gap in cylinder - not enough for the amount of oil thats being burnt, I would think.

I'm going to put in the 850 pistons for the extra cubes - they do look significantly bigger side by side with the 750 pistons - and they've got new rings and been honed.

A valve job is a bit beyond me at the moment, so we'll see what happens.

 

[Footy.]

I would say that the first pic does show that some oil is getting past the rings. The classic sign is the "clean" areas at the edges of the piston. These are because there is always fresh oil coming up at the edge, that does not allow hard carbon to form like in the piston centers. The oil is cleaning the edge area.

We used to have a piston chart that described this, (40 years in engine reconditioning)

Footy.

 

[Steve]

A valve job is a bit beyond me at the moment, so we'll see what happens.

Chances are that you don't need a "valve job", but while you have things apart, you should really change the valve seals and lap the valves to clean the seats. You have to remove the valve springs to do either one of them, and the best (actually, the only) time is when the head is already off, like it is NOW. BassCliff has a video on his site (that I made) that shows how to make a very inexpensive valve spring compressor that allows you to remove the valves for lapping and changing the seals.

You probably got new valve seals with your gasket set, USE THEM.

.

 

[hampshirehog]

Looks like rings and I'd concur with Footy.

However, as Steve says, you might as well do the valve stem oil seals - it's only an extra hour on the job. While you've got the valves out you can easily measure the stem diameter to see if that's in spec. If you've got a dial gauge you can also measure valve stem / guide clearance.

You can go a bit over on the limits - I've run bikes with up to 0.15mm clearance and they've been ok.

If the specs are out you can get a head shop to pop in some bronze inserts and it's not that expensive - had mine done last year for ?120.

 

[Macmatic]

I just wanted to add that you don't HAVE to pull the head to do valve stem seals. I did them on my GS750EF with the head on the engine a couple of months ago and it worked out fine.

/\/\ac

 

[Texasgs]

re

re

You should always do a leakdown test if your bike is burning oil. It will tell you if a cylinder is going south, and if it is the valve seals or the seal of the piston, or rings in the cylinder. Harbor Freight has a cheap leakdown tester that is OK. It can save a lot of work to know what is going on before you pop it open. At least you will know what you need to do, and how far to tear it down.

 

[Guest]

Thanks for the comments guys. I must say I thought it a hell of a lot of oil to be coming down the valves.

BTW I changed the valve seals last year along with head gasket.

Looking at the cylinders in daylight (been doing everything outside after dark til today) there is a lot of vertical scoring in all the cylinders. Not that deep but all the way round.

So now I'm pretty keen to get in these larger pistons