Jump to content
Sign in to follow this  

XR100 Engine Rebuild

Recommended Posts

I am currently rebuilding a 1994 XR100 motor and have a couple questions. The motor was purchased on ebay and looked pretty sad on the outside and ran a little rough so I decided to tear it down and inspect it. There was a lot of shinny sludge in the oil and settled all over the inside of the cases. However, all the gears, shift drum and forks showed almost no wear. Only one bearing on the small end of the output tranny shaft looked a little rough. I will be replacing that bearing and all seals as well as gaskets and other bits.

My question is with the crankshaft bearings. They seemed to have a rather loose fit between the outer races and the crankcase. In fact there is evidence that they had been spinning a bit on the outer race. I realize the fit is supposed to be loose enough for the bearings to stay on the crankshaft when disassembling, but I found no spec on how loose they were allowed to be. Also, the side to side play on these bearings in the crankcase housings seems a lot, around .060" or so. No info in the manual about reassembly either. Should a bearing set like Locktite be used of do they need to float side to side for proper alignment?

Also, the flywheel took a good hit from a drive chain derailment which of course blew out the case and cover too. Not a problem since I will be replacing the cover and having the case welded up. The issue is with the flywheel. The ignition trigger tab on the outside of the flywheel is about 1/3 gone on one side from jamming on the chain. Could this effect the function of the alternator in any way, or should I not worry about it?

Share this post


Link to post
Share on other sites

Post a picher. It stands to reason if the trigger tab is missing material it will mess up the timing. Retard it if the leading edge is 1/3 gone. If 1/3 of the width is gone it might be ok.

After rereading your post you say its missing 1/3 off one side. It'll probably still work. Check how it lines up with the pick up.

Share this post


Link to post
Share on other sites

My question is with the crankshaft bearings. They seemed to have a rather loose fit between the outer races and the crankcase. In fact there is evidence that they had been spinning a bit on the outer race. I realize the fit is supposed to be loose enough for the bearings to stay on the crankshaft when disassembling, but I found no spec on how loose they were allowed to be. Also, the side to side play on these bearings in the crankcase housings seems a lot, around .060" or so. No info in the manual about reassembly either. Should a bearing set like Locktite be used of do they need to float side to side for proper alignment?

The bearings should be a very neat sliding fit in the cases and they are press fitted on the crank . Excessive crank float or walk is not good news as it will affect the conrod and cam chain alignment causing unwanted side loading and resultant accelerated wear due to friction . If the cases are worn , a good machine shop can measure the internal case width with centre gasket in place using an inside micrometer and then measure the overall crank width from outside main bearing to outside main bearing then remove and replace them shimmed out to compensate for the amount of excess side play and bring the assembly back to the manufacturers specs . Conrod centreline to bore centreline and crank timing sprocket to cam sprocket alignment must be taken ito account when determining where to shim . If the bearing outers have been spinning in the cases and there's not too much wear , new main bearings will help tighten things up a bit and Loctite can be used to further prevent the outer races from spinning . I hope I haven't come across as a condom-scenting KIA !!!!!!!!!!!!!!!!!!!!!

Share this post


Link to post
Share on other sites

BTW , Crank float can also affect the ignition and the oil pressure feed plus wear to the primary drive gear !

Share this post


Link to post
Share on other sites

Thanks for the input XR Evolution. I couldn't find any specs in the factory service manual on main bearing side play and ID/OD tolerances. I have the means to measure everything, but it doesn't mean much without some numbers. The ID/OD clearance is not so much that the bearings are loose and clattering in the crankcase, but they do move freely from side to side. I don't see any evidence of wear on the crankcase where the bearings slide in, but the outer races of the bearings have some spinning type wear. I suppose a fresh set of bearings would give the proper ID/OD set, but it would be nice to have an idea how much side to side play it needs so I can look for some shims if I need to. Is loctite a good idea if the bearing are supposed to "float". I'm just looking for more info than I found in the manual, which was almost nothing. The set up doesn't feel like what I'm used to in an engine. Not that it's wrong or anything, but I don't want to just slap it back together without at least knowing there's nothing wrong.

Share this post


Link to post
Share on other sites

End float should be no more than 4 to 6 thousandth's of an inch , the less the better . I have an XR 75 and an 80 engine with exactly the same problems as your engine . The 75 is minimal and repairable (thank god for the oil spinner) but the 80 is cactus because it's too worn due to dirty oil and lack of filtering protection. I've checked brand new engines and they have no crankshaft end float that I can feel . You'll find that as alloy cases build heat , they will expand and increase the end float . The steel crank won't expand at the same rate . Any excess play can allow "HAMMERING" or chattering to occur so the least amount of float as possible at operating temperature is more desireable . The main bearings have to be the minimal sliding fit otherwise you wouldn't be able to put the cases together . The rod side clearance and gudgeon pin float on the little end will allow the piston and rod to find their own operating centres ! A sliding fit is 1 thou or more over , a press fit is size for size and an interference fit is 1 thou or more under and they are all adjusted to suit a particular application as necessary . They don't list the clearances in bike repair manuals because it is the job of the Honda engineers and machinists to get all the tolerances right in the first place ! What they would tell you is that when these clearances go out of spec , it's time to replace the parts !

Share this post


Link to post
Share on other sites

The set up doesn't feel like what I'm used to in an engine. Not that it's wrong or anything, but I don't want to just slap it back together without at least knowing there's nothing wrong.

That approach is what makes you a good mechanical FITTER ! A "mechanic" is a grease monkey who just slaps part "A" together with part "B" as if assembling a jigsaw puzzle without fully understanding the importance of tolerances and clearances and checking to make sure they're within specs ! A good fitter also "reads" the wear patterns on parts and investigates why a failure occurred in the first place with the intention of correcting the situation to avoid a repeat performance ! In the engineering industry a mechanic is jokingly known as a fitters a$$h*le !

The last time I assembled and ran the K1 75 engine , I loctited the outer races in the cases and it worked great for a while but it only had about 10 thou side play not 60 . The loctite held for a while then the side play returned . Now that the rod has failed to pass the rap test , I'm going to fully rebuild the crank and adjust the main bearings to minimise the sideplay while I'm at it ! I was going to buy some used replacement cases off ebay in the hope that they're better than mine but decided not to take the punt !

Share this post


Link to post
Share on other sites

Thanks again for the help XR Evolution. It's nice to see that at least a couple of you will handle the tougher questions around here. I have engineering training in my background and it sounds like you do to. I have also split a number of cases and modified and repaired engines as a teenager, but it's been a while.

It's obvious that this engine has been abused. It appears that it had a top end failure at some point that damaged the main bearings and a couple of the smaller trans bearings. Amazingly though, the trans, shift forks, kick start shaft all look brand new with no wear marks at all. You should have seen the crude in the cases though. Probably never had the oil changed or topped up before the top end failure.

I took a closer look at the cases and there is evidence of hammering on the ends, but not accounting for all the end play. There looks to be only about 5 to 8 thou of wear at either end, so I'm hoping the rest is in the bearings themselves. Hopefully I exagerrated the .060" initial end play a bit since I didn't actually measure it. I think at this point I will get some new bearings and partially assemble the cases with a new gasket and see if I can determine some shim thicknesses that will center the crank and keep the end play down to almost nothing, say around .001"-.002". The way this engine is set up the clearance should get looser as it warms up and not tighter. As in your experience, loctite will probably not be effective over the long term, so I would think shims would be the only way to go here.

Here are a few pics for your viewing pleasure:

http://www.tinypic.com/f0n8jr

http://www.tinypic.com/f0n8mh

http://tinypic.com/f0n8na.jpg

Share this post


Link to post
Share on other sites

OK , This is what I've measured with the '74 K1 75 , the main bearings both measured 56 mm O/D and the case bearing journals both measured 56.05 mm I/D and they're a very neat fit with clearance just under 2 thou. The outer race surfaces look highly polished like chrome.

Internal case width : 72.00 mm

Case gasket thickness : .55 mm = 21.6 thou

Overall case width 72.55 mm

Crank o/s main to o/s main 72.10 mm

Total crank endplay/float .45 mm = 17.71 thou

The manual says that the conrod big end bearing side clearance should be within 0.10 - 0.35 mm (4 to 14 thou)

Aside from the end play this engine is in fairly good shape for its age . The cam tower seems to have a small amount of side play but everything is still round . The crank and cam end float combined with cam chain stretch accounts for the uneven wear pattern on the cam sprocket . Both the sprocket and chain are being replaced ! The above measurements should give you an idea on the condition of your engine . As far as I'm concerned my engine is in as good a shape as can be expected , it was extremely clean with no sludge or crap in the sump and is in the process of being rebuilt back to as new . I've had delays on parts from over there in the US !

I checked your pics out and your cases look about the same as mine . As with your flywheel , as klxd said , you can only try it out and see . It doesn't look like it would be too affected but there's only one real way to find out . I'd be really interested in seeing how your case and crank measurements check out . End play usually feels like it's more than what it really is but like you , I feel that it should be close to zero with the engine cold ! I wonder if the 45 mm stroke crank will fit the early 75 cases and accept the centrifugal oil filter ?

Share this post


Link to post
Share on other sites

Retard it if the leading edge is 1/3 gone. If 1/3 of the width is gone it might be ok.

If one third of the leading edge of the trigger tab on the flywheel was missing , then theoretically you'd have to rotate the pick up clockwise by the amount of crank degrees that the missing metal represents to get the engine to fire at the same time as it did before the damage occurred . That means you'd have to advance the ignition . Crankshafts in XR 100 's rotate counter clockwise when viewed from the flywheel side of the engine .

Share this post


Link to post
Share on other sites

I was saying that if the leading edge was missing material it would retard the timing. I was not telling him to retard the timing. Perhaps I should have said "Retarding the timing". Sorry for the confusion.

Share this post


Link to post
Share on other sites

KLXD

Default Re: XR100 Engine Rebuild

I was saying that if the leading edge was missing material it would retard the timing. I was not telling him to retard the timing. Perhaps I should have said "Retarding the timing". Sorry for the confusion.

That's OK mate , I understand that sometimes you can't quite put what you are thinking and really mean into print and it ends up being misconstrued . I have worn discs in my neck and have to take painkillers everyday so my ability to concentrate can be severely affected at times . I've recently taken to correcting my posts after hitting submit and realising that my comments could be taken the wrong way !

Share this post


Link to post
Share on other sites

For instance :

On reading my post about advancing again , it is technically wrong . You would correct the situation by rotating the pickup clockwise as if advancing the ignition but you would actually just be restoring it to where it was before the damage occured which is neither advancing or retarding ! :banghead::banghead:

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×
×
  • Create New...