Preparing for top end overhaul

[zalophus]

Well, after 25k miles my '81SH looks due for one. Now I'm doing regular 10 mile runs at 60-70mph, oil consumption has gone from negligible to 500ml per 100 miles. Compression is 160LH 170RH dry, and little different (maybe +10 each side) wet.

So it sounds to me as though the bores are pretty much ok, maybe new rings and the main area for work will be valves and guides. Does this sound right? I'm surprised that, although it's clearly burning oil, neither plug seems fouled. I was also expecting lower compression numbers.

 

[pa23driver]

you may just need new valve seals. are those compression numbers in PSI? if so they are on the high side, might want to check your compression gauges out.

 

[retiredgentleman]

Probably the head gasket is passing oil from the camchain tunnel into the cylinders. The compression has gone up because the heads and piston tops are coated with carbon.

Here's a pic of my engine with a passing head gasket. Its not pretty, but a top end re-build and my engine runs like a new bike.

 

[zalophus]

Thanks both, very helpful (and encouraging). The comp gauges were loaned from a professional mechanic who I've found to be reliable, so I suspect the readings (which are in PSI) are good. RG's explanation sounds logical. I'll post pics when I strip it down later this year, as it sounds as though what I find will be yet another example of just how bullet-proof these amazing engines are.