Anyone ever rebored/overbored a 5/6th gen ?

Discussion in '5th Generation 1998-2001' started by Mohawk, Jun 14, 2013.

  1. Mohawk

    Mohawk New Member

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    As per the title, has anyone done this, just want to know, if it's possible & what's required to get a suitable finish for the piston !

    Thanks
     
  2. mastergregor

    mastergregor New Member

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    I have not overbored the cylinders, but what I have done is hone them when I was rebuilding the engine.
    If I am not mistaken, sleeves are made from sintered aluminum, and as long as you do not overheat them while machining, they can be machined/bored/honed as regular steel sleeves. This is also stated in the service manual.
    Honing the cylinders was no different than any other engine. I used 73mm flex hone, the kind with lots of balls :) Since I did not have any other damage or worn out parts, honing/de-glazing was enough to seat the rings and get some better oil retention on the walls.
    I know this is not an answer to your question, but hope it helps somewhat!
     
  3. CandyRedRC46

    CandyRedRC46 Member

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    mohawk did i reply to this already on vfrd or something? it might not have sent, but i was saying that you need to talk to millennium technologies and superbike mike.
     
  4. Mohawk

    Mohawk New Member

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    Thanks Guys, I've got the same or very similar info from other sources, the concensus is that the MMC type liner can be rebored as per normal steel/iron liners. So if I get round to it this winter, I might have a big bore 825cc engine, using CBR929 74mm pistons. They are 246grm compared to 238 for the standard VFR. I will loose some weight from them to make them the same as the VFR ones so no rev limit penalty etc. This will give a 12.2/1 CR & restore the dynamic CR to 10.91/1 with my cams compared to the orignal of 10.86/1.

    The alternative is a 954 piston set at 75mm & 848cc Hmm a Honda 848 sounds nice :) , but might require some reinforcing of the cylinders, easy to do, just use a collar to replace the lost material !
     
  5. tinkerinWstuff

    tinkerinWstuff Administrator Staff Member

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    Looking forward to pictures
     
  6. Scubalong

    Scubalong Official Greeter?

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    Mohawk must be bored :rolleyes:
    Looking forward to your new project, I am always enjoys your mods :thumbsup:
     
  7. Durk

    Durk New Member

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    Go for 848. Super excited to see this.
     
  8. Badbilly

    Badbilly Official VFRWorld Troll Of The Year!

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    :guitar:
    Long stroke..
     
  9. mastergregor

    mastergregor New Member

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    This is an interesting project, and like others, I am curious to see how it turns out :)

    Thinking out loud, I am not sure you will have enough of a sleeve to go 75mm. Even 74mm will be close, as I think that first and second size over pistons are 0.5 and 1mm wider, so 72.5 and 73mm. I could be wrong though, and I am sure you will check that out. It will be interesting to see what you find and what you are able to do.

    Another thing to consider, crank has 3 main bearings instead of earlier generations 4, so you might get faster wear on the bearings, particularly on the power stroke side. Then you get crank walk and all sorts of other issues too :)

    Still, this will be a cool project to do! Keep us posted if you decide to do it.
     
  10. steven113

    steven113 New Member

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    I have a 98 block with one destroyed cylinder that I would send to you for the cost of shipping if you want to let your machine shop use it ad a guinny pig before they try it on an undamaged block. Let me know if you want it... I am not on here that much anymore so just call me at (850)698-5689 if I don't reply here.
     
  11. squirrelman

    squirrelman Member

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    Wouldn't it be best to re-balance the crank with lighter pistons. And, with all that extra work, would the increased 3-5 hp be worth it, really, in practical $$$ terms ??
     
  12. Badbilly

    Badbilly Official VFRWorld Troll Of The Year!

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    I always wanted to stuff an Allison or a RR Merlin into a Vanagon. Had an ol boy in Texas tell me onct that if the damn motor ain't bigger than the damn car, we can git er in there.
     
  13. Mohawk

    Mohawk New Member

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    I have a spare engine to work with already so thanks for the offer. All this work is only an option because of the spare engine. I bought it to get a spare set of cams its from a crashed bike so has some case damage etc, but low miles.

    I've just changed job, so hope to be home more, so should be able to work on this project starting in December :) Once I've torn the engine down, I'll see if its possible.
    As far as balancing goes, I will be using the 929 74mm pistons that are heavier, so I'm going to modify them to be the same weight as the VFR ones.
     
  14. Scott_Lilliott

    Scott_Lilliott New Member

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    I contemplated this mod a year or so ago, got a used set of cases and brought them to my local speed shop (Harry's Machined Parts, Northboro, MA) and they said it would take the 3 mm overbore - but no more. As Mohawk stated, the 954 pistons would be ideal - IF - 1. the valve pockets are the same location/dimension as the 800 (I'm betting they aren't), and 2. the wrist pin diameter must be the same. There's also the issue of the deck height - wrist pin center to piston crown - I have yet to read or hear definitive evidence of what the valve-to-piston clearance is on the 5G. I even priced out a set of JE Pistons 954 kit ...

    So - with that in mind, how far along are you ?
    ;-)
     
  15. Mohawk

    Mohawk New Member

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    Just at the about to strike metal phase. Having trouble finding soewhere that can handle reboring a V4 bike engine case. The V8 car places use the main bearing pockets as a datum & mount, but there minimum bore for the bearings is 45mm & the VFR's mains are 37mm ! None of the bike places I've contacted so far have a mount for a V4 case :( I have a lead on a place that might do it, but need to talk to them today.

    This will take a few months to complete ! my aim is to have a running engine by end of March ready for testing in our new biking season.

    The biggest problem with the rebore is the MMC sleeves shape, it has locating rings moulded in that lock into the alloy case/sleeve, so if you bore more than 74mm then you will removed all the MMC, but have a coupls of 2-3mm deep rings left in the alloy that remains, these can't just be nikasil'd over, so have to be bored out, thus taking the bore to 77mm in an 88mm alloy sleeve support, leaving very little meat so a steel sleeve would be required.

    Honda standard oversizes for the VFR are .25 & .5mm so maximum 73mm bore. TTS in the UK who used to offer VFR big bores tell me that 74mm will fit in the standard MMC liner, 75mm would not leave enough liner no to crack at the location ring locations. I may buy a spare crankcase to disect to get actual numbers. But I'll attempt a 74mm rebore regardless as its a spare engine !

    More news when I have some. I just started a new job so I'm home lots more, so should get this kicked off with the boring t beginnig of Jan, as I still have to strip the spare engine.
     
  16. Mohawk

    Mohawk New Member

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    I just had a word with TTS re the rebore & the info I was given that claimed to come from TTS appears to have been flawed :( TTS use an american firm to do the reboring & nikasil plating on the VFR800 big bores they have built. They rebore the VFR750's steel liners themselves & I may send them a sacrificial crankcase top to test Hondas claims that they cane be rebored as per steel if NOT over heated during boring.

    I think a sacrificial upper crankcase is required to section & calculate the MMC thickness. Does anyone have a spare broken engine case ? Located in the UK, or if abroad, one that can either be shipped or sectioned locally ?

    Thanks
     
  17. Scott_Lilliott

    Scott_Lilliott New Member

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    There are several on ebay at pretty reasonable prices but located in US. I was looking at the ebay UK site and didn't see any locally.

    When you start into the spare motor, could you take an extra few minutes and measure the piston-to-valve clearances at TDC ? In. and Ex ?
    I would love to know this (and so would you!) when planning the piston deck and crown height. TIA!
     
  18. danny_tb

    danny_tb New Member

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    In one of my old car magazines there's an old something-or-other that has had a RR Merlin engine put into it. It was the most powerful road registered car in Australia, with over 2000hp... The car was mainly engine, and not really much else.
     
  19. Mohawk

    Mohawk New Member

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    Scott, I can almost tell you what you want to know. When I pulled the heads I did a test with NO head gasket, the piston just touches the head, but will pass TDC, the valves never touched my test tape. I'll look for a scrap engine, hard to find I know :(
     
  20. Scott_Lilliott

    Scott_Lilliott New Member

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    Thanks Mohawk, I am regurgitating what my speed shop motor builder talked with me about when he built my roadrace engine:
    'There are two ways to measure the p-to-v clearance. Open the motor and clay the pistons (put modeling clay on pistons),
    reassemble with gasket and torque down. Turn the motor over at least 2 or 4 revolutions, then take it apart. Measure the depth of the
    compressed clay to the piston dome. That's what we do. The other way is to leave the motor assembled, valve cover off,
    turn the motor to TDC then use a dial indicator to measure the distance the valve can be 'pushed' until it contacts the piston.
    That's kind of tricky since the dial mount can move a bit and the spring force is pretty high. Race motor clearances are usually
    in the 2mm to 2.5 mm range.'

    I offer this up because you'll want to know what clearances you have to start with before putting in a different piston(s), with different
    deck and crown heights. Also, most head gaskets compress to a nominal 1mm of thickness. The bottom line is, as you know, with
    gear driven cams, there's no head surface planing because of the gear fitment clearances, so every other clearance matters!

    I'm pretty excited by your undertaking this project (as you can see, I've done some homework on it ...)! Thanks for sharing!
     
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