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Old 09-01-2019, 10:35 AM   #1
Devildog-1994
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Default 93 Marathon 295 Knocking...won't come in

Hi All,

I've searched many threads and cannot find my exact issue so I need to post...

I noticed a knocking when the engine is running. It doesn't seem to be making the knock sound on every revolution but it's definitely sympathetic with engine RPM's.

I have read that this story usually ends with a loose rod cap or some other worse issue but I have completely disassembled the engine and cannot find anything obvious. I checked the following, wrist pins look and feel fine, main bearings tight rod bearings are nice and smooth, valves were adjusted and look fine. cam and bearings fine, I ran the engine without the clutch belt which completely collapsed the clutch and didn't detect any sound from there. I didn't put a caliper on any bearings or bearing surfaces but everything feels tight and smooth.

Is there anything else I can check or I may be missing. have not measured the ring gap but could worn rings possibly cause a knock?

The one suspicious item was the smoke right after starting for the first time each day it was used. When I disassembled the head I could see that the ex valves were burnt well (see attached). I suspect that stem seals were allowing oil to settle into the exhaust passage and caused smoke. The head gasket did not have any evidence of failure/blow by that would be an obvious source of the oil/smoke.

I've attached a pic of the valves but I'm wondering if the carbon buildup on the valves may have caused the sound.

The guy I got it from said it didn't burn any oil and claimed to have accidentally put mixed gas in which was causing the smoke... All the projects I've sold to people I have never once tried to BS anyone or lie about the actual condition of the item. If the guy had said he suspects a head gasket or stem seal I still would have gone for it.

I appreciate any thoughts observations on this. I plan to install the new stem seals and maybe valves if I cant clean them up and see what happens.
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Old 09-01-2019, 10:49 AM   #2
trig123
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Default Re: 93 Marathon 295 Knocking...won't come in

Valves that are set higher than say .006 will cause a knocking sound. I worked on one last year that I thought had something bad wrong with it. I used my harbor freight cheap stethoscope and found the problem.
Set the valves on .004
I would rebuild this thing ( rings and seals with new timing belt ) and put it back together. You have the old style valves. Not good. I would replace the valves and keepers with new style.
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Old 09-03-2019, 08:45 AM   #3
Fairtax4me
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Default Re: 93 Marathon 295 Knocking...won't come in

Looks like it needs rings. Knocking may be piston slap if the pistons skirts are worn or there is excessive piston to bore clearance.

Its hard to tell in the photo, may just be the lighting, but those valves look like they have grooves worn in them. Check them for side-side play in the guides. If they have any wiggle replace them.
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Old 09-03-2019, 08:58 AM   #4
CP241
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Default Re: 93 Marathon 295 Knocking...won't come in

Quote:
Originally Posted by Fairtax4me View Post
Looks like it needs rings. Knocking may be piston slap if the pistons skirts are worn or there is excessive piston to bore clearance.

Its hard to tell in the photo, may just be the lighting, but those valves look like they have grooves worn in them. Check them for side-side play in the guides. If they have any wiggle replace them.
Great advice.

Just to add, I would measure the wrist pins carefully. I've had worn wrist pins cause a knocking noise as well.

If you've got it torn down that far, I would measure the cylinder bores (or take it to a machine shop and have it measured). If it's good, replace all the moving parts. Pistons, rings, wrist pins, rods, valves (especially the intake valves, they tend to wear thin on these motors), seals, gaskets. You'll have basically a brand new engine then and it won't cost a lot of dough.

If the cylinders need to be bored, have the new pistons and oversized rings ready before they bore it so they can match everything like it should be. Any machine shop that will bore a cylinder without having the pistons/rings in hand should not be in business.
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Old 09-04-2019, 08:05 AM   #5
Devildog-1994
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Default Re: 93 Marathon 295 Knocking...won't come in

Quote:
Originally Posted by Fairtax4me View Post
Looks like it needs rings. Knocking may be piston slap if the pistons skirts are worn or there is excessive piston to bore clearance.

Its hard to tell in the photo, may just be the lighting, but those valves look like they have grooves worn in them. Check them for side-side play in the guides. If they have any wiggle replace them.


Thanks Man.

So, when I gapped the rings, which should be .008-.016, and I was able to get almost .075 in there. I’m assuming this is severe wear, yes? I was blaming the stem seals for the oil because of its proximity to the valves but could the severely worn rings cause the knock and the smoke? Nevertheless I have new rings and new stem seals on the way.

And yes, a small amount of scuffing on the skirts. Doesn’t appear to be missing material you can just tell that it was touching something.

All the valves, despite being done Pittsburgh style, are actually really smooth.
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Old 09-04-2019, 08:09 AM   #6
Devildog-1994
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Default Re: 93 Marathon 295 Knocking...won't come in

Quote:
Originally Posted by CP241 View Post
Great advice.

Just to add, I would measure the wrist pins carefully. I've had worn wrist pins cause a knocking noise as well.

If you've got it torn down that far, I would measure the cylinder bores (or take it to a machine shop and have it measured). If it's good, replace all the moving parts. Pistons, rings, wrist pins, rods, valves (especially the intake valves, they tend to wear thin on these motors), seals, gaskets. You'll have basically a brand new engine then and it won't cost a lot of dough.

If the cylinders need to be bored, have the new pistons and oversized rings ready before they bore it so they can match everything like it should be. Any machine shop that will bore a cylinder without having the pistons/rings in hand should not be in business.


Thanks for the advice, hope to not bore them.
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Old 09-04-2019, 09:04 AM   #7
Devildog-1994
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Default Re: 93 Marathon 295 Knocking...won't come in

Picture of the worst scuff that I could find. Hoping new rings will tighten things up.

Also included as a bonus for playing along, a picture of a manifold bolt that broke off which seemed too easy so I decided to drill it out and break off an easy out inside the hole just to add a little more challenge.

I plan to have the hole filled with a tig then drill and tap new new holes. There are two other broken bolts here but I will not be using an easy out on those...just going to drill/grind until I get them opened up...
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