The_Shark

I removed my jammin JPX .21 for a routine cleaning, and I noticed how little compression this engine has, I got it a few tanks old and has always had little compression, but the engine runs like mad, VERY powerful and is very reliable, I just really took notice comparing it to my almost new nova p5x

whats going on? is this normal? I'm going to check how many shims are on there, it has about 3 gallons through it on 20% with byrons race formula always run a tiny bit on the rich side (but never boggy)

just doesn't make sense to me!

1QwkSport2.5r

Well, used car engines are IMHO always iffy. The first tank is most critical, especially the kind of fuel used and how it's tuned. If it runs good and doesn't overheat and quit, the piston/liner fit is still good enough to have good transfer velocity. The idle is the first thing to suffer so when it starts idling like crap or not at all, that's when the piston and liner needs to be replaced. Pull the head off and check the shims. If it has more than one, I'd go down to just one shim and check compression. Also, use a long reach glow plug if you aren't already.

Of course I'm not the expert around here so take my words with a grain of salt...........

supertib

pull the front bearing and check for dirt behind the bearing...I am seeing this on many offroad bigblocks..especially powerful ones like the JPX .21

here is what I am seeing on multiple engines, and it is resulting in extremely rapid engine wear

The_Shark

ok, I will tear it down later, still, i'm baffled because it will idle all day long...[]

pull the bearing.... will it come out easy or will I have to bake the case?

The_Shark

well I lost my pitman arm puller (makeshift engine flywheel puller) but i opened up the engine, no scouring, pitting, so signs of detonation or contamination, however looking at the crankshaft to piston arm bushing and i can see slop in the parts, not a whole lot, but its concerning me, I know aluminum swells with heat but what is a acceptable gap?

oh yeah, there where two THICK gaskets on there, i removed one and compression is a fair bit better.

downunder

That's the fit of the conrod to the crankpin. A new clearance is generally about .002" but anything up around .006" is usually considered acceptable. Most rods use a brass bush as the bearing surface but some rods are unbushed so use the rod material. And be aware that the conrod only fits one way onto the crankpin. One side of the rod (or bush) has a tiny chamfer and that end has to be fitted back on the crankpin first so it goes up against the crankweb. The reason for this is that there's a very small radius ground on the crankpin where it comes from the crankweb and the chamfer gives some clearance to that radius.

The_Shark

got it, so it appears to be ok then, I will run it till it won't run any more. Ha! feels like im going to auto school all over again

I remember checking bearing clearance by tightening the bearing with a piece of wax string in-between.

I don't know why i said piston arm instead of connecting rod??, I was thinking about the wrist pin at the time.