dkr

Member
I have a 5 STar I just rebuilt. M5 jugs bored to 4.8" with Ross pistons and new rings that came through Ross. Engine ran ratty so I checked and found #3 & 4 dead. Compression was 0. #1 & 2 were 240.Expecting broken rings or burned pistons, we removed the pistons and found everything good except we could see where the valve impressions could barely be seen on the piston surface. The rear of #3 piston was worn at the top with obvious scraping and a little scarring on the cylinder. We couldn't see anything wrong with #4 piston or cylinder. Does anyone have an idea what might be my problem. Thanks for any help.
 
Thanks, JD. I noticed they need replacing. But, would that account for the lack of compression?
 
im not sure of that but would try that first. i had a 6 cylinder moline that would skip on 1 head and we changed the intake and exhaust gaskets and it ran fine. if they look like they need replacing that could be the problem
 
Had that happen once on a U. I got in a hurry when installing the push rods and rockers. They were not seated in the lifters correctly and after it started they fell into place. Resulted in failure to open the valve. I sure was glad it was that easy. Hopefully yours will be that simple as well???
 
If everything looks good up top, it may be a valve not seating or hanging open some. I like to make an adapter for the compression tester to put compressed air in the cylinder. Lets you get an idea where the leak is.
 
Jason, we crancked it over while watching the valve action and all the valves were coming back to the top. I agree with you that for some reason the valves are open when the piston comes to the top. How else could there be valve marks on the pistons. You would think valve timing, but #1 & #2 are fine. Scratching my head here.
 
We thought about that and actually watched the valve action when it was running. They all worked normally. I'm lost.
 
MMDEL, this is gonna sound stupid, but, I can't remember. Immediately after I tore it down my wife died and I sorta lost interest in it for a couple of years. It's been about 2 - 1/2 years now since I tore it down so I really don't remember, which is not unusual for me. I had to change out the timing gear so I guess there's a chance I did remove it. What might be the downside of that?
 

Are you sure the head gasket made a good seal??

There isn't a chance it is/was blowing between the 3 & 4 cylinders, is there??

Did the valve adjustments seem to feel right when you set the valves??


Howard
 
it may be 180 degrees out of time and on the compression stroke the exhaust valve is opening and letting the compression out the exhaust.
 
Thanks, Howard. There was nothing to indicate a bad seal on the head gasket. A fire ring was broken on #3 but I think we did that removing the head. We shook the rocker arms, sprayed the guides with Kroil and tapped on the valves with a rubber mallet. The valve action looked normal when we ran the engine. I think I can solve my problem when I determine why the valves are open on top stroke. I'm just totally out of ideas.
 
I thought of that and I believe the answer is yes. I was helped by Ken Foland and he advised me of the gear to use and actually sent it to me. I'm pretty confident I have the right one. But, wouldn't that affect all cylinders?
 
i dont think it would effect all cylinders. we bought a engine for a ub special and the bottom end was together when we got it. we put the heads and all on top and set the timing and it would fire every now and again. we found out that the g crank had the wrong gears on it for the new style front cover. we could never get it to do anything. we got it on compression stroke and found out that the exhaust valve was opening instead of the intake. it would only fire on every other time on one or two cylinders and was on the exhaust side at compression stroke. after we changed the gears and got the intake to open on compression stroke it ran like a champ. does yours have the new style front cover or the old style. i take it you got a 6 inch crank in it.
i think with the old style cover it should have a 70 35 tooth gear and a new style should be a 72 36 tooth gear.
 
I would check the cam timing, Are the gears correct? Are you lining up the timing marks? Go through and check that stuff first.

My point to pressuring the cylinders is you can feel the intake, exhaust, and crankcase for air flow. It lets you determine where to start.
 
Gonna take the front cover off today and see what's going on in there. I think we're narrowing it down now and I really think I'll find the problem inside that front cover. THanks for the help.
 
Well we are grasping at straws here, and this is probably a stuipid theory. I have not studied the possibilities. Is it possible that the cam is 180 degrees off, and due to the firing order 1 & 2 are working, and 3 & 4 are way off base????????? When you get to the end of the rope make more Rope.
MMDEL
 
Didn't see Page 2 before I posted. I would think that if the gears are wrong It would go in and out of time, and would not run steady on 1 & 2
MMDEL
 
Ha!! Agreed on the rope. I took these pics today to show the timing marks. I put some white stuff on the crank gear to show where the tining mark is but the mark shows up well. You see the O on the cam gear. These two marks don't match up exactly but as close as I can get them. You can see the cam gear marks on a tooth and the crank gear marks on a tooth. These marks can never match exactly. Is it possible I have the wrong crank gear? that's the one I replaced.
a151629.jpg
 
i was told that there should be a line back from the zero and that should be lined up with the line on the crank gear. also the gear needs to be a 36 tooth on the crank and a 72 tooth on the cam. try this and see if it helps
 
Crap, so I possibly have it 180* out. That O was the only mark we could find on the cam gear. I'll check it again tomorrow. Thanks.
 
(quoted from post at 02:05:52 03/28/14) Crap, so I possibly have it 180* out. That O was the only mark we could find on the cam gear. I'll check it again tomorrow. Thanks.
a four cyl cant have the cam out 180 degrees only the ditributor. is this a gas? It isvirtually impossible to open the valves far enough to hit the piston unless something wasmajor assembld wrong
 
if the marks on the cam are straight across from each other then it is 180 degrees off. we looked at a cam gear and the zero and the line is across the gear from each other so that will be way off.
 
I found it guys. Just like some of you said it was 180* out. I found the right mark straight across from the O. I'm not sure how I did that, but I sure did it. Thanks for your help. Feeling a little stupid right now.
 
that would be me and my brother molinepuller55. we went through the same problem with a ub special. glad you got it fixed. we talked about it the other day and that is when he posted the pic of the gears with the 2 marks.
 
(reply to post aft 11:37:55 03/28i/i14)
fthe lines are straight across from each other turn the engine 180 degrees and they will line up cam to crank ratio is 2 to 1. The 0 is for aligning the accsesory drive for using a mag.
 
Glad you found the problem!!

Now just post a video of it running smooth on youtube so we can all hear it!!

:)

Howard
 
Well I guess you found enough rope. I have one question I am puzzled about. On that model engine how could the valves possibly hit the top of flat top MM pistons? Or do you have aftermarket domed pistons?
MMDEL
 
when we did our ub special we were told that the zero meant nothing. it will be on left side of engine at top on oppsite side of accessory drive cause the lines go at a angle on the gears.
i dont know about the valves hitting the pistons we didnt have that problem. we had flattops to the top of the case with 31 heads at the time
 
I'm sorry, I thought I was replying to dkr. He said the valves were hitting the tops of cyl 3 & 4. with tha cam out of time.
MMDEL
 
I can't answer that question. I really don't understand why the marks are there, but there were 3 very capable mechanics there who saw the marks and agreed that they were caused by the valves. The pistons are flat.
 

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