Author Topic: IT400 over sizes?  (Read 418 times)

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Offline OvardIT400

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IT400 over sizes?
« on: October 06, 2018, 12:52:22 PM »
I've got a '76 that needs re-bored. ebay has a lot of NOS pistons and rings available, but the rings will say "1st over, 2nd over" etc., while the pistons just give a size in mm(.5 over...). can anyone help me with some info on the over sizes? The original owner I got the bike from had it re bored once.
Thank you for any info.

Offline Devondan

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Re: IT400 over sizes?
« Reply #1 on: October 06, 2018, 11:33:47 PM »
A few points to note, the PO might have bored it to more than one size over, but all is not lost, either measure the bore or clean the top of the piston and see what size ir is, it should be stamped on the top.

When it comes to reboring, you need to look at the condition of the bore, if it?s badly scored or rusted you might need to go up more than one size. You might also find that the piston or bore is within spec and all you need is rings?!

I think Wiseco and Wossner both do pistons ( not sure about for the 76) and you need to be careful because they have different clearances to Yamaha pistons.  My advice would be get it measured (if you can?t do it yourself) if it?s out of spec go to the next suitable  available size.
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Offline OvardIT400

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Re: IT400 over sizes?
« Reply #2 on: October 07, 2018, 01:26:56 AM »
I wish it was just rings. When I took the header pipe off I could wiggle the piston inside the cylinder.
 What I'm having trouble figuring out is, when looking for a new piston, and the description only says .5 over(I assume that is 1/2mm over), am I looking at a first over piston, or second or third, etc.?

What is the original piston diameter for the 76 400? Online, I do not see any that are stamped with anything other than an arrow. 

I am taking the cylinder and piston in to the machine shop, and want to give the machinist as much info as possible, as he hasn't worked on something this old for quite a while.

Online Martin I Henry

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Re: IT400 over sizes?
« Reply #3 on: October 07, 2018, 04:08:18 AM »

Welcome to the forum OvardIT400.

A standard size Yamaha supplied piston normally just has the arrow that points to the exhaust port.

.25 or 25 is first oversize.
.50 or 50 is second over.
.75, third over.
1.00 is 1.00 mm oversize.

The numbers relate to mm not thousands of an inch !

However, if the piston is US made, they can be marked in 1/1000 !



This link is to the introduction page: http://www.yamahait.com.au/forum/index.php/topic,1.1330.html Please join in and say hello ! We like photos, especially of IT400C's !

This page is a register of IT400C serial numbers. Please feel free to add yours: http://www.yamahait.com.au/forum/index.php/topic,7098.60.html


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Offline OvardIT400

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Re: IT400 over sizes?
« Reply #4 on: October 07, 2018, 08:45:58 AM »
Thank you so much! That is exactly what I was looking for.

Much appreciated.

Online Martin I Henry

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Re: IT400 over sizes?
« Reply #5 on: October 07, 2018, 07:57:18 PM »

And when you get time :

This link is to the introduction page: http://www.yamahait.com.au/forum/index.php/topic,1.1330.html Please join in and say hello ! We like photos, especially of IT400C's !

This page is a register of IT400C serial numbers. Please feel free to add yours: http://www.yamahait.com.au/forum/index.php/topic,7098.60.html

Cheers,

Dave R.

Offline oldbiker

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Re: IT400 over sizes?
« Reply #6 on: October 08, 2018, 04:49:26 PM »
i could only get Woosner piston set for the  400F and fitted fine. Just handed the cylinder and piston set to the machine shop and they did the rest.
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