ok quick story 325 hemi now in an eary dodge truck was running, but not well three rebuilds later it has 30 pounds of compression spray a little oil in the cylinders that will go to 45 pounds Build by a reputable builder new hothead rings and spec pistons different brand of rings used on last build, helped (not much) Crank, rods, lifters etc replaced. Cam reground Pump 30 psi of air into a cylinder and it sounds like it leaks into the pan took off the fancy new timing chain set and replaced it with the old. fiddling with the timing got 5-10 lbs more compression used a indicator on the rockers and it appears the valves are shutting correctly will start and run (poorly) temp stays aat 180 with elec fan running ran for 1/2 hour then later 3/4 hour. compression 50-60 when warm then back to 30 when cold stumped, ready to throw in towl, got any ideas? thanks
You mentioned that the engine had been rebuilt several times before! The air escaping into the oil pan hints at several possibilities. One scenario, is that maybe the pistons are the wrong bore size for the block(pistons are .030 over and block is .060 over)! Or possibly, the rings are also wrong for the bore,creating way too much ring gap and barely sealing against the side of the cylinders! It sounds as if you will need to tear back into the engine a little deeper to find the real problem. If you do it yourself, study the rings individually as you take them out of the block, and put the rings back into the cylinder one at a time to verify actual ring gap, and that they are right for the bore size you have. Also,see if you can see any size stamped onto piston tops to also verify that are also correct for the bore you have. Good luck, hope you find the problem....
acording to the engine builder, nice fellow presently tearing out his hair, the this was the first time the block had been bored. since then it has been taken apart and reassembled three times, same problem each time no compression Pistons were made to spec. (don't want to mentions whose as pointing fingures is not the goal) Do you know what the ring and/or piston numbers should be for a 325? it time to do some double checking. Build no. 4 on the way.
Hi,sorry I hadn't responded sooner to your question! I thought for sure others would have added their comments by now. I checked several sources and the all confirm the stock bore as 3.687". maybe this will give you and your engine builder some reference to start with. Hope you find the problem on the 4th go around....
What is cranking compression reading supposed to be on this old engine? Do you have a spec? These engines didn't have real high compression like the 60s engines. I purchased adjustable pushrods to make sure my valves were closing. They are a pain, but with mixing matching new "repo" lifters etc, I didn't want to take the chance. Fiddling with the timing should not influence compression reading, unless it is the timing at the chain. Sorry to hear about the problems.
325 comp ratio is 9.25 OEM, should have same pressure as any other engine with simular comp., I'd think. Oil bringing up the pressure indicates bad piston-bore match or something wrong with the rings.
what type of rings, chrome? moly? they both require a different finish to the bore prior to assembly. did you check ring gap prior to assembly? how long has this one been together? did you observe proper break-in procedure? a chrome ring will take longer to seat than a moly ring. from what i understand, a chrome ring requires a more perfect bore finish, where a moly ring is more forgiving.
That's an awful lot of blowby for just rings leaking. Assuming the pistons are fitting the bore, check the piston/deck measurement. Maybe its just not making any compression due to low CR.
Maybe the cam? If the cam was badly ground, it might partially open the valves and dump compression. Are the compression readings even? Is the gauge good?
When you say engine builder, I assume this guy has rebuilt engines successfully in the past, so should have the skills to figure this out. Did he check the end gap of the rings in the cylinders before putting them on the pistons? Proper gap in ring lands? Are the valves closing completely? Correct head gaskets, and good seal there? Are you checking compression with a known good gauge, and do you have the throttle wide open when checking? What are the cam specs? Lots of overlap will lower the cranking compression reading. Do you have adjustable valvetrain, or is it fixed-length pushrods and non-adjustable lifters?
So it may be obvious that the proceeding post are ancient history, and they are. Its a real shame that this motor sat in the corner of the garage for years while a couple of other projects came and went. But finally, due to the combination of frustration and the need that at least some of the cash invested in the thing was recouped, it was torn down again. Everything checked, bore, rings, etc when the problem was found! (don't you love a happy ending!) Although we had checked valve lift we had not checked the duration. The cam was ground wrong. Ok so it took the search for another cam and another trip to a cam grinder, but she runs. Well it should be noted. It took a long time to finish this one, thanks to all of those who took the time and concern to offer an opinion.
Thanks for posting the solution to the problem. A lot of people can use this as reference if they experience a similar problem.
Are you certain the cam is for the 325. If it was a low deck cam it wouldn't index properly I believe that's the term valves wouldn't shut completely
You're spot on. Although the cam was specifically ground for this motor, and the cam card with all the specs identified it as a 325 cam, in fact it wasn't.
Glad to help. I have a soft spot for hemi builders. I was ready to fire up my 270 and found a water leak into the oil pan that could not be detected with magnaflux. Will pressure test any engine I build in the future
My understanding is that if you put a high deck cam in a low desk, or vice versa, @ least in a Chr, the engine will run, just not right supposedly because of the change in angle that the lifters have. Don't understand why that makes a difference. Haven't heard of it keeping valves open unless there's something else involved like too long a push rod, ect.
It really wasn't an issue of duration but perhaps overlap. In any case, the engine builder should have know to remove the pushrods completely - to ensure the valves were closed - then check for leakdown.
I like the idea of rings being too small for the bore. Even if the slugs are correct an if the rings are even just .010" or .020", the ring gap isn't the only problem. The outside diameter of the 2 circles are different and there will always be lots of room for blowby. I had a friend down in Louisiana that bored a 350 chevy .060" over, but used .030" over pistons. He figured it would have less cylinder drag and make for power. It ran for a few minutes and that was all. When you tear it down again, put a ring in the bore and square it in the bore with the top of a piston. Check the ring gap to see if it is in spec. Then put a light source under the cylinder to see if any light comes through the side of the ring anywhere. If all looks good I would check cam timing. My 341 uses a 318 timing chain kit. I think yours does too. I get my info from an old Motors Manual. Let me know if I can help you with it at all. Bill
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