Author Topic: RD400C slight engine seizure  (Read 1264 times)

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Offline vectisitch

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Re: RD400C slight engine seizure
« Reply #45 on: June 19, 2021, 01:18:28 PM »
Crazy !

https://www.cruzinimage.net/2017/08/10/yamaha-rd400-pistons-set-oversize-1-0mm/

Usually on ebay too but I just buy direct. Forsetti are on ebay:

https://www.ebay.co.uk/itm/131162467200?hash=item1e89e46b80:g:dF8AAOSwvlVgSwfd

Those cruzininage pistons  are crazy cheap. Is that really for both?

Online c50

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Re: RD400C slight engine seizure
« Reply #46 on: June 19, 2021, 01:19:17 PM »
Forsetti without the gaskets etc

https://www.ebay.co.uk/itm/152277935323?epid=7004348428&hash=item237478e4db:g:hEIAAOSwdg1gNLz0

Don't forget there is a 50/50 chance the piston will be slightly smaller than your existing one making it useless. But price of especially the Cruze version isn't a big loss plus you can sell them on ebay.

Both "look" equally well made, far better than a Mitaka 4L0 set I have, they look the the windows were cut out by a 2 year old with bow saw. We have only used the Cruze pistons on a few bikes now to air cooled clearances, all with trouble free results. Waiting to use my Forsetti's.

Online c50

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Re: RD400C slight engine seizure
« Reply #47 on: June 19, 2021, 01:20:33 PM »
Crazy !

https://www.cruzinimage.net/2017/08/10/yamaha-rd400-pistons-set-oversize-1-0mm/

Usually on ebay too but I just buy direct. Forsetti are on ebay:

https://www.ebay.co.uk/itm/131162467200?hash=item1e89e46b80:g:dF8AAOSwvlVgSwfd

Those cruzininage pistons  are crazy cheap. Is that really for both?

Yes!

That's what happens when you cut out all of the middle men. vast majority of the cost of a piston is middle men. He will still have at least 30% on them.

Online c50

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Re: RD400C slight engine seizure
« Reply #48 on: June 19, 2021, 01:23:12 PM »
And that is why rebore shops don't want you to use them, they want you to use whatever they have or can get to maximise their profit regardless of their suitability. ARD excepted, he will gladly rebore with pistons you supply.

Offline Richard Ramsay

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Re: RD400C slight engine seizure
« Reply #49 on: June 20, 2021, 08:20:01 PM »
Hi
Well it seems I was incorrect in blaming the apparent lack of chamfers.
This info from someone way smarter than me.

"The Exhaust needs a proper chamfer on the timing edge and the port bottom simply to gently push the bulging ring back into the groove.

Transfers do not need any " chamfer " at all - just the sharp edge removed with a cotton mop to prevent shear of the oil film off the skirt wall.

Chamfers on the port edges creates turbulent vortices on the outer edges of the column of gas flow exiting the duct "


So ignore me and continue on your quest.  :D
Thanks
Richard
" I fart in your general direction! Your mother was a hamster and your father smelt of elderberries! "

Offline vectisitch

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Re: RD400C slight engine seizure
« Reply #50 on: June 20, 2021, 09:41:51 PM »
Hi
Well it seems I was incorrect in blaming the apparent lack of chamfers.
This info from someone way smarter than me.

"The Exhaust needs a proper chamfer on the timing edge and the port bottom simply to gently push the bulging ring back into the groove.

Transfers do not need any " chamfer " at all - just the sharp edge removed with a cotton mop to prevent shear of the oil film off the skirt wall.

Chamfers on the port edges creates turbulent vortices on the outer edges of the column of gas flow exiting the duct "


So ignore me and continue on your quest.  :D
Thanks
Richard
Yes thats how my ports are done.

Offline ajh

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Re: RD400C slight engine seizure
« Reply #51 on: June 20, 2021, 09:47:32 PM »


Transfers do not need any " chamfer " at all - just the sharp edge removed with a cotton mop to prevent shear of the oil film off the skirt wall.

Chamfers on the port edges creates turbulent vortices on the outer edges of the column of gas flow exiting the duct "[/b]

Amazes me how they can get to this level of detail!
thanks for the info
in Chiswick, West London

77 RD250d, 80 DT175mx, 81 4L0 350LC

Offline Numbskull

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Re: RD400C slight engine seizure
« Reply #52 on: June 21, 2021, 01:51:09 AM »
I will continue to chamfer my transfer ports.

Rob
The lyf so short, the craft so longe to lerne.

Online 5port

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Re: RD400C slight engine seizure
« Reply #53 on: June 21, 2021, 07:34:36 AM »
Hi
Well it seems I was incorrect in blaming the apparent lack of chamfers.
This info from someone way smarter than me.

"The Exhaust needs a proper chamfer on the timing edge and the port bottom simply to gently push the bulging ring back into the groove.

Transfers do not need any " chamfer " at all - just the sharp edge removed with a cotton mop to prevent shear of the oil film off the skirt wall.

Chamfers on the port edges creates turbulent vortices on the outer edges of the column of gas flow exiting the duct "


So ignore me and continue on your quest.  :D
Thanks
Richard

I don't know where you got your port chamfering "advice" from Richard but, it is not correct for a standard RD.  Maybe it is for some other engine?
The RD250/350/400 need various chamfers on the ports.  All fresh bored cylinder port edges need a fine de-burr to ensure that there is no raised sharp edge or burr. The horizontal port edges need real chamfers on them to guide the piston rings back into the bore shape as they bulge into the ports in passing.  The wider the port, the more important the chamfer, the top exhaust port edge being the most important, it is wide and the piston speed is very high as the rings pass it. The bottom of the Inlet port does not have rings passing in use but, the piston lower edge has to be guided there and needs a good chamfer.
NOS Yamaha cylinders have good chamfers. But, I have seen used cylinders with ridiculous excessive chamfers that changed the porting.  However, none of the rebore cylinders I have ever seen have had satisfactory chamfers, except those from Mick Abbey that have very nice chamfers!

Cheers
 5port
5port

Offline vectisitch

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Re: RD400C slight engine seizure
« Reply #54 on: June 21, 2021, 02:32:43 PM »
Just back from Taymars and he thinks the same as me. Just not enough miles done and the 17 year old me being a hooligan didn't help. Just a hone and new piston is all i need. I spoke to him about the mitaka pistons and not wanting to open up the great piston debate again he says the mitaka pistons are ok but the ones with TKR printed inside are the better ones. Which mine are. He sets the clearance at between 1.5 and 2 thou usally when he rebores the aircooled engines. He told me to check the air nozzle wasn't blocked on the carb which i just did , it is clear.I'll check the oil pump setting too just to make sure but it was imo just me being over zealous on the throttle on a very hot day with sustained speeds for too long

Offline Numbskull

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Re: RD400C slight engine seizure
« Reply #55 on: June 21, 2021, 02:55:36 PM »
Check the air nozzle hose fitting
at the airbox. I had one clog up
with dead bugs and dirt once.

Rob
The lyf so short, the craft so longe to lerne.

Online c50

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Re: RD400C slight engine seizure
« Reply #56 on: June 21, 2021, 05:12:58 PM »
Just back from Taymars and he thinks the same as me. Just not enough miles done and the 17 year old me being a hooligan didn't help. Just a hone and new piston is all i need. I spoke to him about the mitaka pistons and not wanting to open up the great piston debate again he says the mitaka pistons are ok but the ones with TKR printed inside are the better ones. Which mine are. He sets the clearance at between 1.5 and 2 thou usally when he rebores the aircooled engines. He told me to check the air nozzle wasn't blocked on the carb which i just did , it is clear.I'll check the oil pump setting too just to make sure but it was imo just me being over zealous on the throttle on a very hot day with sustained speeds for too long

Bit of finger in the air and hope there! It had done more than enough miles for a rebore, more than enough ! Too tight a clearance with the wrong piston is almost certainly why it seized. 1.5 thou with LC pistons is too tight for them when used in an aircooled engine. The hone will add some clearance and probably some rattle and hopefully just enough clearance to prevent another seize. Is he going to loosen the other side up too? If not keep your hand over that clutch!

Online c50

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Re: RD400C slight engine seizure
« Reply #57 on: June 21, 2021, 05:17:08 PM »
A blocked air intake will make it rich making it far less likely to seize  :-\

Offline Numbskull

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Re: RD400C slight engine seizure
« Reply #58 on: June 21, 2021, 09:43:18 PM »
A blocked air intake will make it rich making it far less likely to seize  :-\

I'm talking about the main jet nozzle hose going up to the front of the airbox. The fitting can get
clogged in the elbow choking off the air to the main jet nozzle. Easy enough to check.

Rob
The lyf so short, the craft so longe to lerne.

Offline Alain2

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Re: RD400C slight engine seizure
« Reply #59 on: June 22, 2021, 12:02:00 PM »

Bit of finger in the air and hope there! It had done more than enough miles for a rebore, more than enough ! Too tight a clearance with the wrong piston is almost certainly why it seized. 1.5 thou with LC pistons is too tight for them when used in an aircooled engine. The hone will add some clearance and probably some rattle and hopefully just enough clearance to prevent another seize. Is he going to loosen the other side up too? If not keep your hand over that clutch!

Why would a LC piston need more clearance? Is the alloy composition different?

I said it before and will say it again, 1.5 thou is too tight to pass the highway test. Do not blame the LC pistons.
1973 RD350
1977 RD400
1979 RD400
1980 RD400
1985 MJ50