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"Cooking" the case for main bearing install

  • hackman101
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My 73 MX250 needed a new main bearing. Instead of freezing the bearing and using a heat gun to heat just the area, I wanted to set the oven to 220 degrees and heat it up that way. My concern is the tachometer plug. Or maybe it is the speedo plug..... I do not want to remove it, I do not want to get another one. I know there is a possibility of that anyway, but looking at the heat and ice method, the 220 degree will make the water boil off as the test shows how hot to get it, but will it damage the tach plug? They are made to handle some heat by default, but that much is a concern to me. Has anybody out there had the same issue, or heated the case in an oven with a seal in place and succeeded? Thanks in advance!!
1973 MX250
Past rides 45 years ago (and longer)
1964 Honda 50cc C110
1960s Yamaha 65cc scooter
1960s Honda 65cc w/ 90cc engine mashup
1971 DT1-E modified to MX kinda...
Last edit: 30 Oct 2021 11:32 by hackman101. Reason: Added more info
30 Oct 2021 11:30 #1

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Not sure about those but on my DT1 or 360's i just press them in cold with oil to help but they have a cast in steel insert that 'never' complains or comes loose. If yours is alloy--check it with a magnet--i'd just warm it up a bit with a little gas torch going around n round for a bit & freeze the bearing then should fall in. Heat gun i see you mention so yes if that's your option. If it has an alloy insert--in the alloy case, stupid idea, you'll likely see oil appearing around the edges as you heat it but i wouldn't worry unless it's loose--as in moves up & down separate from the case. Case will be formed over the edge of the insert so not easily seen. If ya lucky there won't be an insert as was the update on my YZ125 case but that was about "78 where the insert 'really' let go.
30 Oct 2021 14:56 #2

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Replied by hackman101 on topic "Cooking" the case for main bearing install

Just to confirm, the cast in steel insert will not have an issue with local heating? I plan to heat a little farther out than just where the bearing goes, but not into the gearbox area. I have the steel insert, as a mention, the alloy, may fail with heating if done too haphazardly? This seems like something that should be done with care and patience, whether it be alloy or steel. Also my heat gun has settings that go to 1000 degrees.... so it says.... I will still get it to the "boiling water point" and no more, the best I can.
1973 MX250
Past rides 45 years ago (and longer)
1964 Honda 50cc C110
1960s Yamaha 65cc scooter
1960s Honda 65cc w/ 90cc engine mashup
1971 DT1-E modified to MX kinda...
30 Oct 2021 18:03 #3

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If steel like my old model i guess the expansion rate is different between 'that' & the case. If steel i just oil them up & press them in. Doubt yours 'is' steel inserted though. Best let others reply before i sidetrack you with my bad habits. Could tap the bearing in a little with a soft hammer on the outer ring to get a feel of how tight its going to be. How did you get the old ones out?. If tapped them out then that'd give you an idea of the fit.
30 Oct 2021 18:17 #4

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Replied by hackman101 on topic "Cooking" the case for main bearing install

I got the old one out with a gentle heating, but it took a good whack or 10 to get the old one out, using a proper sized socket as a driver tool with a hammer.
1973 MX250
Past rides 45 years ago (and longer)
1964 Honda 50cc C110
1960s Yamaha 65cc scooter
1960s Honda 65cc w/ 90cc engine mashup
1971 DT1-E modified to MX kinda...
30 Oct 2021 18:20 #5

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1. Service Manual says 200 degrees
2. If you don't remove and clean everything carefully, you could have a fire or even explosion using an oven, especially gas oven.
3. Sounds like one bearing?  The others can come loose and even fall out when heating case
4. Never hammer on inner race when installing new bearing into case. 
5. Most important is keeping straight. 
6. Press is better than hammering.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
30 Oct 2021 19:30 #6

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Replied by hackman101 on topic "Cooking" the case for main bearing install

@ Mark T, I am hoping that the heat and ice trick will not need any driving at all. I did drive out the old bearing by the inner race after I heated it up a bit, but it was toast anyway. It came out evenly, as I hope the new will go in  evenly as well without hammering. If it does not drop in like I seen in other posts, I will start over with the same process AND with the proper driver in hand. I have a pic that may shed some light on what I am working with. It does have the steel insert, my rare earth magnet jumps onto the ring that is cast in.


As a side note, all the black paint looking stuff seems to be just that. It is like the prior owner painted the cases before they were assembled. The other half of the case has it on the mating surface as well. I am going to see if brake cleaner will remove it. Otherwise I have a very fine steel wire wheel that would do the job. As a side note, all the black paint looking stuff seems to be just that. It is like the prior owner painted the cases before they were assembled. The other half of the case has it on the mating surface as well. I am going to see if brake cleaner will remove it. Otherwise I have a very fine steel wire wheel that would do the job.


It looks like the picture will not upload. I will try later since I have tried a few times and it comes up with an error each time.
1973 MX250
Past rides 45 years ago (and longer)
1964 Honda 50cc C110
1960s Yamaha 65cc scooter
1960s Honda 65cc w/ 90cc engine mashup
1971 DT1-E modified to MX kinda...
Last edit: 03 Apr 2022 22:02 by hackman101. Reason: Post came out all screwed up
30 Oct 2021 19:53 #7

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Replied by Pedalcrazy on topic "Cooking" the case for main bearing install

Just another experience on the topic. The heat/freeze method was recommended to me by a machinist that I was talking to about replacing the bearings in 76 DT400 a few years back. Turns out you don't need a machine shop after all. In fact they were hesitant about working on the 45 year old obsolete parts. So after some research and discussions here, in went the cases at 225 degrees for maybe 10 minutes. Oh yeah, I put the bearings in the freezer the night before. And no plastic or rubber plugs in the cases. MarkT is right in that the bearings can fall right out ...but some need a little tapping. But I was replacing all the bearings so no worries about some coming loose that weren't intended.

Heat the cases back up and pull the bearings from the freezer. Some went in right away and some only slight tapping. So, I'm sold on the heat/freeze method. Froze the crank overnight too before pulling it in. Work smarter not harder I've heard.

Also, interesting about working temp for this work...My digital dipstick for my oil tank on my 2016 Harley commonly reads 230 degrees or higher, which isn't abnormal, and that's the OIL temp! Probably different materials used today than back in the 70s but I'm just saying.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
1968 DT1 (3)
The following user(s) Liked this Post: MarkT, Ht1kid, Sneezles61, hackman101
Last edit: 31 Oct 2021 08:52 by Pedalcrazy.
31 Oct 2021 08:47 #8

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Replied by hackman101 on topic "Cooking" the case for main bearing install

Thanks for your experiences. The oil dipstick markings are quite interesting, if the oil gets that hot, I imagine the metal may as well. I could only guess that a transmission case on a 2016 bike would would be no hotter or cooler than a 1973 transmission. Unless the Harley trans is an automatic?? If it is not automatic, then I would figure that an 73 MX transmission would possibly be hotter from the stress the trans goes through, at least during competition....
1973 MX250
Past rides 45 years ago (and longer)
1964 Honda 50cc C110
1960s Yamaha 65cc scooter
1960s Honda 65cc w/ 90cc engine mashup
1971 DT1-E modified to MX kinda...
31 Oct 2021 13:39 #9

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Replied by msavitt on topic "Cooking" the case for main bearing install

i put cases in oven at 250F for 30 mins and the room temp bearings drop right in without any pressure

I do this without any seals or wife present, but with shifter cam plug in place which has no issue at this temp

I have tried 200 on cases and freezing bearings but the "in/F" for aluminum is way higher than steel and therefore heating cases is much more effective than cooling bearings
The following user(s) Liked this Post: MarkT, Snglsmkr, hackman101
01 Nov 2021 17:12 #10

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