Help Rebuilding Engine

[phorton]

Hey there,

Wondering if anyone wants to start a dialog. I just rebuilt the motor on a 1972 Yamaha LT2 (97cc with 4800 miles) replacing the piston, ring(s). and clutch plates. I was able to come up with a new gasket set, so the head, base, and crankcase gaskets are also all brand new.

In general, I did not replace any bearings or seals. I re-used the piston bearing, pin and clips on the top end. I cleaned every part carefully with gasoline and re-assembled it to the best of my ability..

As far as the bottom end the only things I did not take apart were the transmission, kick start gear assembly, and the main crankshaft (together, these are the things that sit in the left hand side crankcase after it has been split).

I rebuilt it when it started making a high pitched whining noise from somewhere near the top end when getting into the power band. The first thing I noticed when disassembling was that one of the two rings was busted in several pieces, with several missing (hence my decision to split the case and get any stray metal pieces out). I also found the broken ends of a circlip in the motor. In the bottom was around a half teaspoon of sludge with a smattering of well ground metal dust.

The circlip ends were from the middle of the kickstart mechanism. The circlip was still in place, and seemed fairly secure, so I left it. It is not a high speed part.

Just so you know, I cleaned every spring and screw. I carefully scraped all the gasket surfaces clean and used Yamabond 4 to reassemble it. It runs now, but the same thing seems to have broke. It whines (read: screams a high pitched metal on metal sound) when it gets into the good part of the power band. It will run for hours at less than 4000 rpm, but try to have fun (i.e. do a donut or accelerate hard) and it makes the most horrendous noise. I'm pretty sure the ring 's broken and/or the top end bearing is gone.

FYI, I broke the engine in by riding it around the neighborhood for 30 miles or so, from gentle to low-mid power band. I didn't hear the noise until I took the bike out this last weekend and really got on it the first time. The first time I heard it I backed off right away, but it was the first day of a 5 day trip. I took the engine up to the power band about three times over the 5 days, each time, hearing a whine like a dentist drill on a blackboard. Yikes. But we had planned the vacation for a while (trail riding in Sequoia at 5000-8000 ft).

I have another Yam LT2 that is in great shape with 1821 miles on it and it happily likes the power band, and has pop and punch. My girlfriend got to ride it and I had to ride the broken one. I put about 100 miles on the broken motor in 5 days, all under 400 rpm, sigh.

Before I rebuild it again, what I'm looking for is some advice (and someone to talk to).


Here's what I think I did wrong.

0. Rode it when it was broke. Imagine trails thru sequoia groves and try not riding. Sigh. I had to. At least I'm honest! I never heard any "thunks" so I don't think anything fell into the transmission. But I sure didn't make the cylinder wall any better. I can pretty much peg when the whining noise will begin and could drive around all day at 3800 rpm without hearing it if I wanted.

1. Did not bore cylinder. The old piston was 1st oversize (.25 over). It had two rings. The new piston only had one ring, but was .25 over. These pistons are not extremely easy to come across, so I decided to use it up, without boring, rather than scare up a 2nd os set. The cylinder had some pretty bad damage, most notably a couple of vertical groves around the exhaust ports where the pieces of old ring had scraped it. These grooves (call it two) were approx 1mm wide and maybe .1 mm deep. I gently took some 600 paper to the walls by hand but didn't really take any material ... just shined them up and touched the edges of the ports slightly to try and micro-deburr them, if that makes any sense :-) When I put it together, turning it by hand, you could hear the oil and air sucking thru the grooves, but everything moved smoothly and it still had enough compression to start and run.

2. Did not repack the bearings. There is a question of whether or not I am capable of removing the crankshaft from the left side and/or splitting it to get at the big-end rod bearing. But I did flush it with gasoline and probably sucked some or most of the grease out of it and all the other bearings. I feel like an idiot, but I wanted to fess up. When I put it back together, I figured everything would be bathed in oil and as I may have mentioned before, I didn't want to mess with the bearings and seals. But I probably should have done what I could to repack them. Including the little end bearing which I just oiled as I was reassembling everything.

BTW, There's another noise that came up when I stressed the transmission, but I know what that is, I think. The clutch basket is held onto the main drive gear by three rivets. There was a little play in the basket when I examined it. I think that when it's stressed it moves a little out-of-plane making a crunching low pitched growl. It only happened once on the 5 day trip.


Status:

0. I have the shop parts book and service manual as well as a Kilmers for the bike. I have studied them extensively.

1. I found the only remaining NOS drive gear - clutch basket in the country and bought it.

2. I am getting new cylinder base and crankcase gaskets from Motogrid. Unfortunately I cannot find another copper head gasket so I plan on re-using the one thats there (that I just put on 3 weeks and 100 miles ago).

3. I have bought several NOS seal sets for the bike on ****. Any needed seals that are not in the set, I will purchase individually when I discover they're missing (when I get the sets and compare them to the parts book)..

4. I have bought NOS 1st os and 2nd os pistons and rings on ****
These each have two rings and are from "Rocky Manufacturing". They come with new pins and clips.

5. I have ordered all new bearings on Motogrid (including rod small and large end, crankshaft main bearings, both transmission shaft bearings. I've never ordered from Motogrid before so it could be weeks and/or I could find that some of them are unavailable. But right now I hope to get them all (over $100 worth of bearings).

Plan:

A. Tear the whole thing down again. Still don't know if I should or can remove the crankshaft from the left side case, and dissasembling the crank itself (to replace the big end bearing) scares the piss out of me. There are apparently special tools to (a) remove the crank, (b) take it apart, and (c) put it together. I can probably use a crank puller I have to get it off and apart, but it does'nt sound like I want to pound the halves back together with a hammer. Yamaha has a special tool that compresses the crankshaft halves back together. Advice on this would be helpful.

B. Replace (and pack) all bearings. Not looking forward to this part. I will probably get a heat gun, cuz I hear that's about the only way to get them in and out. Anyone with experience on replacing bearings on small cc engines can help out here. I imagine they have to be micro aligned when put back in, but is sounds like you almost need a sledghammer to put them in. I've never changed a bearing before. What I'm hoping is that someone will say"go ahead, it's really easy", and it will be and we'll all live hapilly ever after :-).

C. Replace all seals. I should be able to do this with my skill level.
If I can get to them (note above about left side main seal and bearing ... I don't know how to take the crankshaft out of the case).

D. Measure the pistons and the cylinder and make a determination using the shop guide and probably the 2nd os piston about what diamater to bore the cylinder out to.

E. Get the cylinder bored out. It's easy, I suppose, if you know where to go. I'll have to find someone from scratch to do the job. My local Yam dealers won't touch this old motor and I don't have a regular ongoing relationship with a machine shop. If anyone knows anyone around San Diego that they would have do this, PLEASE LET ME KNOW.

F. When I bore it out, I will probably worry about the change in the port sizes, but do nothing about it.

G. Get someone to mess with the head regarding the new bore size. At least to take the lip to the new diameter. Don't know about increasing the bowl size to adjust the compression ratio downward to compensate for new piston size. These seem like pretty specialized machining tasks and I wonder if I will have luck finding someone to do it.

H. Clean and install the oil pump. Note that I am currently pre-mixing this bike between 16 and 20-1. When I bought it, the guy gave me the oil pump and parts in a plastic bag. It looks fine and I think it probably does a better job of lubing the bottom end than mere premix (although with the leaky rings and high mix ratio, I'm pretty sure the crank got some lubrication). So I want to re-install it and test it as I put the bike back together.

I. Put it back together using the new clutch basket and now slightly used clutch plates I put in last time. They're working fine. Put as much grease on bearing surfaces as I can. Oil the sh*t out of everything else. Seal it all up so that not a drop of oil comes out.

J. Set the timing. Had a little problem with this last time. Manual says 1.8 mm btdc ... but I couldn't figure out how to hook up the ohm meter. Seems the coil or condenser was in-circut and messing up my readings, so I just eyeballed it. I'd like to get this right when I reassemble it. I had to way advance it to get the bike to run decently at 8000 ft. Now that I'm back at sea level it sounds like its pinging (pre-burning), esp at low rpm. But I don't think this has anything to do with the screeching noise in the power band.

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So that's it. I know this is a long post. If nothing more I hope you enjoy reading it. Truly however, I'm hoping advice on the issues noted above and any other general comments or discussion would also be appreciated.

Thanks very much,
- Pat

 

[LWillmann]

I'm in a similar boat in that I'm looking to replace all bearings and seels in the engine and reassemble it all.

However, my engine and transmission are fully disassembled (except the crank is still together with the rod attached), so my job is already partially done, except that I'm not the one to tear this one down, so I have to figure out how it goes back together.

Either way, I'll be very interested in this thread.

 

[viking20]

Changing the bearings in the case is really not that difficult.
You sound like you already know about applying heat to the cases , and dropping the bearings in the freezer overnight.
With a suitable drift they should tap out quite easily.
If you disassemble the engine and check all the bearings , you might be able to get some of them from a bearing supplier instead of oem. You dont have to align anything as far as I know , and the bearings will be very easy to install when they have been in the freezer , and you heat the cases for a minute or two.
You might want to use a press , instead of a hammer.
That will also make it easier to get the crank out.
Any machine shop that does engine work can help you out .
The crank halves will have to be aligned after the conrod-bearing has been changed . Again , any shop that does engine work can do this for you.
Remember that the rod in itself is the outer bearing surface for the lower bearing ( unless your engine is very special ) , so it will be best if you change the bearing and rod as a set.
I know this sounds like difficult work , if you havent done it before , but its really not that complicated.
My sons have a habit these days of dragging home 20 or 30 year old 50 cc mopeds , in desperate need of repair.
We repair cranks , replace bearings and so for lots of other kids and dirtbikers etc.
Its really not that hard , if you think about what you are doing , and know what your limits are. You cant fix the crank yourself , only a shop with the correct aligning tools can do this for you , only takes an hour or so...

 

[viking20]

Oh , yeah , if the head gasket is all copper , you can just anneal( sp?) it , heat it to cherry red , and drop it in a bucket of water. Remove all loose particles with a wire brush , and it will be good as new.
You dont have to mod the head because of a bore job , only if you increase the cc´s much more , by say a 2 or 4 mm overbore.
Dont know what its like in the US , but my guess is that if you show up with a cylinder that needs an overbore they will do it regardless of the age of the bike.Probably the same if you give them a crank and parts , they will probably just help like they would if this was a new bike.
The guys that wont do work on old engines are probably just scared of having to spend a lot of time chasing seals , gaskets and other parts , only for the engine not being picked up , when the price for the job is known. ( often more than the old bike is worth )

 

[phorton]

Thank you LWillmann and viking20 for the replies.

Good to hear someone say the bearings are not too much of a pain. Freeze the bearings, heat the cases. Got it. As far as a press for the bearings go, can I use wooden blocks in a vise or is there a special tool called a "bearing press"? Seems to be a lot better to move them slowely than by pounding on them.

And also nice info on annealing the copper gasket ... it IS all copper and I'd heard about the treatment you describe. Your confirmation of the process means it's a for-sure for me.

I will also probably take your advice on not messing with the head due to the re-bore. It's an old bike anyways. I just want it to run good, not super-top-notch. If it looks like the piston might touch the head, should I dremel the head out a tidge?

After reading your post, the crankshaft still holds some mysteries for me. As per your advice, I will see how Motogrid does with the rest of of my order and probably order a new crank pin, just to have it in case I do the crankshaft. I will be especially gung-ho to do so if I can find someone who says they can put the crank together (align it, as you say), after I take it apart. We'll see ... I'm batting 0 for 2 on the motorcycle dealers service shops I have called so far. I've got a couple calls into independent cycle repair shops, but they havn't returned my calls yet.

Is it likely a general automotive machine shop would be able to do the boring and crank-aligning?


Also,

On each side of the crankshaft, between the bearings and the crank itself, are shims that mate with the bearing and provide proper clearance to the crank case. Apparently they come in 6 thicknesses to compensate for wear and tear. I noticed on Motogrid that the middle and larger sizes are backordered, indicating that these are common wear items. The question is "Any chance these are screaming?" doesn't sound like it, but maybe you have some experience that says they too should be replaced.

Finally, I could also ask the same question about the spacers, spring washers, etc that go on the crank and transmission shafts and in the clutch .... in general they don't look too bad, and when worn, probably each incrementally adds to the racket the engine makes, but I doubt they are the source of the "screaming", which I believe to be the top end bearing and or rings-scratching-walls. In general, does it sound like I'm on the right track?

 

[viking20]

Engines can make some weird noises for sure ,...If you have a bad conrod bearing the noise will be a knocking sound , it will get worse real quick , and it wont run for more than minutes , before you feel something is really bad , or it will seize.
Main bearings will give a knock at low revs , or a rumbling sound. Screaming , or whining noise is most likely a gearbox bearing , or something in the clutch.
Piston skirt slap , will also give some sort of knocking at idle and low revs.
A press is a hydraulic device , a stationary thing used in workshops etc. Do not attempt to use the vice at home for most of the things you need to do.
It will be much better to use a hammer and drift on the bearings , be sure to only hit on the outside of the bearing surface when you install the new bearings.
Reg the crank work , go to Eric Gorrs Forward Motion site ( link on the left )
Find the article on 2 stroke lower end rebuilding and crank rebuild. It will explain everything in detail.
The shims used for adjusting side clearance on the crank , is simply added , or removed to give a bit of side clearance in the cases.
Assemble the cases , and feel for play side way on the crank. What you want is just so much play that you can feel it . If there is no play at all , you will need fewer/thinner shims , and so on .
You can use two thin shims instead of a thicker one , no problem.
Dont forget to have the crank centered between the bearings first( the Eric Gorr article will explain )
The boring of cylinders and crank aligning can be done in many shops. Be sure to have a new piston ready for the shop to measure , and dont be afraid to tell them what the piston clearance is supposed to be , if you know the correct piston/wall clearance.
If you have trouble getting work done locally , maybe Eric Gorr can help you out.

Good luck !

 

[phorton]

Thanks again for the reply Viking20 ... key points to remember. Will read the Eric Gorr article completely.

Called several more places after last post, and found a machine shop that says they can do the work. 1 hour for the bore, and ? 1.5 for the crank alignment. hmmm .... $70/hr .... hmmmm .... bring the piston, etc. Will probably call a few more places as time passes, but at least I have a starting point.

Viking20, it sounds like you are saying that the scream (and it is definitely a high pitched noise) is likely not from any of the bearings, but is more likely in the gearbox or clutch. Yikes ... this runs counter to my whole theory (ring and or piston pin/bearing) about the high pitched noise. I'll know more when I take a look ... if the ring is broken or badly scored it will jive with my feelings. I don't know how to evaluate the little bearing/piston pin, but I'm replacing them anyways.

Regarding the clutch/transmission, I guess I will examine everything as carefully as I can ... maybe disassemble and re-assemble the transmission and look for anything real bad and obvious. Stuff that wiggles around is hard to judge. The new clutch basket might help ... and there will also be new thrust bearings and plates. Maybe the noise is from a thrust bearing?

This is the hard part ... KNOWING what you're doing ... and only comes from experience. Somewhere there's probably somebody that could hear this engine noise and just say "oh yeah, these old Yamahas ... I know exactly what that is" .... and I wouldn't be messing with the good old working parts of this motor ... only the bad old ones ... But this is also how you learn, so I'm going to end up putting a lot of stuff into the engine it might not need, and may still miss the key part(s) that are causing me grief. LOL ....

(p.s. I thought briefly about trying to make a sound recording of the noise, but breaking the motor just as I'm passing a tape recorder at 20 mph is not likely to result in any usable information lol)

 

[viking20]

I meant that the sound you describe is probably a gearbox bearing or similar, not nessecarily the gears....If you change all the bearings , and check that nothing looks like anything has been rubbing against other parts ( incorrect assembly , or badly worn parts ), Im sure it will be fine.A badly worn clutch basket can make all kinds of sounds.
You are right , experience is important when rebuilding engines , unfortunately there is only one way to get experience , LOL...