No compression after rebuild

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golden boy

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Cant get my 408 to start after rebuild. Fires.timing is about 12 degrees. Leakdown test show air coming out thru crankcase. 0 compression. Rhone with 220 stones new rings
Pistons. 3 thou average clearance. Ring gap is 32 thou top 26 second ring.cant understand why no comp
Fires but won't start. Any ideas???

20180924_152431.jpg
 
My guess would be a problem with the rings. Did you off set your gap? Maybe it was not gapped properly? Maybe a stupid question but did you do the assembly yourself?
 
No compression on all cylinders ? Cam/crank timing off causing valves to be open on compression stroke or bent valves. Pushrods too long, valves not closing. Rockers too tight, valves not closing. Only reason for "0" compression is a valve train problem. But you say it fires ? Can't with "0" compression unless you are just igniting using an ether type of starting fluid and that is igniting. There is more to the story than we are being told. We need more info about the build it self starting with the components used and the assembly procedure.
 
MoparLeo said:
No compression on all cylinders ? Cam/crank timing off causing valves to be open on compression stroke or bent valves. Pushrods too long, valves not closing. Rockers too tight, valves not closing. Only reason for "0" compression is a valve train problem. But you say it fires ? Can't with "0" compression unless you are just igniting using an ether type of starting fluid and that is igniting. There is more to the story than we are being told. We need more info about the build it self starting with the components used and the assembly procedure.
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I agree with you but he said it's leaking out of the crankcase that tells me the rings are not sealing.
 
I had similar problem one time I didnt have any compression and I pulled the engine back out and disassemble it and the builder forgot the bottom rings.
 
Did you check piston to valve clearance when you assembled? If you had insufficient clearance, the valves would hit the first time around and then no more compression.
Pull the rocker arms and see if the valves are obviously not closed. You can also put and air hold tool in the spark plug hole and see if the cylinder will hold pressure. If it blows by freely you likely have a bent valve.
 
You didn't specify a Leakdown result, so I might assume you did it at BDC, and that cannot work unless you back off all the rocker gear.
But you give a confident big fat zero for compression, and that also is not possible, unless maybe the intake is completely isolated from the atmosphere, but the comment "fires but won't start, precludes that. At least you didn't say backfires and lit the car on fire,lol. So in that case, at least the intakes are closing.
So far we know the pistons are going up and down, the intakes are closing, and somehow with zero compression, she fires but won't start.
Hmmmmmmmmmmmmmmmmmmmmmmmmmm
My guess is your gauges are lying,charge the battery, pull the plugs, squirt a lil oil in each hole, crank it over to distribute the oil and blow out the excess. Yeah it will make a mess so take steps to minimize that. Then repeat the compression test with a different screw-in tester, or do the LD test with the pistons at TDC-compression,each one in it's proper sequence. Or if you cannot get the pistons to hold still, sometimes it's just easier to do the LD test with the pistons at the bottom and the rocker gear backed off. Unfortunately, this does not tell us anything about the valve seal AFTER the rocker gear is re-installed; it only tells us if the chamber can or cannot be sealed, not is sealed.
Then post up the results.

BTW; some LD into the cc would be normal, say up to 2 or 3%, I mean the air will find the ring-gaps pretty quick
 
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golden boy said:
Cant get my 408 to start after rebuild. Fires.timing is about 12 degrees. Leakdown test show air coming out thru crankcase. 0 compression. Rhone with 220 stones new rings
Pistons. 3 thou average clearance. Ring gap is 32 thou top 26 second ring.cant understand why no comp
Fires but won't start. Any ideas???

View attachment 1715229949
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Extra parts after assembly look like this? :poke:

steel-piston-rod.jpg
 
Ring gap may be OK if these are KB hypers. They call for a large top ring gap. Regardless, it should fire and run fine with those gaps.

I would typically assume the '0 compression' means readings from a compression gauge but that is not clear. I seriously doubt the cylinders are at zero unless there are holes in the pistons or the rigns were just no there. And the symptom of 'air in crankcase' does not mean a lot unless we know if it is a lot of air or just a little air leakage. Especially since it wants to fire.

Is the fuel in the car old, like even just a few months old, from sitting for some months?
 
THERE IS NO POSSIBLE WAY that "ring gap" will ever prohibit a start or be so bad as to be "zero." Throw a few drops of oil in each cylinder, crank it, check compression, see if it comes up. I think you have "something else." Likely first thing I'd try is loosen/ remove the valve gear and re-run the leakdown
 
diymirage said:
What if his timing chain is off 180 degrees?
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If the timing is showing 180 off then crank the crank shaft 1 turn and it is lined back up.
The cam turns at half the crankshaft turns.

2 turns of the crank to 1 turn of the cam.
 
If I undertand the question correctly,180* off is no big deal

I like to start at split overlap and #1piston at TDC.... because I know where split overlap is supposed to be.
So,now, at split overlap, the cam is finishing the exhaust stroke and beginning the intake stroke, so if the heads are on,both valves are slightly off the seat.If the heads are off, then both lifters for #1 cylinder will be nearly equally off the base circle.
180 more crank degrees, the cam has turned only 1/4 turn, and the piston is at BDC, finishing the intake stroke, and soon beginning to build compression.
Another 180 crank degrees, and the cam has now turned 1/2 turn or 180*, and the piston is back to TDC finishing compression.
180 crank degrees later, the cam has turned 3/4 turn, the piston is at BDC of the power stroke, and the exhaust stroke has begun.
Another 180 crank degrees and back to split overlap,where you started, the cam having turned 360 degrees, while the crank went 720, or two turns.
 
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When the pistons hit the valves at cranking speed, it makes a horrible clack-clack racket and if your ears have never heard it before, your brain will be tardy letting go of the key. If you have heard it before, it's too late anyway, the engine is cranking at 6 revolutions per second, so the damage is done, and the heads have to come off.
BTW; the sound doesn't go away, so if your brain doesn't figure it out right away, and you hit the key again well, clack clack there it is again! And as long as the engine is spinning the clacks will follow at 6 revolutions per second, 4 clacks per revolution,lol. Don't ask me how I know these things lol. And if you laughed, stay away from the screen at least three feet,lol.
 
zero compression ? Interesting, I will wait for the outcome, it should be simple. a couple minor tests should tell the story.
 
Jadaharabi said:
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If the timing is showing 180 off then crank the crank shaft 1 turn and it is lined back up.
The cam turns at half the crankshaft turns.

2 turns of the crank to 1 turn of the cam.
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To put this simply, a cam cannot BE 180 off. It can be any other amount of degrees, but not 1/2 a turn
 
Before you rip the front off to double check your cam timing, just check split overlap. Pull a valve cover off,whichever is easier and check the valves for #1, or #6 cylinder at TDC. If one valve is open and one is closed, just turn the crank around 360*. At split overlap, BOTH valves will be open close to the same amount. Rock the crank back and forth and watch the valves; one will be opening and the other closing, and when you reverse rotate the crank, the valves will switch directions.. If they are NOT both open approximately the same amount within about 5 degrees of TDC............... then it's time to take the front off.While that cover is off, you can check the pushrods.
 
AJ/FormS said:
Before you rip the front off to double check your cam timing, just check split overlap. Pull a valve cover off,whichever is easier and check the valves for #1, or #6 cylinder at TDC. If one valve is open and one is closed, just turn the crank around 360*. At split overlap, BOTH valves will be open close to the same amount. Rock the crank back and forth and watch the valves; one will be opening and the other closing, and when you reverse rotate the crank, the valves will switch directions.. If they are NOT both open approximately the same amount within about 5 degrees of TDC............... then it's time to take the front off.While that cover is off, you can check the pushrods.
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So! as it turns out , a little thought goes a long way. Thanks to those who quickly pointed out it had to be the valve train. it seems that i had all the valves adjusted just enough to keep them barely off their seats. this causes the engine to sound weird (like a broken timing chain)when it turns over . but the interesting thing is that i checked #1 with rockers backed off @ TDC and 0 compression /leakdown was air pouring out of the crank case. So ,for those who are curious. freshly rebuilt engines have no compression until they run enough to seat the rings. who new? got it going last night sounds great!!!
 
Glad it is all good now! Woo hoo!

Odd... now you have me wondering what was going on in yours.
  • Any info on the rings?
  • Were torque plates used in boring/honing?
  • Piston-to-bore clearance clearance?
  • Maybe a stroker thing, what with the short pistons?
 
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