JoshDT91

iTrader: (1)

I was asking the service tech at a place I was getting my state inspection if the engine was interference or not. Well he tells me it probably is and that on my engine I probably have plastic or some sort of plastic timing gears. Of course I think he is full of it when he tells me plastic timing gears with a metal timing chain. So two questions: 1) Is my 1988 2.8L an interference engine or not and 2) Are the timing gears really plastic?

91rs4life

They aren't plastic exactly. They are a poly-something nylon tooth gear. It will break like plastic yes. But technically it's not plastic. Possibilty it could be fractured or a piece broke off. Metal replacements can be cheap.

TomP

Strange; my original '86 timing set was all steel.

JoshDT91

iTrader: (1)

Alright I checked my 1988 GM Service Manual and it says the cam and crank sprokets are hardened sintered iron, I should have checked my manual first. Maybe the newer engines had some sort of plastic type gears. The only thing I wonder now is if the chain were to break would a valve smack into a piston. From what it looks like I would assume GM engines in third gens are non interference but that's just my guess.

Dale

iTrader: (12)

My dad had an old 81 chevy full size, which was the first thing I ever drove. It use to have steal center, with plastic teeth. And yes, they broke.

Every 2.8-3.4l I have seen was all metal, no plastic.

TomP

The pistons are flat-tops, there's no valve reliefs cut into them- well, as far as the 2.8 goes. Not sure about the 3.1.

Lee7

On a 2.8, if your timing chain broke, i doubt the pistons would hit the valves, this is mainly because on a stock engine, the valve lift is only like 0.223 or something small like that.

Now... if you upgraded to a cam with more lift, it probably would hit.

SSC

Gm was quite fond of plastic (yea poly/nylon/bs) in early model engines. I havent seen a plastic cam sproket in anything newer then 85 so I always assumed gm droped it for 86+. It's actually very durable and quiet, last one I seen stock had 190,000 miles on it and it finally gave out. But I havent seen any on 90*V6's.

As far as being an interference engine its a 50/50 shot. Thier not typicaly interference but the valves have been know to kiss the pistons when a timing chain went but not pop a hole through.

JoshDT91

iTrader: (1)

Let's see here I have all this data in this GM Service Manual. It says this (partial listing):

Camshaft:

LIFT
Intake.......................................6.67
Exhaust....................................6.94

Valve System:

STEM CLEARANCE........................0.026-0.068


I wish I knew all these specs here then I might understand a little bit better. I am assuming these measurements GM uses are in inches or is it millimeters? It says the BORE is 89 and the stroke is 76 so I would assume that is mm and not inches. Does anyone know of a good book to learn about all these sorts of engine terms?

Well it's good to know from this book I have iron sprokets and not plastic. I have 165,500 miles on my engine and the Service Tech at the Chevy dealer told me it's not worth changing the timing chain unless it's making noise. He said it's not some import plastic crap or lame belt it's made of a chain, refering to the chain. I thought that was funny but anyways how hard would it be to change your timing chain yourself? I mean I know it requires draining the radiator, oil, and removing the water pump, the power steering bracket, and the front cover. Once you get in there though how hard is it to install a new chain and I guess sprokets while you're in there?

Doward

Bull!! Change that chain every 80k miles or so... you'll feel a nice increase in power, from your timing coming back inline of where it should be...

Ovrclck350

What in the hell do you mean by interference engine?

TomP

I think he means that if the timing chain snaps, and it's an interference engine, the pistons will smack the valves.

Might as well not even worry if it's an inteference motor or not, and just change the timing chain if it's got a ton of miles on it. I changed mine out at 180,000 miles; I replaced it with the Cloyes true dual roller set from Summit, $90.

Ovrclck350

Oh, ok. I had no clue WTF he was talking about.

91rs4life

Yes interference means that the piston comes awfully close to the valves and they are so precisely timed that if your chain snapped you could make your pistons and valves share a thought. Think of it as the pistons could "interfere" with valve operation if timing is not maintained. I may be wrong on this but, if I remember correctly interference engines usually have a timing belt and not a chain. Though they may have used a chain on early interference designs.

The gear itself is iron, yes it is the teeth that are made of nylon. I have a rare 2.8L... It is a late model 87 berlinetta LT and it was given a 1988 engine. equipped with the dual ram air snorkel that came on LT1. (If anyone is inquisitive I have pics to show you.) It is equipped with one of these garbage nylon things. I'm not saying the manual is wrong but, it may be inaccurate.

p.s. the above mentioned is non interference. (1988 2.8L)

TomP

That's what I meant by "that's strange"... I wonder if this was some kind of "change" by GM in 1987, to use nylon teeth. Sure I never took a metal file or magnet to my old timing gears, but they looked/felt like metal.

JoshDT91

iTrader: (1)

Tom: I know you rebuilt your engine so tell me, how hard is it to replace the chain and gears yourself?

91rs4life

Not hard just time consuming...kinda. I would explain it here but, more than likely it is explained in your manual. The whole job takes about 2-3hrs. Just relax and take your time. you'll be done before you realize how much time you spent. One thing I will say here that it probably doesn't say in the manual is that, once you get at the gears, get the crank gear marking to the 12 o'clock position before removing the gears. This way you won't have to find TDC after the job is done. (TDC = Top Dead Center)
And move your cam gear marking to 6 o'clock before removing the old gear.

TomP

Well, I never rebuilt the engine... not yet, anyway. It's not technically hard but it's a bitch. Worst part was scraping the old gasket material off of the aluminum (gouges easily, and scrapes = leaks) timing chain cover. Oh and don't use the center crank bolt to install the crank gear or harmonic balancer, either. I cracked the bolt off flush with the crank snout, and my mechanic had to weld my balancer on. I used a complete gasket set from FelPro; I think it was $15 or $20, came with everything I needed.

And to "second" 91rs4life's statement, unplug that negative battery cable before you start. That way you can't accidently bump the starter to mess up any valve timing.

It took me more then 2-3 hours... scraping is a pain, and don't forget you've gotta drain that damned radiator (I hate coolant; too sticky) too. While you're in that far, you might as well replace the lower hose since it's gotta come off anyway.

91rs4life

Like Tom said, try not to gouge the mating surfaces. if you do by accident...a little RTV copper will hold real nice. lol and yeah I paid about $22 through autozone for my felpro gasket set. And replacing the lower hose isn't a bad idea either. About $10 through autozone.

JoshDT91

iTrader: (1)

Yeah the manual does mention making sure that the #1 cylinder is at TDC and to line up the marks on the gears before removing them. Of course the manual does make it seem easy.

1991tealRSt-topGuy

your forgetting about the size of the valves.....................


i would put my money on the valves being able to tongue tie the pistons

Ovrclck350

Took me around 12 hours.


Broke a few parts, went ahead and cleaned the intake since I had it completely off.....I probably didn't need to take it off in hindsight, but it made getting to everything else a lot easier.


Main thing that sucked was I broke a coolant valve off the intake and had to JB weld it.

SSC

LOL, Tom, all the tools you have and still no rolloc connector for a die grinder (scotch brite cleaning pad for those who dont know). Makes the gasket removal a 5minute job instead of an hour. Well worth the investment.

SSC