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Q on Piston/Wall clearance

  • pabdt
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Q on Piston/Wall clearance was created by pabdt

So, measured my piston and cylinder today.  Both "look" like they are in good shape.  Need a sanity check on this data.   Used a bore gauge to get the following.   1) does this make sense.   2) which piston setup should I be looking at? 

I'm thinking the max clearance is around 0.2925, so I need a 0.50 overbore setup?? Also, the piston appears tapered.  Is the
cylinder wall supposed to be the same diameter top to bottom?  The only place its 80mm is at the sleeve at the very bottom.
service manual states 0.045MM max.  relatively speaking thats about 0.0017" or about the same as a stock piston in
a stock LS1 Chevy.  

Note, piston imprinted, a 74 w/arrow exhaust side and a 96 on port side.  Is that a 79.74mm piston??  what do the 74 and 96 numbers mean?
must be the OEM piston.

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74 DT 360 ... resto in progress.
Last edit: 06 Apr 2021 23:23 by pabdt.
06 Apr 2021 21:55 #1

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Replied by RT325 on topic Q on Piston/Wall clearance

I'll probably confuse things & confuse myself. But my take on it is Std is 80mm. They go up in 0.25 oversizes. If you go to 0.50 i guess you'll be plenty safe at 20 thou as would be pretty worn out to 'not' clean up at that. I guess if you measure it at points all the way up & in particular just above the exhaust port. So question 'is' do you need a maximum of 5 thou wear at that point to 'be ok' to clean up at 10 thou--or can you be safely closer to 10. isuppose it depends how much wear there is taken from a center line as when its bored it'll be taking 5 thou each way from a center line. So if you've got 5 thou wear & it's all on the exhaust side then ya in trouble at first over 0.25--10 thou. Told ya i'd confuse things lol. Now i'll edit & fix mistakes.
07 Apr 2021 04:35 #2

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Replied by RT325 on topic Q on Piston/Wall clearance

Std bore size is 80mm for a 360 by the way, & clearance is on the piston, so if it has 79.96 on top i "think"? that's 0.4mm under 80 which is too tight by a whisker so i think the pistons are matched to the bore from the factory so the cylinder might be 0.005mm over 80 or 'two tenths of a thou' from new on a particular bike--not sure how to write 2-10ths in numbers & got too used to metrics but still convert it in my mind to view haha. Help me someone if i'm tallking rubbish!!. Anyway i was running an 82mm piston in one of my RT2's from a "79 YZ400F.2X5 kit.
Last edit: 07 Apr 2021 04:56 by RT325.
07 Apr 2021 04:50 #3

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Replied by RT325 on topic Q on Piston/Wall clearance

I get confused looking at numbers when i'm tired but i guess bottom line is all your cylinder measurements are a fraction under 80mm so just order a Std kit & carry on into the sunset.--i thnk.
07 Apr 2021 05:02 #4

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Replied by MarkT on topic Q on Piston/Wall clearance

Please read the manual. 

Cylinder needs to be checked for taper and ovality as well as size. Specs for maximum taper and ovality are in the manual.

Piston is only measured in ONE place, a little way up the skirt.  That measurement is compared to bore size to determine clearance. 

Pictures help but 96 generally means a standard piston that would have measure 79.96mm measured at the specific place on the skirt shown in the manual.  Original bore would have been exactly 80mm or very, very slightly over (80.005mm).

So that kind of puts your measurements in question...  bore should wear larger, not shrink.  But relative values may be more important than absolute sizes depending on what tools you have...  still something to look at because it's very easy to measure the bore and have the sizes come back "small".  In my experience a good dial bore gauge and micrometer are critical to have for this.

Guessing you're working on a DT360??  If you mentioned it I missed it.  The min/max given in the table is the "fresh bore, new piston" clearance. (0.04mm to 0.045mm or 0.0016" to 0.0017")

Maximum wear allowed is 0.1mm or 0.004".  This spec and the correct measurement technique can be found in the text of the manual,
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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Last edit: 07 Apr 2021 17:50 by MarkT.
07 Apr 2021 06:34 #5

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Replied by pabdt on topic Q on Piston/Wall clearance

Just realized piston total clearance is divided by 2 so those numbers above would be half indicated and split on both sides of piston equally .  

...and  btw thanks rt325 for trying make sense of this...

looks like I better remeasure...
 
74 DT 360 ... resto in progress.
Last edit: 07 Apr 2021 06:56 by pabdt.
07 Apr 2021 06:44 #6

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Replied by MarkT on topic Q on Piston/Wall clearance

pabdt wrote: Just realized piston total clearance is divided by 2 so those numbers above would be half indicated and split on both sides of piston equally .  

...and  btw thanks rt325 for trying make sense of this...

looks like I better remeasure...

 

Piston clearance is simply the bore minus piston size.  No division by 2.
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
07 Apr 2021 07:20 #7

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Replied by pabdt on topic Q on Piston/Wall clearance

Don't forget you need clearance on BOTH sides.   For what you say to be true, that would be 0.045/2 and I have never seen a piston that tight.
74 DT 360 ... resto in progress.
Last edit: 07 Apr 2021 12:01 by pabdt.
07 Apr 2021 12:00 #8

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Replied by RT325 on topic Q on Piston/Wall clearance

Me making sense, that'd be a new one lol, but thanks.
Clearance is clearance & only have just under 2 thou in a big bore if you slid a feeler gauge in with it. If i gave the impression it's two each way then no sorry. But did say something like if they're boring it they would go 5 each way--meaning 5 all around--to get 10 or first over in yamaha speak. So if you're boring it you need to be sure of how & where the wear is. Many a time the machine shop has called work 'yamaha shop' to say it won't quite clean up so need the next piston, so we obviously should have got it measured first before sending it off to be bored. Suzuki go in 20 thou steps so unlikely to run into that problem. Piston tapers a lot from about 18mm up & also is cam ground across the gudgeon area so measures less across there compared to front & back. Sorry, heading off on a tangent. Better stop while i'm ahead--or not.
I would say though--if a cylinder was 80 std the discrepancy would be something like "0.005mm" over 80 which is about 2-10ths of one thou so a gnats whisker & not two thou over 80 for a new cylinder out of the box. Think i've seen etched marks on the liner not sure if on my bikes but probably says what the finished size was from the factory.
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Last edit: 07 Apr 2021 17:29 by RT325.
07 Apr 2021 17:18 #9

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Replied by MarkT on topic Q on Piston/Wall clearance

pabdt wrote: Don't forget you need clearance on BOTH sides.   For what you say to be true, that would be 0.045/2 and I have never seen a piston that tight.

 

Bore ID minus Piston OD equals clearance and yes it is that tight.  I posted a fairly detailed post a few back that you might have missed but the best advice I can't repeat too many times is to look in the text of the manual where they explain the tolerance and measurement process in detail.
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
07 Apr 2021 17:48 #10

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