Rebuild question.....

[chrisexv6]

I picked up an engine at the LHS that was used, the owner was having a lot of trouble tuning it so pretty much just handed it in for a Sirio.

Anyway, I tore the engine down to see how the piston and sleeve are, and I can't push the piston thru the sleeve (only within about 1/8th inch of the top of the sleeve) so I figure she's in pretty good shape. Piston is still the nice aluminum color, and not burnt or anything.

So after tearing it down and cleaning everything with denatured alcohol, I put it all back together but for some reason the piston seems to stick at TDC. If I turn the crank with everything except the backplate on, it goes fine thru most of the 360 degrees, but right when it gets to TDC there is some "play" where the crank turns but the piston doesn't move. The conrod looks OK to me, but I havent totally removed it from the piston (I dont have spare wrist pin clips for when I lose this one ).

Any ideas?

Thanks!!!
-Chris

 

[wdavidhicks]

That almost sounds like an engine that hasn't even been run. Either that or the previous owner took it apart and damaged it somehow.

That crankshaft play is normal as long as it's extremely minimal. By minimal I mean barely detectable. That's the connecting rod boss on the crankshaft moving from Nth degree BTDC to Nth degree ATDC.

Have you now reassembled it and attempted to start it again?

 

[chrisexv6]

I know its been run, but there was no evidence of it being torn down before (that I could see).

I slid the sleeve out again and turned the crank and piston with the sleeve inserted at different depths. The problem didnt show up until I had the sleeve all the way in. Pushing a little downward on the piston while it was going up towards TDC made the problem go away, but when I lifted pressure off again it came back. Reinstalled the cooling head and it seems to have gone away.

I'm wondering if the conrod was just cocked somehow on the wrist pin. I havent started it up again, it looks like one of the flywheel threads on the crank may have been bent a little (possibly by me, but I never really manhandled it). I just need to figure out how to straighten it out and Ill reinstall the engine.

Its not imperative I get it running, I'm just bored during the current truck downtime (original engine just got sent back to Traxxas after losing quite a bit of compression after barely a gallon).

-Chris

 

[wdavidhicks]

I can't explain why pushing on the piston eases the movement. I know new engines should have a tight fit at TDC.

The tolerances on the conrod/wrist pin/piston should make it impossible to install the pieces in a cocked fashion. If you can smoothly assemble it, it's ok.

It's interesting that you mentioned compression loss on your 2.5. I have the same engine with just over a gallon through it also. And it's a bitch to start when it's hot. I religiously followed their break-in procedures, too. So I may be sending mine back after the Picco Outlaw goes in.

 

[chrisexv6]

Yeah, at this point I'm torn on the 2.5. My brother in law got his a week after mine and his is running perfectly. It even had the "performance gain after one gallon" syndrome.

Mine, on the other hand, has been tough to deal with ever since after break-in. In fact the first time I went to use it after break-in the engine wouldnt start. That turned out to be a bad carb body (end cap was stripped). Problem being I dont know how long it was running with the air leak that may have caused it to run too hot (didnt have a temp guage at the time), so once I replaced the carb I now have an engine that ran for a tank and now has very little compression. It went back to Traxxas.

So there it is.............1 good (not mine) 1 bad (mine). I'm not trying to be a conspiracy theorist, but I also noticed that now (all of a sudden) the 2.5 Maxx is very easy to come by. Before a lot of places were out of stock or on back order. Wonder if they had a production problem. Or maybe its just coincidence.

Either way, I dunno what I wanna do. Use the "fixed" 2.5 that I get, or just sell it and use the $$$ towards a Fantom or Sirio. Its a little more $$$, but they actually seem to hold a tune pretty well and dont have problems with composite carbs.

This hobby is beginning to get on my nerves

-Chris

 

[wdavidhicks]

I think that the major difference between nitro and electric is that nitro owners MUST enjoy wrenching at least as much as bashing and racing.

I bought mine about six months ago. Within a month I stripped the glow plug threads in the head. After waiting a month for a stock heatsink I wised up and bought one from New Era. I called the LHS and told them to cancel the order and they still called me three weeks later and told me it had arrived.

I'm gonna have a buttload of spare Maxx parts.

 

[chrisexv6]

Yeah, I dont mind working on the truck at all. What bothers me is that I did everything "by the book" and this is where it got me.

Pretty disappointing when you follow everything the manufacturer tells you to but the thing still kills itself too early.

-Chris