No life (a.k.a. DattoMaster) 
Joined: 2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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I've read a few places that chrome rings won't bed in on lpg, but frankly I'm not sold on it. The biggest 'need' for rings to bed in properly, is cylinder pressure. It's not spring tension, but combustion pressure which shoves them hard against the bores adn gets them to wear in to each other's groove (so to speak).
So to do that, the main thing is to run it with decent throttle opening, enough to create sufficient combustion pressures, during the ring bed in period (which is about 20-30 minutes for plain rings btw). After that, you don't want to 'keep' doing that as much, for the sake of the bearings to bed in which takes longer. After that initial ring bed in, just drive normally, not revving astronomically high, and every 20 minutes or so of driving, give it a hard squirt from low down in the rpm range to moderate/high rpm (not quite redline, lets say no higher than 5500 if it's redlined at 6500) - doing that every so often will help any final bedding in of the rings without massive risk to anything else.
One of the key things is also to run (typically) a mineral and MONOGRADE oil. The biggest reason for monograde is that the friction modifiers and vi improvers in oil tend not to do well with high temps/shear, and combustion gases getting past currently not bedded in rings (i.e. the fresh engine) is enough to compromise them, so the monograde oil won't deteriorate in its spec as quickly under those conditions.
If you wanted a schedule for running in the rings - say the eventual redline will be 6500 rpm for arguments sake. Get your cam run in, change the oil and filter, and out on the road. Start with whatever rpm low point the car will handle some throttle without hunting, let's say it's 1500rpm, load the car up, the heavier the better, then you can do it in the lower gears and not risk a fine) take it out to about half throttle and let it rev up to about 3000rpm, back off to 1500 and repeat a few times. Then go from 1500-3500 with about 60-70% throttle, repeat a few times, then go from 1500 to 4000 at about 80% throttle, repeat them 1500 to 4500 and 90% throttle and finally do a bunch of runs from 1500 to 5500. Basically repeat each run enough times that the total ring bed in time pans out to about 20-30 minutes. It'll do the job. The specific throttle and rpm % don't have to be exact, I'm giving a general sort of an idea, 500rpm either way or whatever won't really hurt.
Another big thing - whilst running in a cam, run the thermostat housing with the outer ring of an old thermostat but no centre valve in there, that gives full flow but still preloads the water pump, and helps keep it all at its coolest which is the go for running in a cam, it's a type of work hardening process, it wants to stay cold.
BUT for running in rings, fit a new thermostat, and with a temperature you intend to run all the time (for me that'd likely be 190F or possibly 195F on lpg because that's where the mixers and convertors were designed to run and meter fuel optimally). You need that temp in the engine to give the bore whatever shape/size it'll have whilst actually running normally and to bed the rings in to exactly that scenario.
After bedding in the rings for the 20-30 minutes, I'd probably drop the oil again (not the filter) and then run it out to around 1000km of the normal driving with a quality mineral oil (it seems anecdotally that synthetic is actually 'good' enough to not allow rings to bed in, btw). If you were still seeing some blowby out of the rocker cover after the 20-30 minute ring bed in, I'd not hesitate to repeat it for another half an hour or so. Chrome rings are hard, but they have been run in aircraft engines and they come out for rebuilds after less flight/operating hours than some have suggested it takes to bed them in on a street car. That can't be right. (I do note that plane engines operate at or near full throttle for most of their lifetime, which certainly doesn't hurt ring bed in of course!)
Posted on: 2010/3/22 6:43
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