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Oversize piston

  • likaroc20
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Oversize piston was created by likaroc20

I have a 1977 D100. Currently it has a stock piston but I would like to install a 1.75 oversize. Can the stock sleeve be bored to accept the oversize or will I need a different sleeve? Thanks in advance.
26 Sep 2020 16:54 #1

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Replied by yamahahaha on topic Oversize piston

Yamaha piston sizes start at standard .00 1st over.25 THEN 2nd over .50 THEN 3rd over .75 THEN 4th over 1.00 which is 4th oversize... In my opinion I prefer not to go past 1.00 oversize.... The more you exceed you are asking for trouble shaving the cylinder wall THINNER and THINNER.... JMHO
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Last edit: 26 Sep 2020 17:52 by yamahahaha.
26 Sep 2020 17:35 #2

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Replied by likaroc20 on topic Oversize piston

I didn't realize that. The 1.75 is a piston I saw for sale on a local forum. I'll stay away from going that oversize. Thanks very much!
26 Sep 2020 17:39 #3

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Replied by Pedalcrazy on topic Oversize piston

If you think you need a new piston you need a machine shop to check out the cylinder. They will tell you what size of overbore will clean up the cylinder. You can't tell by just looking. Maybe it will take a third of fourth oversize or maybe just a hone and new standard piston. If you don't have it professionally examined your going to toss time and money up in the air. Just my experience anyway. I thought my RT1B cylinder looked good...no bad scuffs....however the cylinder was ovalled out at the bottom and took 2nd oversize to make it all round again. I never would have suspected that with the naked eye.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
1968 DT1 (3)
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26 Sep 2020 18:05 #4

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  • MarkT
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Replied by MarkT on topic Oversize piston

Go only as much oversize as you need to.

Boring it as big as you can won't give you any noticeable power increase... in fact, you OFTEN lose power with a two stroke because the port size and timing changes as the bore is increased... you'll need to port the cylinder just to get back to stock specs.
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
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26 Sep 2020 18:12 #5

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Replied by Rick C. on topic Oversize piston

It's a dance.
Got any pictures you can show? Top of the piston and such. yeah, clean it so we can see the numbers, may not need it.
Let's not go crazy yet.
Let us help on this.
Reason being, if you have a standard piston, the setting you want will be a waste of cylinder to no reason.


Listen to these guys . I have been for years and years.
Last edit: 27 Sep 2020 21:19 by Rick C..
27 Sep 2020 21:15 #6

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Replied by RT325 on topic Oversize piston


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Replied by likaroc20 on topic Oversize piston

Thanks for your message. Here's a few pictures. This piston and head came from a parts bike and I thought it a good idea to change out the piston and rings before I put the engine back together. What does the piston look like to you? Does that burn mark mean anything?
28 Sep 2020 05:51 #8

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Replied by likaroc20 on topic Oversize piston

Thanks for the message. Yes, a DT100.
28 Sep 2020 06:37 #9

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