Loose valve guide?

[tomkilde]

My completely rebuilt engine has only a few hundred miles on it, but recently it started "losing" oil - nearly half quart of oil with every ride.  I don't think it's burning oil, as there is no blue smoke and compression seems fine.  The oil is not staying in the engine, as the return to the tank is healthy and it doesn't wet sump.  Of course there are a few oil leaks, but the amount that seeps out is not significant.  After scouring the forum for clues, I think it might be a loose exhaust valve guide.  I pulled the headers off, and there is an oily film inside the right side exhaust port, header and muffler.  On the left side these are sooty but dry.  Is this conclusive, or are there other things I should be checking?  I do have a hazy memory of one of the new guides being much easier to install than the others when I put the head back together.

[CheeserBeezer]

There's obviously oil burning on that side of the engine so it must be coming up past the rings or down the exhaust valve. If you say the valve wasn't tight it seems the only way forward is to whip the head off and check. Oversize valve guides are available. It might be worth doing a compression test. Significantly lower compression on the culprit side would indicate bores, assuming the valve seats are OK.

[muskrat]

G'day Tom.
Looks to me as the rings or the valve/guide clearance. If it was the guide in the head there would be evidence higher up on the guide.
I just bought +1, +2 and +4 guides for when I need them.
Cheers

[chaterlea25]

Hi Tom,
As Muskie said if the guide was loose there would be oil at the head / guide joint
Take the carb off and look for oil in the inlet side.
What do the plugs/ piston tops look like?
I would also check the cork gasket in the breather valve, these can settle in and cause oil problems

John

[Jules]

I'm guessing that if its truly coming up (the bore) or down (the guide) then you must see that amount of smoking when riding I would have thought?? If so, a quick check of when it smokes might give you a good idea of whether its coming up or down could help you understand the issue better ie wide throttle, heavy load (low vacuum) burning oil = rings/bores vs light load/overrun/idle (high vacuum) burning oil = guides, as a general rule of thumb...

[BSA_54A10]

Rule of thumb
Smokes on acceleration = rings
Smokes on the overun = Guides

[Greybeard]

Something I learnt when I was a motor mechanic: Do a compression test then pump some oil down the plughole and do the test again. If the pressure has gone up it indicates worn rings. The oil makes a temporary seal.

[Jules]

I agree with that idea Greybeard, in principle, but in reality I've never actually found compression testing to be an accurate guide to anything!!

[BSA500]

I would recommend a leak down test rig. I had a smoky engine and it found the leak across the cylinders and a leaky inlet valve.

[BSA_54A10]

I agree with that idea Greybeard, in principle, but in reality I've never actually found compression testing to be an accurate guide to anything!!

Compression testing is a good way of monitoring wear
However you have to keep track of the compression readings over time
Thus you will know when new rings are are needed well before you start laying smoke screens
A sudden large drop will usually mean a gasket leak
When I ran the SR500's & XJ 750's for work they all got monthly compression tests
The SR's would start off about 175 psi and by the time it dropped to 120 it was new ring time and I could usually get 3 or 4 sets of rings per rebore
With the XJ's the magic number was 95 psi.
However the tests have to be done the same way using the same tester as there is quite some difference between them, particularly at the domestic end of the range

You are right about a random test of an unknown engine, all it will tell you is either it is too high so pistons are probably going to hit the valves or head or too low so it won't start on petrol.
Bt a calibrated calf muscle will alert you to both of them as well .

[tomkilde]

Thanks all for the advice and information.  Here's an update.  The results of a compression test (roughly 80/100) told me that my amateur DIY engine rebuild was not a success, so I took the head and barrels to a proper machine shop for evaluation.  The initial inspection showed enough unevenness on the face of the head to prevent a good seal, and some faint scratches on the bores.  Additional measuring of the bores is pending.  The valves did pass a vacuum test, so at least I can pat myself on the back for that.  The head will be skimmed and the cylinders will be honed next month, and it looks like I will need to go shopping for a set of oversize pistons.

[sean]

Thanks all for the advice and information.  Here's an update.  The results of a compression test (roughly 80/100) told me that my amateur DIY engine rebuild was not a success, so I took the head and barrels to a proper machine shop for evaluation.  The initial inspection showed enough unevenness on the face of the head to prevent a good seal, and some faint scratches on the bores.  Additional measuring of the bores is pending.  The valves did pass a vacuum test, so at least I can pat myself on the back for that.  The head will be skimmed and the cylinders will be honed next month, and it looks like I will need to go shopping for a set of oversize pistons.

have you measured the guides / valves be easier to replace now while its down and give you peace of mind .

[tomkilde]

I replaced the valves, guides and springs when I rebuilt the engine, but the machinist didn't seem impressed when I told him that.  He plans to take everything apart and check all clearances, etc.  He will also grind the valve seats if they are not perfectly concentric with the new guides.

[Swarfcut]

 It was always practice to regrind the valve seats after replacing the guides. The new guide is used as a datum for the conical grindstone, carried in a spindle and applied to the seat. One of the most simple (and money making) machine shop procedures, completed in most cases quicker than you can read this twice. Dunno, but most guys in this sphere of activity seem miserable buggers, when in fact when the job's done well they actually sell happiness.....

 Oversize pistons will require the cylinder to be bored to suit. Honing is a process which at best removes glaze, at worst renders the bore useless for existing pistons and rings. It is more usually the final finish to a boring operation, smoothing the crests of what is in effect a wide bore fine internal thread. The standard of tooling used to hone cylinders ranges from precision equipment to a lavatory type brush with abrasive balls. Ask to see the hone.... there may be some reluctance to show it.

  Swarfy.