Thicker head gasket chain too short???

[POKTROKT]

Hi all, I've recently refurbished a head for my 2002, it has a cartech turbo kit and was preigniting really bad so I decided to reduce compression ratio by using a thicker MLS head gasket (Cometic 2.5mm).  Now my timing chain is too short, anybody ever have to deal with this?  

Edited August 3, 2018 by POKTROKT

[2002Scoob]

That’s pretty thick... ya sure you’ve got your chain tensioner fully compressed?

Does that also have any effect on cam timing?


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[POKTROKT]

it is fully compressed.  

Engine has piano top pistons with a E12 head, 2.5mm head gasket would bring me down to 8 to 1 ratio.

 

You're right, it would have an effect on cam timing.  If I keep this setup I would need an adjustable cam gear.  

[Hans]

hard to believe 1 mm more would fully extend chain

[POKTROKT]

my thoughts exactly!

[2002Scoob]

Have ya tried fully removing the tensioner/piston, install the chain/cam gear and reinstall?

Maybe shine a light down the cover and confirm your timing chain is fully engaged on the crank cog.


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[kbmb02]

1 hour ago, POKTROKT said:

Hi all, I've recently refurbished a head for my 2002, it has a cartech turbo kit and was preigniting really bad so I decided to reduce compression ratio by using a thicker MLS head gasket (Cometic 2.5mm).  Now my timing chain is too short, anybody ever have to deal with this?  

 

Trying to understand this...

 

IF the crankshaft hasn’t moved (since disassembly), and IF the camshaft is in the exact same spot (since disassembly), AND you’re trying to install the cam timing gear using the locating pin after installing a thicker head gasket (about 0.040”), then YES - the chain will act like it is ‘short’ (on the non-tensioner side, for lack of a better explanation). You’d need to rotate the camshaft forward a bit so the cam gear fits with the locating pin (you’ve then changed (advanced) the cam timing).

 

That’s my one guess for the week (month and year). -KB

[Preyupy]

The Thicker headgasket will ADVANCE the camshaft if you try and use the stock cam sprocket. 

 

The real problem with 1mm thicker gasket is that as far as the chain is concerned it is 1mm too short on the tension side AND 1mm to short on the slack side.  I have never tried to use a gasket that thick especially on a head that has not been "shaved" A LOT!!  I don't believe you will have enough chain length to make it fit.  Why domed pistons on a turbo engine?  How much boost are you trying to run?   

[POKTROKT]

Good feedback all!

@2002Scoob: the tensioner is out, I'll have another look in there to make sure I'm fully engaged. 

@Kbm02: Didn't rotate the cam but have rotated the engine to take up the additional length, no go.

@Preyupy: The head hasn't been shaved, I have added 2mm of length and I agree the chain just isn't long enough, it doesn't help the sprocket needs to go over the bore of the cam at an angle.  Domed pistons are not ideal, they were in the engine when i got it (already turbo'd) so I thought i'd use them with the thicker head gasket, they are custom Venolia pistons. 

Edited August 4, 2018 by POKTROKT

[tzei]

15 hours ago, POKTROKT said:

head gasket would bring me down to 8 to 1 ratio.

That is not much. What it was with old gasket?

[Hans]

Putting aside the piston C/R debate, I'm having trouble understanding how a 1mm thicker gasket would take all the slack out of the chain, even with  a virgin head. If it was refinished, I assume it was milled a bit. I'd be making sure the chain was seated properly on the lower sprocket.

[TobyB]

Huh- you know, with a full- height head, new sprockets and a new chain,

I suppose it might NOT be long enough!  I've only put together a couple that had

129mm heads, and there's not a whole bunch of extra slack.

 

My only thought would be to take the front covers off entirely, because, as you say,

with the lower cover on, you're asking the top sprocket to go on at quite a bit of an angle.

With it off, it's easier to straighten the chain and get all of the available chain slack to the top...

Maybe even slide the crank sprocket forwards a bit...

 

Oh, it's off by like, 0.5mm, right?  It's possible for the chain to 'droop' on the lower sprocket

and catch a small loop down at the bottom of the crank sprocket...  That makes it more like 5mm short.

 

hth,

t

 

[Son of Marty]

I'm just spit-balling here but you just might get that little bit more stretch out of a single row set up

[TobyB]

...and I bet you could post a WTB for a set of worn sprockets and stretched chain!

 

(just kidding...)

 

t

 

[FB73tii]

I just went through this. A few years ago I had a valve to piston clearance problem (valve pockets were not deep enough) and the band-aid solution was a .120" (3.05mm) Cometic MLS. It was tight but the chain went on, the head had been shaved (don't have the measurement now), and all works fine. Fast forward to another engine I had built for me and it turned out there was a piston-head clearance problem. I thought "great, I'll just slap another .120 gasket in there and be on my way."

 

Well, that was not the case. The timing chain (new, as were both sprockets) was too short by a lot and some research suggested somehow a link was hiding down in the front cover (i.e., turn the engine over both ways and maybe it will free up) as Toby mentioned above. That didn't work. I pulled the front cover, and somehow thought I could modify the tension rail to create more clearance. Well, I finally got the chain to fit, only to realize the rail now interfered with the front cover--it just was not going to work even with tension piston removed (I slotted the bottom of the rail to move it in, and also tried grinding off some rubber).

 

Finally I realized the engine builder probably used a full height head. The engine was never designed to use a much thicker head gasket with a full height head. Now a winter project is to pull the pistons and get them shaved. Compression was 13.8:1, so it is fine with me to cut the pistons--want it more like 12.5:1 for this engine, which will run a Schrick 336 cam. --Fred

 

Edited August 4, 2018 by FB73tii