questions on reringing

[Basscat1]

I got a 94 225 omc that im not likeing the compression #s on. Motor has about about 20 hrs since rebuild, running fine but #6 is showing about 87 while #1 is about 107. I done the rebuild right i thought with 3 cylinders bored and 3 honed with cooling mods done to the block. Used new Wiesco pistons with new bearings and used but excellent condition rods. I doubled oiled till about 15 hours. I had a hard time getting the motor running right at wot after rebuild but have now solved that issue. Ive sunk alot of money in this motor and want it to be a good reliable motor that will last me a few years. Can I just rering it without honing it back out and get a good job? Also what could some of the problems be for the bad #s. Ive checked the compression 3 or 4 times in the last 4 hours and although it varies some here is about what it is from my memory. #1 105 to 113 #3 103 to 110 #5 97 to 103 #2 96 to 102 #4 96 to 102 #6 85 to 87<br />These are off the top my head but i have exact #s in the garage. Any suggestions on where to go from here.<br />Thanks

 

[Hooty]

Re: questions on reringing

I don't have any advise but maybe a thought or two. <br />First, how does it run? <br />Second, when reboring for forged pistons, more piston to cylinder clearance is required because of the expansion rate of the forged piston. Was the engine up to temp when you checked the compression?<br />Is the engine running in the 140* temp range? Here I'm thinking about coked up rings. <br />If you find it necessary to re-ring, I'd check the bores and if they were ok, I would hit it with a ball hone.<br /><br />c/6<br />Hooty

 

[WillyBWright]

Re: questions on reringing

Were the holes that weren't bored at least honed? You didn't reuse any old rings, did you? Otherwise, I'd give it more hours at regular mixture to break them in. If you reused rings, it could take hundreds of hours for them to wear-in, especially if they found themselves in a different hole.

 

[rodbolt]

Re: questions on reringing

hello<br /> there can be many reasons for lower compression. not knowing who or how it was rebuilt I cant say what was done. I wont hone a cyl unless I bored it. no reason to. if its scratched it means I have lost my .oo4" maximum piston to wall clearence. this is the max. this is where the factory considers it wore out. now the rings on a 2 stroke really dont wear into the bore. they are a very low tension ring. if the cyl surface was correct at assy it will hold less than 5% on a leak down test the day it is assembled.I would suspect a looser than normal fit on the piston to wall. but instead of a compression reading take it to a known good shop. automotive or outboard and have a leak down test run. a cyl leak down is far superior to a compression in determining the sealing abilities of the cylinder. the major reason for break in is to allow the microscopic burrs on the cyl to erode and to allow the piston skirts to wear into the cyl. a piston is not round but rather elliptical. its called cam grinding. the cyl, when bored is round. when honed the cyl is whatever the stones floated around to. if a flex hone is used the bore diameter will taper and egg just like it was to start but will now be shiney.so if the taper and egg is different than the cam grind on the piston it alters wall clearence. if the intake and ex ports become chamfered to large it can cause the rings to walk around when passing over the port area.since the engine was not rebuilt I would noit get overly concerned with compression numbers but would rather see the % of cyl leakage on a leak down tester. just my opinion from years of rebuilding everything from weed wackers and R/C airplane motors to 1200 hp big block chebbies. it will give you peace of mind to know if the leakage is within the 12-15 % range. that is the max. if it is more tear it back down before it tears itself back down. if it is within 7-10 % then redo the compression test and record which cyl has what leakage and compression numbers for future reference. a good range is 5-7% you will always have some leakage past the ring end gaps. btw did you measure the end gaps? every now and then I will find one very loose.<br /> at 20 hours I would not think the rings are coked. forged pistons require a minimum of 5 min to warm up and you must keep the t-stats in place or a cold siezure can result. sorry about the long post <br /> good luck and keep posting but for peace of mind do the leak down.

 

[Tinkerer2]

Re: questions on reringing

Basscat<br /><br />You start by saying it's "running fine but #6 is showing about 87 while #1 is about 107" and at the end you mention different readings on different cyls.<br /><br />Sounds like your problem isn't the way it's running but the variable readings you're getting on your compression gauge.<br /><br />Would it be worth getting compression checked professionally to see if the same readings come up?<br /><br />Be a pity to rebuild it again if your compression gauge is at fault.

 

[Basscat1]

Re: questions on reringing

Thanks for all the replies. All cylinders were fresh with rebuid and new rings were used. Although i didnt do it myself, rings were gapped. Ive checked compression from dead cold to warm about 45 minutes after beeing run on water.Guess thats what with all the diffrent readings. The thermostats are new and it goes through the fast idle stage when its first started,then idles back down, so i assume its reaching the right temp. What is the right way to take compression test? I feel the guage is pretty close to accurate. The leakdown test I been wanting to do but dont have access to one. How much do they cost and where would you get one? Also how exactly do they work? Could one be made out of a compression guage?<br />Thanks for the help.

 

[R.Johnson]

Re: questions on reringing

When they bored, and honed 3 cylinders, did they measure the other 3 for size. Just because those other 3 were'nt scuffed or damaged does'nt mean they are still to size. Did you put all new piston's in, or just the three on rebore. The factory states that compression of the v-6's on the bottom two cylinders will be lower, they don't say how much lower, as they don't publish compression figures. A leak down tester consist's of 2 pressure guages, a air regulator, and a sized orifice. One guage goes on the inlet side of the regulator, and the other on the outlet. I really don't know what they cost, as I made my own. Where I live we have a business that deals in industrial air systems, so finding the parts was easy.

 

[Basscat1]

Re: questions on reringing

The block as took to a supposably really good marine machine shop. I know they measured the bore, all 6 I cant say for sure. It was supposed to be done right. All 6 pistons and rings were new. I have access to guages and regulators and would be interested in making a leakdown tester if someone was willing to walk me through the specifics on doin it.

 

[Basscat1]

Re: questions on reringing

What is going to be the end result of running the motor like this. Does diffrent compression get it out of balance and make it hard on bearings or something?

 

[Dhadley]

Re: questions on reringing

What was the problem with the top rpms? Did you correct the reason the motor had to be rebuilt? <br /><br />Rings can and will coke up in 20 houre especially on double oil if the motor is lugged.<br /><br />The new Wiseco's (31 series & coated) do not require extra clearance when boring. Is number 6 a bored cylinder?

 

[Basscat1]

Re: questions on reringing

The high end timing adjuster stop came loose and backed all the way out. Scorched #1 cylinder, so we blamed it on this. After rebuild it would kind of smother at 3/4 throttle. Run fine up untill then but get to 3/4 and it would just gain no more and sometimes even loose afew rpm. Tried diffrent props and it didnt matter what the rpm was, just wouldnt take the last bit of throttle. Went through all electrical and fuel system and everything was ok. Ended up putting a new style air breather on it and it took off. I guess it was starving for air on high end. #1,3 ,5 bored, 2,4,6 just freshened and cross hatched. I really dont have a clue when it comes to bore sizes and tolerances. So i cant say for sure they were sized to the right tolerances and rings gapped right. I personally know the guys that rebuilt and done the machine work, not saying they are above a mistake, but i feel confident they have the knowledge to do it right. Ive also been running Amsoil synthetic in it since rebuild.<br />Thanks