compression Test Results

[Ldub]

I can't find it now, but wasn't there a reference to LPG conversion having a tendancy of burning exhaust valves?....

[PK]

I can't find it now, but wasn't there a reference to LPG conversion having a tendancy of burning exhaust valves?....

That there be my post in another thread.

http://vehicross.info/forums/showpos...93&postcount=7

When LPG conversions first came out, they recommended that you changed the exhaust valves to those that had been made of?? or treated with?? Stellite.

IIRC it is a change in materials that makes the valve harder and allows it to withstand hotter temps.
But gee - that memory is about 20 years old - might have all changed since then.
Might also only work down under where the exhaust gasses fall out of the cylinders.
You guys up on top have to push the gas out.

Although, looks appropriate in this instance.
It would be nice to measure the adjustment of a few valves before dismantling, and then dismantle to check condition. If condition worsens with valves out of adjustment, that would just about pin it down.

While I am on my soap box - I agree with Scott - replace the head bolts if you can get them.
Cylinder head gasket crush is one of the hardest things to get right when assembling an engine, and new bolts just add that little bit of extra insurance.
I have done it with old bolts, but only on older cars (pre 1980) when the torque values are lower, and the cylinder pressures less.
On those engines, I had to re-torque the head after about 1000 miles.
Some of these newer engines even have a "waisted" neck on the bolt (reduced diameter) to control the plastic deformation.
That type of bolt cannot hold the same tension when used again.

PK

[Ldub]

No wunner I cunna find it......

Thanks PK...

[Ldub]

Here's a discussion on the subject that i found when i goog'd "LPG conversion valve burning"

http://forum.australia4wd.com/index.php?showtopic=5330

I found this post informative...http://forum.australia4wd.com/index....ndpost&p=42009

There is more...LOTS more

[psychos2]

We reuse head bolts all the time . Some of the motors are 1300 hp. Some stock big blocks. If it says they are reusable then reuse them. Put it this way do they replace the main bolts when rebuilding? No and they are more stressed than the head bolts. You can tell when there is a problem with the bolt as it will not torque properly as the bolt is actually stretching and if you keep trying to torque it ,it will break. Just make sure to clean the threads in the block and the bolt threads so you get the proper torque. shawn

[SlowPro48]

Thanks for the info Scott. I guess coking isn't the proper term since coke comes from coal but that's what I've heard it called when oil gets turned to pure carbon on a hot engine part. Maybe we should call it "cooking" instead! Anyway the reason I asked is because I had an old four stroke dirt bike once that had that type of valve face pitting and I always suspected leaking valve seals was the cause. It was a "second string" bike that I kept after getting a new one because I wanted something to plod around on that I wouldn't feel guilty not washing after every ride. It smoked for a minute or two after cold starts but then cleared up and ran just fine so I didn't want to take time to install new valve seals. Then it lost compression. Not completely but enough that it was down on power and hard to start. The valves had always been adjusted - if anything a little on the loose side. As they say a ticking valve is a happy valve. It had rockers. I don't think they say that anymore now that shims are so prevalent. But anyway when I pulled the head, inverted it and poured some gas in it, it came out the exhaust port in a matter of seconds. Aha! Leaking exhaust valves. When I removed the valves I discovered that they were being held slightly open by baked-on carbon. It wasn't the nice, even, fluffy coating you'd get with running rich though - it was in one area and hard and crusty - took a knife and a Dremel with wire brush to get it off. My hypothesis at the time was that oil from the leaky valve guide seal was running down and pooling up in the same place every time I parked the bike and then cooking on the valve when I fired it up. Most of the leaked oil went out the pipe as a blue hydrocarbon haze but some stayed on the valve and cooked. The cooked carbon layer just got thicker until it finally interfered with the face/seat seal causing a slight leak - and a hot valve - because as you said the valve depends on conduction for cooling and there's not much conduction going on if the valve and seat aren't touching. So then I had overheated, leaking valves - and that's probably when the pitting started. My valve faces looked exactly like FuddyMucker's. Didn't replace 'em though - there was plenty of margin left so I just lapped 'em and slapped 'em back in there. Definitely replaced the seals though...

Figured it was probably a lash issue but now you know why I was wondering about valve guide seals and coked oil.

Those pictures sure make me want to check valve clearance! Guess I can add that to the list of things to do at 100K miles - which is looming - less than 500 miles away now..

OK that's my long-winded, thread jacking post for tonight.

[FuddyMucker]

Hello again Guys, sorry it has been a while since my last update but life has been a bit sh*t recently and I have had other things on my mind..

I managed to rebuild the head and fitted new valves and seats where needed. I also removed the other head and inspected it and it looked like it had already been done, it had an aftermarket head gasket fitted and the valves looked like they had only done a few thousand miles.
I rebuilt the engine with a complete head gasket kit and also fitted a new cambelt and tentioner for peace of mind.
I have done 1500 miles in it since the rebuild and am happy enjoying the extra few horses it has found.
I even had the LPG system re-tuned to make the most of that new power, I love running this VX on LPG because it drives the same as on petrol but at half the price, makes a real diffrence here in the UK with petrol at £1.25ppl and I'm only paying £0.58ppl for LPG (guess it is still expensive compared to the US though).