New valves in slightly-used guides/seats?

[svcetiquette]

Good morning all,
I did some searching but only found some older threads that didn't directly apply. What are options for installing new valves in slightly used guides and seats? Sounds like hand-lapping is no longer recommended.

Situation is thus: stock 3.0 aside from PMOs and headers in a track car that's only used for DEs. Engine has just under 12hrs since it was completely rebuilt, 6500 red line. The heads and valves were in good shape so just installed new guides and the seats and valves were ground. My engine builder also advised that safety collars were a better alternative to carrera pressure-fed tensioners. During the last event, the 1-3 tensioner & said safety collar failed at 5800 rpm allowing the cam to slip a few teeth... You all know how this turns out... intake valves hit pistons, all 3 rocker arms broken.

I'm borrowing a leakdown tester this weekend to see if the valves are bent, but given the track only nature of the car, I'm inclined to replace all 3 valves given the impact they suffered. What say you all for the best course of action to get back on track? Thanks!

[svcetiquette]

[dannobee]

Guides are often damaged in the process. And they’re a wear item on 911’s anyway. Check for wear and damage or have your machinist do it.

[Ollies930]

Strange, would the camshaft not usually retard if the chain slips and therefore increasing the intake valve clearance? Should it not be the exhaust valves hitting in that case?

[Walt Fricke]

I'd let the leakdown be your guide. Any valve leakage from any head on that side (this assumes the other side can serve as a baseline)indicates pulling that head. At that point, the bent valve can be replaced, and the guides there checked. And you can check the others if you want, just in case. The extra work isn't all that much compared with what it took to get just one head off and back together. You could also take the three heads off as a bank with the cam carrier still bolted on. No need to pull the Ps and Cs - you can inspect the piston tops and decide if you want to go deeper.

I see you had the exhaust cover off, so no broken rockers there? Interesting. But my experience (all 12 rockers broken) was with a stripped IS gear. But sort of same deal - instant death. As soon as a rocker breaks, the valve (unless bent and stuck down) is safely out of the way, so attempts to limp to pits or get engine refired didn't hurt anything most likely.

[Walt Fricke]

At my 2d DE in 1985, I hit 1st instead of 3d upshifting from 2d in my '77 2.7. The engine ran a bit strangely. I fiddled around with ignition timing (complicated by the fact that the fan pulley locating pin had sheared, and the fan timing mark wasn't where it should be, which was quite a puzzle with the timing light until I figured that out) and the CIS air flow reading flap, and got it running smoothly again. Didn't break any rockers.

Fast forward a couple of years (of nice improvements in my lap times)and I took the car in to have tranny synchros replaced. When done, the mechanic drove the car, and said it seemed down on power. I pulled the engine and gave him the heads for a valve job. He said all the exhaust valves were bent into an S shape. The leakdowns, when I got the device and did that, had been decent. Because the valves had sawed away at the guides to allow the valve head to pretty much center and seal on the seat.

New valves, new guides. Had I done that immediately, maybe no need for the guides?

So my prediction is that you are going to find intake valve leaks on the damaged side, and not on the other side. That will tell you what you need to do.

In my case, the hell of it is that within a year or less of driving my now more powerful engine, with my somewhat improved skills, on the track, the head of a valve fell off and trashed the engine. What, after a shop redid the heads? Well, sometimes things happen.

[svcetiquette]

Hey Walt, thanks for the follow up. I did the leakdown test over the weekend - shockingly all cylinders were in the 20-30% range with zero leakage coming through any of the intake valves. 1 and 3 had a nearly imperceptible amount pushing past the exhaust valves, but I bumped each open a couple times with a prybar and even that went away. Plan to just put new rockers on and go with it. Excuse me while I go buy a lotto ticket in case my luck is still holding.

[Walt Fricke]

Er, on my leakdown tester, 20-30% would be pretty bad. Not as bad as, say 100 (valve stuck open, hole in piston), but you'd hear a lot of air from one or more of the orifices. Unless it were a daily driver, a guy would immediately plan on taking the heads off, and maybe at least re-ringing, depending.

2 or 3 percent would be just fine. A decimal place issue?

My tester has two gauges. I adjust the first to read 100 psi. Subtracting the reading on the second gauge (say 98 psi) gives a 2% leakage. But there are one gauge systems and other differences.

Anyway, sounds like you dodged a bullet.

[bpu699]

Originally Posted by svcetiquette

Hey Walt, thanks for the follow up. I did the leakdown test over the weekend - shockingly all cylinders were in the 20-30% range with zero leakage coming through any of the intake valves. 1 and 3 had a nearly imperceptible amount pushing past the exhaust valves, but I bumped each open a couple times with a prybar and even that went away. Plan to just put new rockers on and go with it. Excuse me while I go buy a lotto ticket in case my luck is still holding.

Yep...20-30% is a shot motor...

Sorry...

But that argues against bent valves somewhat... a bent valve would register a zero...

[Walt Fricke]

PBU - something not quite right. This was a one bank issue, right? So if all cylinders read the same, that doesn't make a lot of sense. Broken rockers on intakes on one side, all other rockers good? This wasn't from an overrev, but the chain jumping timing on one side.

I think we need to see how he came by those 20-30% leakage numbers. You always hear some noise from the rings, but shouldn't ever hear any from the valves absent something bent or burned. Or, as he intimates, a bit of carbon on an exhaust seat.

[E Sully]

When I had a tensioner fail, in the end I needed all the valves and guides replaced on that side of the engine. The valves were all damaged. As for the guides, I cannot say for sure they needed replacement from the bent valves or the fact they were worn from age. I would think that guides would suffer some damage from a bent valve. I had no rocker arm damage, maybe due to the tensioner failing at moderate rpm.
Either way, I think it would be a mistake not to pull the head and have the shop check everything. Murphy's Law.

[icarp]

When doing a leak down test, it is advised to use only 80psi for the in pressure. Then do the math for the held pressure . More than 80psi in is not fair to the rings , this should help with more accurate numbers .

[Walt Fricke]

Icarp - is this an FAA spec? I like 100 because it removes the math, but otherwise have no idea how it might affect accuracy. My first compressor couldn't maintain 100, so I used 50, for the same reason. Both seemed to work about the same. Air is slippery stuff, so I'm unsure just how much the pressure would affect leaking. I'd suppose 100 psi isn't going to get past a valve where 80 would not. Pressure behind the top ring helps seal it, but a good running engine has well over 100 psi on the compression stroke.

I sort of view leakdowns like compression - what is the cylinder to cylinder variation? And as to where the leaks are, just sending the air directly into the cylinder lets you figure where it is hissing out. The numbers do matter some. I liked Total Seals because I generally had 0% leakage, but my gauges weren't accurate enough for that to be Bureau of Standards accuracy. Just said the engine was ok for the moment.

[bpu699]

Stupid question...I think...

Bearings are actually soft. Or at least the top coating is that touched the crank...

You apply 100psi to the cylinder. That’s 1200 pounds of pressure or so applied to that rod bearing at the big end...

How does that not damage the bearing???

Now, I have done tons of leakdowns...and I know the bearings are fine... I also understand the motor combustion applies tons of force, but that’s not static. That force rotates the crank...

[svcetiquette]

Originally Posted by bpu699

Yep...20-30% is a shot motor...

True for fresh jugs tested near operating temp. I reused the original P&Cs (alusil) of unknown history in the rebuild, they had obvious wear commensurate with high mileage. They only need to make it through another half dozen or so DEs before they come out to make a race engine.

[dannobee]

Peak combustion pressures in high performance engines top 1500psi. In diesels it sometimes tops 2500psi. Your 100psi is merely a fart in the wind. Parking the piston at TDC when performing a leakdown results in absolutely no wear. Zero.

Originally Posted by bpu699

Stupid question...I think...

Bearings are actually soft. Or at least the top coating is that touched the crank...

You apply 100psi to the cylinder. That’s 1200 pounds of pressure or so applied to that rod bearing at the big end...

How does that not damage the bearing???

Now, I have done tons of leakdowns...and I know the bearings are fine... I also understand the motor combustion applies tons of force, but that’s not static. That force rotates the crank...

[safe]

Originally Posted by svcetiquette

True for fresh jugs tested near operating temp. I reused the original P&Cs (alusil) of unknown history in the rebuild, they had obvious wear commensurate with high mileage. They only need to make it through another half dozen or so DEs before they come out to make a race engine.

20% is really bad even for a well used engine.

My stone cold 3.2 with an unknown amount of miles below the heads and 30000 miles in the heads is around 0-3%.

[safe]

Originally Posted by bpu699

Stupid question...I think...

Bearings are actually soft. Or at least the top coating is that touched the crank...

You apply 100psi to the cylinder. That’s 1200 pounds of pressure or so applied to that rod bearing at the big end...

How does that not damage the bearing???

Now, I have done tons of leakdowns...and I know the bearings are fine... I also understand the motor combustion applies tons of force, but that’s not static. That force rotates the crank...

Its not uncommon for turbo engines to run 30psi of boost, then add 10:1 compression ratio. Thats 300psi even before combustion....

[bpu699]

Originally Posted by safe

Its not uncommon for turbo engines to run 30psi of boost, then add 10:1 compression ratio. Thats 300psi even before combustion....

Great points...

On a microscopic level, how does the relatively soft coating not get deformed?

The coating is very easy to scratch.

The bearing itself is steel, but isn’t the coating a softer lead or something similar?

Take a piece of lead and apply 100psi with press, it easily deforms...

Now take 2000psi+ and apply it through the crank onto the bearing over what is likely 3 square inches of bearing...

I know it doesn’t cause damage, just can’t wrap my mind around HOW it doesn’t cause damage...

[safe]

Originally Posted by bpu699

Great points...

On a microscopic level, how does the relatively soft coating not get deformed?

The coating is very easy to scratch.

The bearing itself is steel, but isn’t the coating a softer lead or something similar?

Take a piece of lead and apply 100psi with press, it easily deforms...

Now take 2000psi+ and apply it through the crank onto the bearing over what is likely 3 square inches of bearing...

I know it doesn’t cause damage, just can’t wrap my mind around HOW it doesn’t cause damage...

You have an oil film that is "soft", the bearing never contacts the crank.