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why the plastic guides on timing chain?

rm787

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Hello everyone,

I have a question I'd like to ask: why did Ford design the timing chain for these vehicles to have plastic guides..or should I say..a plastic cassette? Especially when it's impossible to remove and replace the timing chains on these vehicles without removing the engine from the vehicle? It seems that timing chain issues are common on these vehicles.

Anyway, the reason why I ask is because my father owns a 1994 chevy 3500 pick-up. The truck has run almost 500K miles on the original engine, and has never had a problem with the timing chain. Never been serviced. Based on the manual, the chain doesn't run on a plastic guide at all. It just runs on the sprockets, with a tensioner in there. I didn't see any kind of guide for the chain mentioned in the manual; guess it doesn't require one.

Why didn't ford do the same thing with this vehicle? It seems like a bad design flaw with plastic guides.

I did take a picture of my 2001 ford explorer under the hood...I'm trying to figure out where to find the front timing belt tensioner. I bought it used..so it's possible that a new updated tensioner was placed in the vehicle. I'll see if I can post a link to it later, as I'm not able to upload a photo here.
 
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SoNic67

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Chain links will stretch during life. Tensioners are welcome, to quiet the noise, have a precise timing...
Why plastic? Well, probably you don't want the chain to wear out?
Why not a rolling guide (bearing with plastic coating) instead of a sliding guide (skate type)? Probably cost...
 
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2000StreetRod

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OHC vs OHV

I suspect the 1994 Chevy 3500 engine is an OHV instead of an OHC engine. Years ago many of the OHV engines had no timing chains. Camshaft timing was done entirely with metal gears. Here's a photo of the type of engine in my 1960 era Volvo PV-544:
Volvo544Eng.jpg

There were no chains linking the crankshaft gear to the camshaft gear which rotated in the opposite direction.

Some manufacturers replaced the metal gears with plastic gears to reduce gear noise and cost. Every mass produced OHC engine I'm aware of uses a chain or a belt with a tensioner and requires periodic replacement. My 1958 Jaguar XK-150 DOHC 3.4L inline six cylinder had a double row timing chain for reliability but the tensioner (and eventually the chains) still had to be periodically replaced. My current 1996 Volvo 850 turbo wagon with DOHC 2.5L transverse five cylinder requires the belt and tensioner to be replaced every 60K to 70K miles.

In my opinion the main deficiency with the Ford SOHC V6 4.0L design is not the use of plastic tensioner to chain wear surfaces but the use of plastic as a structural element. Due to heat cycles and mechanical stress cycles the plastic eventually fractures and then breaks. The cassette guide assemblies should have been built of metal with plastic only used for chain to guide wear surfaces. The other significant deficiency, as you pointed out, is that both cassettes should have been in the front instead of one in front and one in the rear. That way replacement of either cassette would not require removal of the engine or the transmission. Ford avoided these deficiencies with the 4.6L SOHC and DOHC V8 engines.
97DOHC4_6.jpg


97DOHC4_6.jpg


Volvo544Eng.jpg
 
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rm787

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I was looking at a picture of the timing chain set-up in the Hayes manual... It almost seems that if the track guides were to go, there would still be enough clearance on both sides (as long as there is still tension on the chain) to keep it from hitting the sides. Perhaps not...
 
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2000StreetRod

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chain deflection

Your comment points out another design deficiency - the excess chain length and deflection which is worse in the rear than in the front. The left guide outboard (traction) side is almost straight and metal with a plastic wear surface.
LftGuide.jpg

The right guide inboard (traction) side is curved and made entirely of plastic.
RCssttV.jpg

The greater the deflection and the greater the excess length the more important the reliability of the tensioner and guide and the greater the wear on them. During engine acceleration the crankshaft sprocket tugs on the camshaft sprocket via the chain to rotate the camshaft causing tension on the traction side and slack on the other side. During engine deceleration the reverse is true - the slack is on the traction side. When the rear guide plastic traction side breaks
DSCN8608.jpg

the chain is long enough to strike the guide upper positioning bolt.
GuidPost.jpg

Chain wear (stretch) is minimal and there is no justification for the tensioner excess range of deflection. Proper design of the chain length and sprocket teeth spacing while maintaining a 2:1 crankshaft to camshaft gearing ratio would have greatly improved the reliability of the camshaft timing. The diagram below illustrates the timing chain configuration of my 1964 Alfa 2600 Spider.
AlfaDOHC.jpg

The chains were double row and there was only one plastic part (item 7) utilized. The engine was extremely reliable and easily revved to 7,000 rpm. The intake and exhaust valve open duration was fixed but the opening for each cam relative to the crankshaft was adjustable with the front timing cover removed.
 
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SoNic67

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I guess Ford used that design because the more sprocket teeth are in contact with the chain links, less stretching force is exerted on the individual links.
Deflection pressure on tensioners is way smaller than the resulting tension in the chain.

From the above picture I see that not wear was the primary cause of failure, but under-dimensioning of the connecting bridge (made of plastic). Compression/stretching cycles stress any material (aging) and favors the sudden breaking in the concentration point. That's why any metal parts have rounded connections - to minimize the stress concentrators.
 
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rm787

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Thanks for the replies.


When I owned a rinky dink dodge neon, the timing belt jammed up on it. The tensioner had given out. I replaced the belt and tensioner; it was easy to re-time. I recall that there were marks on the camshaft/crankshafts
that one could align to get the timing correct. Very simple. I did not need special tools. Just ratchet and sockets to rotate the pullies so they align to the markings. The Ford Explorer doesn't have marks on the crankshaft/camshafts that can be aligned by sight, to make the timing process easier? I understand the the ford explorer requires special tools to time it.

Also, I read that one would need to pull the motor, or drop the transmission, to replace the rear cassette. Is it possible to replace the broken rear cassette without having to pull out the timing chain and then re-time
everything? It's not possible to do all of this through access from the wheel well?

thanks
 
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2000StreetRod

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no timing marks but slots

There are no timing marks on the camshaft or jackshaft sprockets. However, there are timing slots on the opposite ends of the camshafts:
cam1.jpg

To time the camshafts rotate the crankshaft until cylinder #1 piston is at TDC on its compression stroke and then position the camshaft so the timing slot is below the axis of the camshaft and parallel to the surface of the head that mates with the valve cover. The timing tool kit assists in aligning and holding things in the correct placement when the sprocket retaining bolts are tightened.

In order to replace the rear cassette guide either the engine or the transmission must be removed. The bolt that retains the cassette and the plug that covers the jackshaft rear sprocket and retaining bolt are covered by the transmission housing.
LwrRtBlt.jpg
 
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Foz1359

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And... mine has performed exactly as pictured below! The break is nearly identical and the chain has begun to impact at the guide positioning bolt. My question is can I now introduce a nylon (p66) covered roller bearing at the guide positioning bolt? I'm thinking about a diameter that will barely contact the slack chain, or perhaps leave a couple mm gap so it "slaps" at the nylon roller rather than spin with rpm. I'm asking a bearing to simply hold a reasonable measure of chain pressure at engine deceleration, right? Can you guys help me engineer the right bearing, bolt, spacer, locking nut setup? I'm thinking something similiar this assy might actually save me:

Double Bearing (Inch) - Fairlane Products

A good quality tire (flat surface) or even a U groove where the chain simply engages the sides of the bearing upon deceleration, the sides of the U groove (V groove better?) serving as a positioning guide.

All we want to do is save the right bank from jumping time and slapping a metal bolt right? Can't we figure out a fix that'll save owners from an extremely expensive repair? I'm almost ready to fasten the broken guide piece to the guide positioning bolt with a pin, so it "hangs" in the correct position, protects the bolt head and allows the chain to slap it upon deceleration. My concern there is it's broken once already (due to acceleration though) and may break or bind somehow.

I bought this good looking golden girl with 140k miles and you can eat off the head, oil looks to have been changed religiously, very clean and no gunk build up on inside. Only reason I found the broken right bank guide is the valve covers were pouring oil and the owner didnt want to pay estimated shop cost for the job. The valve covers are a piece of cake in my driveway but there's no way I'm pulling the engine or tranny and if I was going to do so, I'd replace all the chains and guides. That's not happening.

Plus, I'm just a working class guy with some decent experience (rebuilt a couple GM motors waaaay back) trying to fix up a car for our son, he needs something good to happen here.

Would you fellas be willing to chime in here? Can we develop a reliable repair to this common problem using some old fashioned "close enough" engineering?

Mike Foster
Harahan, LA
504-346-9825


The right guide inboard (traction) side is curved and made entirely of plastic.
View attachment 76806
The greater the deflection and the greater the excess length the more important the reliability of the tensioner and guide and the greater the wear on them. During engine acceleration the crankshaft sprocket tugs on the camshaft sprocket via the chain to rotate the camshaft causing tension on the traction side and slack on the other side. During engine deceleration the reverse is true - the slack is on the traction side. When the rear guide plastic traction side breaks
View attachment 76807
the chain is long enough to strike the guide upper positioning bolt.
View attachment 76808
 
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shucker1

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This would be an interesting thread for @imp .

He is a retired bearing and seal engineer and might be able to chime in.
 
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shucker1

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BTW,
@Foz1359 welcome to the forum!

Harahan, LA?

Right around the corner...

Go Elite member for $20.00 per year and you can post all the picture you want.
 
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2000StreetRod

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My question is can I now introduce a nylon (p66) covered roller bearing at the guide positioning bolt? I'm thinking about a diameter that will barely contact the slack chain, or perhaps leave a couple mm gap so it "slaps" at the nylon roller rather than spin with rpm. I'm asking a bearing to simply hold a reasonable measure of chain pressure at engine deceleration, right? Can you guys help me engineer the right bearing, bolt, spacer, locking nut setup? I'm thinking something similiar this assy might actually save me:
Double Bearing (Inch) - Fairlane Products

For your broken cassette replacing the guide upper positioning bolt with an appropriate sized nylon roller may prevent the chain being damaged by striking the positioning bolt and prevent the chain from slipping on the camshaft sprocket.
GuidePost.jpg

However, it may not prevent the chain from slipping on the jackshaft rear sprocket. The best location for a roller or friction chain tensioner is at the midpoint of the chain on the slack side. This is why in my opinion the tensioner on the OHV V6 is superior to that of the SOHC V6.
OHVTimingKit.jpg
 
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fast_dave

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2000StreetRod

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@2000StreetRod Epic response - bravo!!! :chug:

Please go on to tell us why FORD, in all it's wisdom, decided to place a water pump internally
on their 3.5 and 3.7 V-6's :banghead:

LINK: https://www.courthousenews.com/wp-content/uploads/2018/08/FordPumps.pdf

Very interesting. I was not aware of Ford's incompetent design of the Cyclone engine. I doubt there is any reasonable cost solution to resolve the design flaw. The engine is appropriately named since it destroys its internal components with water.
 
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fast_dave

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Very interesting. I was not aware of Ford's incompetent design of the Cyclone engine. I doubt there is any reasonable cost solution to resolve the design flaw. The engine is appropriately named since it destroys its internal components with water.

@2000StreetRod,

No problem - I assumed that with the excellent post re: SOHC vs OHC timing chains that you would have known about this glaring defect with 5th Gen Explorer "Cyclone" V-6's and thus appreciated the sarcasm on my part. Regardless, I am glad to enlighten you on the subject, as well as the class action lawsuit.

Hoping that FORD corrects the internal water pump error with the release of the RWD 6th Gen Explorer (and 2020 Bronco).

Also hoping for a American built (as opposed to Chinese built) manual transmission on the 2020 Bronco.

Carry on -
 
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Foz1359

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Yes, neighbors we are! Thanks for the welcome. Well I decided to polish up (round the edges) my salvaged piece of plastic guide and pin it permanently onto the guide positioning bolt, then filled the cavity (covering the pin, a rib shank paneling nail) bedding it all in JB Weld. My logic is... the acceleration, heavy pressure side of the guide is still intact, it's a full length metal rib lined in plastic that still has a hydraulic tensioner holding it up high and a bolt holding it at the bottom. In theory it appears to be functioning normally. The deceleration side is now merely acting as a chain stop up high, I'm hoping the remaining portion of guide/ side rail is not going to catch a link in any way. After the rig cures I'll bolt it up and give it another hard look before installing the valve cover. Still like the "cam follower, roller bearing, etc" idea but I'd like to get her going soon. We'll see.

Who Dat?!!!!!!!

BTW,
@Foz1359 welcome to the forum!

Harahan, LA?

Right around the corner...

Go Elite member for $20.00 per year and you can post all the picture you want.
 
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shucker1

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10-4 on The "Who Dat".

Boys better step up their game after last week.

But that's the Saints for you. "Always able to snatch victory out of the hands of defeat!"
 
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imp

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........ This is why in my opinion the tensioner on the OHV V6 is superior to that of the SOHC V6.
View attachment 166563
@2000StreetRod
Your image is of an OHV camshaft drive chain? Why did it have a tensioner at all? Umpteen millions of Overhead Valve Engines have been built and used without any tensioner. If the idea was to decrease loss of "design valve timing" by tensioning the chain, as the links wear, the loss of timing accuracy presents itself anyway.

I don't understand something here, evidently. imp
 
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shucker1

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@imp,

The OP wants to come up with a Nylon Roller to replace the slack side guide?

What do you think?
 
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