Topic: compression test on my rebuilt turbo motor...questions

[cafebob]

So I finished rebuilding the motor for my turbo project. (836 kit, 750 DOHC rods, F2/3 head, CR 7.9:1, F2/3 internals in a 76 case so that i can use a 520 chain)

im going to put the motor in the bike tomorrow, and wanted to see how the compression looked before I get the motor off my bench.

hook up the battery, kick the starter over and 60-40-40-60.... 

#$%* myself, thought about it a little, and put some marvel mystery oil in each cylinder.....retested and 120-120-130-150..... 

valves are adjusted and triple checked.  pistons and cylinders were measured and within spec. I have rebuilt motors before, and usually the are around 90 right after assembly, and come up to 150 or so after they are run in.

the only thing i did diffrent in this build is to use assembly lube during cylinder assembly, instead of motor oil.....my thinking is that normally i add more oil during the assembly, and the rings get better compression. 

so what do you all think put it in the bike, or tear it back down?

bob

[JLeather]

Are they new rings and a fresh bore?  Sounds to me like the rings need to seat.  If I was you I'd setup a makeshift run stand and break the rings in and retest.  Did you make sure the ring gaps are spaced away from each other?  The fact that oil jumps it up means it's definitely something with the rings. 

[cafebob]

its almost easier to just put it in the bike...I have to have the motor mounted to get the header made...hence the push. my gut feeling is everything is fine, especially  since it jumped so high when i added the oil...just really sucks that im not 100% about the motor....

bob

[Terry in Australia]

Well that'll teach you NOT to check the compression before you fire up that beast Bob, ha ha!

But seriously, something is wrong. I just finished rebuilding my 836cc engine, and even before it's first run, I nearly had to stand on the kicker to turn it over! I still haven't done a compression test, but it doesn't blow smoke, so I'll wait until it's bedded in some more before I do.

You say that pistons and cylinders were measured and are within spec, does that mean that you've re-used used parts? That's not normally a problem, mine are used Arias pistons in used bores, but there was a thread here where a couple of "experts" confused everyone regarding the method for  measuring correct piston to cylinder gap, just to recap, the correct gap should be around .001 - .002 on one side only, i.e., with the piston hard up against the other side of the bore.

One other thing, and I'm not trying to insult your intelligence, but are you sure that your compression rings are up the right way? The top (chrome) ring should have some numbers or letters stamped on it and these should be facing up, and the same for your second (dull) compression ring. If they're fitted the wrong way, you'll certainly have compression problems.

As you're (sensibly) running lower compression pistons you'll have to expect that 150psi is about the best you'll hope to get, but 40-60 is no good, and my advice is to flip the head off again and do some more measuring! Cheers, Terry.

[cafebob]

yeah, i may just let it sit for a few days, and think it over.  The pistons were used, but i think the rings are new (at least thats what i was told)  And they seemed new, as they "sprung" away from the piston groove.  On most used pistons I've seen the rings kinda sit in the groove with out much seperation at the ring gap.  These guys had alot of spring, and were a pain to get the cylinder over.

The weird thing is the rings did not look like normal 750 rings.  They did not have the typical chrome / beveled ring combo.  But I have seen other after market rings (MTC and Arias) that don't match this set up. In fact I measured these rings and they were both the same thickness, and looked identical to each other.  no numbers or marks on the rings..I think these pistons are from Arias, based on the 3 groves on the bottom of the piston...but i could be wrong.


then again I may just say what the hell, and see what happens...my gut says the rings just need to seat.  I could also hook it up to a car battery and run the starter for an extended time...and see if the compression drops after the oil is pushed around.  i have tons of starters so if i burn one its no big deal.

Damm me for being to careful, a younger me would have said just fire it up....

bob

[Jim F]

Well that'll teach you NOT to check the compression before you fire up that beast Bob, ha ha!

But seriously, something is wrong. I just finished rebuilding my 836cc engine, and even before it's first run, I nearly had to stand on the kicker to turn it over! I still haven't done a compression test, but it doesn't blow smoke, so I'll wait until it's bedded in some more before I do.

You say that pistons and cylinders were measured and are within spec, does that mean that you've re-used used parts? That's not normally a problem, mine are used Arias pistons in used bores, but there was a thread here where a couple of "experts" confused everyone regarding the method for  measuring correct piston to cylinder gap, just to recap, the correct gap should be around .001 - .002 on one side only, i.e., with the piston hard up against the other side of the bore.

One other thing, and I'm not trying to insult your intelligence, but are you sure that your compression rings are up the right way? The top (chrome) ring should have some numbers or letters stamped on it and these should be facing up, and the same for your second (dull) compression ring. If they're fitted the wrong way, you'll certainly have compression problems.

As you're (sensibly) running lower compression pistons you'll have to expect that 150psi is about the best you'll hope to get, but 40-60 is no good, and my advice is to flip the head off again and do some more measuring! Cheers, Terry.

Dang Terry..........You run that much clearance between the cylinder and piston
I Did all my engines at a total of .0005 and .0008 (.00025 and .0004 per side)
My 836 Motor has always had between 175/180 PSI ranking preasure
My 840 Motor with Arias pistons is the same
but I do like things tight.
But It does sound like he needs to pop the head off and see whats up


cheers to you and Marry Christmas
Big-jim

[Jerry Rxman Griffin aka MuthaF'er]

Bob,

Did you check the ring end gap?

[cafebob]

sure did, it was correct.

[Terry in Australia]

Dang Terry..........You run that much clearance between the cylinder and piston
I Did all my engines at a total of .0005 and .0008 (.00025 and .0004 per side)
My 836 Motor has always had between 175/180 PSI ranking preasure
My 840 Motor with Arias pistons is the same
but I do like things tight.
But It does sound like he needs to pop the head off and see whats up


cheers to you and Marry Christmas
Big-jim

No worries Jim, if it works for you that's great, but I used to do a little part-time work in my cousins bike shop boring out small engines (lawn mowers and dirt bikes mostly) and we'd always  give a minimum .001 clearance, and sometimes more. (race applications were closer to.002)

Jerry Griffin has a NOS 900 kit, and I'm sure the instructions call for .005 clearance! There is a formula (which escapes me) about the clearance being in direct proportion to the diameter of the piston, so I guess there is room for some variation. Merry Christmas Mate! Cheers, Terry.

[MRieck]

 Forget the compression test and do a leak down. If you have problems sealing you'll hear where it is coming from.

[JLeather]

The clearance also depends heavily on the piston material.  Cast pistons should be .0005"-.001", and big forged pistons (like 1000cc kits) should be .002-.0025 or so.  Cast aluminum doesn't expand as much when heated as forged aluminum.

For instance, on my car (chevy 350) I used hypereutectic pistons.  They're like cast, but with more silicon for strength.  The spec'd clearance for the bore on those was .0010 (measured at the skirt, not the top).  Similar forged pistons for the motor require anywhere from .0025-.0045.

[Lumbee]

...I guess I'm the only one that didn't see anything wrong with the comp numbers    Seriously...on most of my freshly rebuilt engines I usually see very low numbers before the bike is ran.  Then after running, the numbers get closer to normal.  Maybe the number don't strike me as odd because its an 836...whats normal comp. for an 836?

[Jerry Rxman Griffin aka MuthaF'er]

"Jerry Griffin has a NOS 900 kit, and I'm sure the instructions call for .005 clearance! There is a formula (which escapes me) about the clearance being in direct proportion to the diameter of the piston, so I guess there is room for some variation. Merry Christmas Mate! Cheers, Terry. "

The kit is a Henry Abe. It calls for .001 to .0015 per inch of piston.

[Terry in Australia]

"Jerry Griffin has a NOS 900 kit, and I'm sure the instructions call for .005 clearance! There is a formula (which escapes me) about the clearance being in direct proportion to the diameter of the piston, so I guess there is room for some variation. Merry Christmas Mate! Cheers, Terry. "

The kit is a Henry Abe. It calls for .001 to .0015 per inch of piston.

So somewhere between .003 and .004 for 67mm pistons Jerry? Are they forged or cast? Cheers, Terry.

[Jerry Rxman Griffin aka MuthaF'er]

Look to be cast to me. I'd like to have one of each side by side to really be able to tell the difference. Look like stock Honda pistons. Are they cast?

[Terry in Australia]

Yeah mate, OEM pistons are cast. Actually JL's post explained something to me that I've wondered about for years, about forged pistons expanding more than cast.

I always thought it was the other way around, that the less dense material will expand more when heated, but JL's post explained to me why forged pistons "rattle" until the engine warms up, because obviously there's more clearance to allow for more expansion.

Geez, I've learned something else! (I wonder what I'll need forget to make up for the space that this new piece of info will take up in my tiny mind?)