250sx engine experts (LOOK!!!!!!!!!)

[Honda_Dude]

85 250sx I rolled the dice on and bought cheap then put a used head on. (PO said had a "stuck" exhaust valve. Engine rotated fine.

To make a long story short, I put a used head on (exhaust valve had "nicked" piston, piston looks good believe it or not, has good compression, now).

Anyway, after screwing with the valve adjustment and a homer of my own(forgot to tighten cam gear bolts)I now see why the engine had piston/valve interference.

After setting the valves and kicking the engine over, I "felt" the timing gear slip.

Now I have had my top end apart obviously. The top gear, auto-tensioner and chain seem fine (unless it's really stretched that bad - is there a measurement????)

It seems my next step is to pull the side case and check the lower gear out ???


I need to do that anyway if the cam chain needs replaced. I guess what I'm asking is..... What is the most common failure when you have a chain jump a tooth/teeth ????? IS it almost always a stretched chain ????? is the bottom gear for the timing chain on a keyway ???


I thought the chain may have felt sloppy. It kept wanting to slip off of the bottom gear while I was trying to install it. BUT, it seemed to tighten up after I put the auto-tensioner in. Soooooooooooooo, I won't buy any parts until I get this thing apart but what would you guess if ya had to??? chain?...and guides ?????
lower gear ?????? And yes, how do you tell if the chain is stretched beyond it's limits? Is there a measurement? I haven't found it in the manual if it's there.
THX, Doug

[Honda_Dude]

Nobody's ever had a timing chain that wants to jump time on a 250sx?! Al-righty then, I'll tear into it and learn as I go. I' probably just replace the timing chain, both guides, double-check the auto-tensioner. Wow, I figured I'd get some replies.

[Dirtcrasher]

I've never seen one "jump teeth", unless the auto tensioner was dicked with and or reassembled wrong. The gears are always fine, but the chain does stretch and both guides should always be changed on a "ticky" engine.

It may have jumped when it hit the piston?? I'm not sure..... Lets hope you didn't harm a valve in the timing issue.

Even when the chain doesn't feel sloppy, with just the valve cover removed, if it was noisy then the work has to be done. It does nearly eliminate any and all engine noise when your done.

EDIT - I looked, but I don't have an engine apart. I'm unsure if it's keyed but allot of them are a press fit and they never slip unless the shaft is undersized.

[Tri-ZNate]

Several years ago when it happened on mine it was because after 20+ years chains seem to stretch. Change the title of the thread to something more specific and you'll probably get more help. Lots of knowledgeable people on here

[Honda_Dude]

Hey thanks guys. I found some info by searching(yah I did look LOL) and found a member with a similar issue. Talking to him through PM's have me assured it's a stretched chain (provided the bottom gear is ok).

I have timed it twice and it has definitely "slipped" enough to cause piston/valve interfenence. It rotated good several times.I have only kicked it, not started it, but it definitely slipped.

I asked in another thread about the sloppiness of the chain during install but didn't get an answer. It was very easy for me even with the tensioner out to install the chain. It was actually difficult trying to "time" the cam gear (getting both bolts in) without the chain "slipping" on the bottom gear.

The auto tensioner took up the slack but there was quite a bit.




If you remember any of my other posts, I bought this cheap($150), looks great, had a "stuck" exhaust valve(according to seller)..rotated fine...bought it,pulled the head and found the exhaust valve MIA. Surprisingly, the piston is only a bit nicked. That's why I've been easy kicking it without trying to start it. I thought it was odd for it to actually slip also , but it definitely is....... I'll post back after I tear into it a bit farther
Thanks for your replies...........