Reassembling engine, cam chain too tight

Hey everyone,

A friend and I are re-assembling an XJ550 engine, and we've come to a snag: we're trying to put the camshafts in, but the chain is way too tight. To be precise, the chain is so tight that, even without the cam chain tensioner, the chain is absolutely taut when the two camshafts are installed. If the engine were to start, i'm certain the chain would break immediately.

A little background on what we've checked and tried,

- Removing the cylinder head to ensure the drive shaft is properly installed.
- Ensuring the drive shaft is TDC
- Ensuring the cam chain is not kinked on the drive shaft sprocket
- Ensuring the front cam chain guide is seated correctly in its base such that it aligns perfectly with the top of the cylinder head.
- Ensuring the rear cam chain guide's rounded bottom is placed properly in its seat, with the bolt holding it in place properly raised to make room.
- Ensuring both cams are in their corresponding exhaust & intake seats, facing the correct direction, and correctly timed.

After a week of disassembling and reassembling the engine, we're at our wits end. We even had a shop mechanic look at it, and he couldn't make heads or tails of it either.

Is there anything we're missing? Has anyone seen this before?

 

first let me say i don't know jack about 550's. you mentioned the rear guide but not the tensioner. could that have moved full out and is keeping the chain tight?

 

@Polock Thanks! I only wish it were that easy. I forgot to add that we removed the cam chain tensioner entirely. We can see a clear hole through the cylinder head leading to the rear cam chain tensioner right where the tensioner should be. I'll do my best to take some photos next time we're in the shop.

 

Are you certain the cam chain is not snagged and is securely seated around the crank sprocket? During assembly the chain will snag if you rotate the crank without holding the chain. Can you remove the chain guide between the cam sprockets and use a flashlight to check? If not you may have to remove the cams. Imo don't rotate the crank until this gets sorted.

Gary H.

 

if this were a 750, i'd unbolt the sprockets from the cams, slide the sprockets off the shoulders then gently try to spin the crank back and forth a wee bit while holding the chain, noticing that the chain moves with the crank.
now if you can tell what side is jammed, maybe you can wiggle it free. If it's on the crank, it probably needs to come off to get un-stuck

 

I was thinking the same way but he'd have to rotate the crank to remove both sprocket bolts if they're installed.

Gary H.

 

Then either your chain is not routed correctly yet, or it shrank upon exposure to the atmosphere.

Here's a question for you....is this a replacement chain or the the original one that was already in the engine? If it's the same one, you KNOW it must fit. If it doesn't, you have it installed incorrectly yet. If its a new chain, then, the first thing I'd say is it's the wrong length.

Can you show us a pic of the top so we can see "how tight it is"?


Dave F

 

@rocs82650 We held the chain taut with our hands and rotated the main shaft to make sure it wasn't snagged. If it rotates smoothly, it should be seated correctly on the main shaft sprocket, right?

@Polock When rotating the drive shaft and holding the chain taut with our hands, both rotate in sync smoothly. We haven't tried while the cams are in though. Do you think that'll make a difference?

@hogfiddles This is the same chain we took off the engine, so it definitely should fit. Can chains actually shrink under atmospheric exposure? We cleaned it and left it out to dry for a few weeks. I'll take some photos later this week.

 

If it's not hung up on the crank I'd look closely at the chain guides.

 

I'm trying to envision how tight this would be. I mean, the chain should be pretty taut when everything is back together, right? How much force was required to bolt the sprockets back on the cams? Did you have to use leverage to get sprockets on the cams? That's usually a process where I have to fasten one of the bolts, turn the crank, and address the second bolt (for each sprocket). N.B. I have a 550 and have done this.

You said you're sure the chain would break immediately if you started the engine, why is that?

What were you guys doing to the engine? This might be a clue... was it a complete tear-down? Did you change anything on the crank end? Any new parts that might be from a different model (donor bike, eBay, etc)

 

I thought chains only shrunk in the southern hemisphere and only in leap years.

I've never installed a chain, but I would think that maybe it might be a little stretched and is jamming on the teeth much like a stretched drive chain.