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Old 12-10-2017, 06:31   #1
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Perkins 4108 pushrod

Recently had problems with my engine making power while under load. Removed valve cover and found two bent pushrods. After purchasingClick image for larger version

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Old 12-10-2017, 08:51   #2
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Re: Perkins 4108 pushrod

It will be easy enough to get the broken parts out but what bent them? The only thing that comes to mind is carbon build up on the pistons. The clearance between the valve and the piston is very very little in a Perkins. You may also have a bent valve face. The piston and valve are square to one another so maybe just bent pushrods. I think you are gonna need to remove the head and clean the piston crowns and give it a valve job.
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Old 12-10-2017, 08:53   #3
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Re: Perkins 4108 pushrod

Just out of curiosity, where did you purchase the push rod from if you donít mind?
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Old 12-10-2017, 09:06   #4
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Re: Perkins 4108 pushrod

I agree wholeheartedly with Guy, were it my engine I'd pull the head first thing and you'll want to check the cam and lifters as well. If the engine were out I'd pull the pan too, 4-108's always need a new oil seal anyway.
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Old 12-10-2017, 09:15   #5
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Re: Perkins 4108 pushrod

To get the old pieces out use a mechanics magnet. (extendable magnet on a rod) You should carry one anyway.
My concern is you got water in that cylinder, then when you started the engine it bent the push rods.
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Old 12-10-2017, 10:01   #6
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Re: Perkins 4108 pushrod

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Originally Posted by Guy View Post
It will be easy enough to get the broken parts out but what bent them? The only thing that comes to mind is carbon build up on the pistons. The clearance between the valve and the piston is very very little in a Perkins. You may also have a bent valve face. The piston and valve are square to one another so maybe just bent pushrods. I think you are gonna need to remove the head and clean the piston crowns and give it a valve job.


A stuck valve is most likely in my opinion, especially with it bent that bad and broken.
Either way the best fix requires removal of the head to either decarbon or ream or replace guides.
Me, Iíd pull the head and take it to a shop and have a full valve job, possibly valve guides replaced etc.
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Old 12-10-2017, 10:07   #7
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Re: Perkins 4108 pushrod

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A stuck valve is most likely in my opinion, especially with it bent that bad and broken.
Either way the best fix requires removal of the head to either decarbon or ream or replace guides.
Me, Iíd pull the head and take it to a shop and have a full valve job, possibly valve guides replaced etc.
Yep, you might be right. Carbon build up on the piston would do less damage.
I had our Perkins head ovhl'd by a good shop here in the USA. With the clearance issues etc., it needed all new valves and came to about $450.
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Old 12-10-2017, 12:43   #8
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Re: Perkins 4108 pushrod

Thank you all for replies. I've been down on the boat with my head in the bilge for the last few hours and can give you some more info.
There were two bent rods. One on each end of the engine. I found both valves Frozen in place. After liberal soaking with PB Blaster and a few sharp wraps on the valve stem they freed up. After turning the motor over by hand several times I used a pencil magnet and was able to retrieve a small piece of push rod from one cylinder. Nothing from the other.
Finally I said what the heck I don't have anything really to lose, so I reinstall the pushrods set the valve clearance and fired up the motor. It started easily and ran smoothly with no knocking. However I was getting exhaust through the air intake. The motor would
run easily however under load it would max out at basically a high idle. Am I right that this means a intake valve stuck open?
I agree with you all that I'll have to at least pull the head to do a proper repair. Because of the awkward position of the motor and my unwillingness to lay on my stomach for the time it takes to accomplish that task, I'm leaning towards pulling the engine and doing a proper rebuild. It will take a little more research to decide if that's something I can do myself or if I should Farm it out to someone who knows more than me.
By the way I purchased the push rods at transatlantic diesel in Virginia. I have found them to be quite helpful and knowledgeable in the past and this time was no exception. One phone call a credit card number and the parts arrived the next day. I am in Norfolk and they are close to me but it still beat the hour drive to pick them up.
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Old 12-10-2017, 15:54   #9
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Re: Perkins 4108 pushrod

You probably also have a bent valve. There is almost no clearance between the valve and the piston at TDC. If the valve was stuck open, it would hit the piston. They might not be still stuck, but bent and then of course not seal.
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Old 12-10-2017, 16:58   #10
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Re: Perkins 4108 pushrod

I'm not trying to get out of doing the inevitable, but is there a way to determine which valve is sticking without removing the head?
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Old 12-10-2017, 17:20   #11
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Re: Perkins 4108 pushrod

I think that you could strike the rocker arms just over the valve stems with a rubber mallet. You would have to turn the engine a bit to test all of them (like you do when adjusting the clearance). You might be able to hear the difference.
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Old 12-10-2017, 17:21   #12
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Re: Perkins 4108 pushrod

Pull the head, I would not run it again. You may be playing with swallowing AKA dropping a valve, do that and the engine is destroyed.
If the valve sticks open and the piston pushes it closed, the keepers may fall out of the retainer and if that happens the valve will fall down into the cylinder bore, next time piston will try to make it fit into the combustion chamber, which it canít of course.
Iíve dropped a valve or two racing and ďfloatingĒthe valves, turning higher RPM than the valve spring can close the valve, so the piston does it.
As Guy said you very likely have a bent valve, see the valve is usually at an angle to the piston so the piston bends it when it pushes it closed.
Often a bent valve will have the valve head come off itís itís a two piece welded valve, then you have the same destruction of the engine.

If you really feel the need to know which valve is stuck or bent, a compression leak down test will tell you, but in my opinion itís a waste of time, pull the head, cause it sounds like you got lucky and nothing is bad, yet,
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Old 12-10-2017, 17:51   #13
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Re: Perkins 4108 pushrod

I agree and understand. I'm really just trying to pick up a little more insight.
I'm still trying to understand the the cause for the break down and figure out if there was something I should have noticed sooner.
For instance, why are two cylinders affected? One intake and one exhaust valve. Both valves seemed to be frozen in place. This boat is no dock queen and the motor is run regularly.
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Old 12-10-2017, 17:59   #14
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Re: Perkins 4108 pushrod

If I had to guess as itís a Marine engine with a wet exhaust, Iíd bet corrosion from water intrusion, possibly from the exhaust elbow.
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Old 13-10-2017, 00:23   #15
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Re: Perkins 4108 pushrod

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Originally Posted by Ribman View Post
I agree and understand. I'm really just trying to pick up a little more insight.
I'm still trying to understand the the cause for the break down and figure out if there was something I should have noticed sooner.
For instance, why are two cylinders affected? One intake and one exhaust valve. Both valves seemed to be frozen in place. This boat is no dock queen and the motor is run regularly.
How many hours on the engine?

If, as I believe it is, the rocker shaft is oiled from the center, failure (sticking, or probably more accurately, galling) in alternate valves at opposite ends of the engine is likely indicative of low oil volume.

You can pull the head and respective valves to confirm this, but I think your proposed rebuild will be the best option. (though if you have low hours, a sticking relief valve might cause low volume, but probably not [in any case, if that were the problem, you'd almost certainly have to pull the engine to fix it, as the relief valve is integral to the oil pump; the oil pam has to come off to access it...]).

if you're a little mechanically inclined, patient and meticulous, it is not that difficult and can be a surprising source of pleasure. Plus parts are comparatively cheap...

If you don't have a service manual, the first step is to get one and read it, cover to cover.

There's a factory manual here, for free.

http://www.google.com/url?sa=t&rct=j...m-VdZ9lnTQGNOs
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