Big Overbore Limitations 4

[jkdmag]

Would be interested in the opinions of the experts on the limit to size of an overbore when the combustion chamber remains the same size. How large can the overlap be at the head/bore junction be before the increased risk of detonation becomes critical. The size of the overlap I am contemplating is 3-4 mm.

 

[dgallup]

Why don't you just pick up a used VFR500?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[jkdmag]

Do you mean a VF500? A much different and technically inferior product. If you are a resident of the USA, you would most likely be unfamiliar with this Honda model as it was not an official USA import. There are some in the USA but they would have been one-off personal import, same as here in Oz. Bike was only officially imported into Europe.

 

[patprimmer]

Where are you in Aus.

I know Jack Nalbandian at KC's Dynamic Engine Balancing and Head Servicing in Castle Hill (Sydney) has fitted Darton Sleeves to Honda car engines recently.

Regards
Pat
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[jkdmag]

Hazelbrook in Blue Mountains NSW.

 

[patprimmer]

Jack is at Unit 17, 7 Carrington Rd Castle Hill 2154

phone 02-98946644

Tell him the crazy old guy with the White Honda Integra said to call.

Regards
Pat
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[jkdmag]

Thanks Pat.

 

[jkdmag]

One thing I forgot ask. Ultimately I have to be able to seal the head to the bore and at some point the sealing area between the bore and water flow holes into the head will become too small. How do I know when I have reached "too small". What minimum distance should be maintained between bore edge and the water feed holes in the head.

 

[patprimmer]

It is not hard to open uo the water holes in the head, but it is pointless opening them past the holes in the gasket. When modifying water holes, give due consideration to where the water tends to flow on a stock and how that and any mods influence uniformity of temperature. Note, the water pump is on one end and one side of the block and the top radiator hose attachment is at the extreme opposite.

Regards
Pat
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[jkdmag]

Pat, had not contemplated modifying or moving the water holes. Was going to use a head gasket with same hole size/pattern just larger bore size. Issue will be that larger bore will reduce sealing area between bore and water holes. How small can the area be before I run the risk of gasket failure.

 

[patprimmer]

about the same or a bgit less than between the bores.

Regards
Pat
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[jkdmag]

Back again with more questions. Having digested the views expressed in this thread, I have had a rethink on this subject and was wondering if a better solution would be to bore out the OEM cast in liners and then nikasil coat the remaining aluminium bore, as the new running surface for the piston.

This configuration would give a wall thickness of some 4.5mm, although it would be all aluminium. The OEM wall thickness, including the cast in liner, is approximately 7mm. Would 4.5mm wall thickness be stable enough without the cast in liner?

The nikasil coated bore appears to be a lower cost/simpler and perhaps sounder option than using wet sleeves to replace the OEM cast aluminium/liner cylinders.

The piston ring material of the intended bigbore pistons would normally run on cast iron sleeve, would they be compatible with a nikasil coated bore?

 

[patprimmer]

I think 4.5mm cast aluminium is to thin.

I think ductile cast iron rings run fine on Nikasil, but don't quote me on that as I never actually did it or fully investigated.

Regards
Pat
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[BrianGar]

I would steer away from coating for a few reasons.
One reason down the list is that it is not easy re hone, and will need to be sent out again if needs be.
Your piston skirts may need an aftermarket treatment/coating to work with Nikasil too, so look into that if you go ahead...
I too think 4.5 is too thin for a bore wall, fine for a liner support/wet or dry, but a little shy for coating.

D.Cast iron rings are ok for coated bores. There are others, but cast iron is a good choice.

Brian,

 

[jkdmag]

Reflecting on everyone's comments regarding re-sleeving the original cylinders or plating, I have decided that a wet sleeve with a top flange supported by the existing outer water jacket is my goal. Basically an open deck to closed deck conversion.

Although ductile iron is apparently preferred for wet sleeves because of its shock load capacity, is grey cast iron acceptable provides the wall thickness is sufficient, perhaps a much as 10mm. I may be able to source grey cast iron sleeves at a good price.

Piston choice raises another issue, for which I would be interested in your opinions. My preference is to use a piston from another model of motorcycle that is the same style as those fitted now, i.e. 4 valve pent top. Again for cost reasons, I am trying to avoid custom made pistons.

However, the piston of choice uses a 15mm piston pin and the OEM rods are 14mm. Is resizing to 15mm considered a safe option. Max rpm for the OEM engine is 14,000 but in real life I would not exceed 12,000 worst case and 10,000 would be the nominal maximum. This model of bike, when raced, would normally only be run between 10,000 and 14,000 rpm.

 

[patprimmer]

There are several truisms in the performance modification industry.

1) Speed costs money. How fast do you want to go.

Cheap, fast, reliable. You can only pick two.

Sleeves are generally a centrifugal cast iron for grain density and strength. I am not aware of the exact composition, but I would guess that it is a composition to give specific surfafe characteristics when honed to give a good wearing low friction surface. It also needs to hold its shape under cylinder pressure.

Regards
Pat
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[pontiacjack]

I'm puzzled. The original question concerned "large overbore" but now you've decide to use cast iron wet sleeves "perhaps 10mm thick"? From your comments, you apparently won't be racing; why is a "large overbore" demanded? Curious minds want to know...

 

[patprimmer]

Jack

Thick cast iron wet sleeves to replace the thin cast iron dry sleeves in an aluminium cylinder is pretty common practice in Hondas.

The individual thick cast iron sleeves normally have flats between the cylinders so they go together like a Siamese bore.

Hondas are also normally open deck, so the tops of the cylinders just stick up like the cylinders on an old VW but without the cooling fins, then seemingly unattached is the water jacket. The MIDi sleeves have a deck built in that locates the top of the sleeve in the water jacket inside surface.

http://www.darton-international.com/midinfo.html

Personally I think the Darton sleeves require the original bore to be excessively removed leaving a weak seat where the sleeves bottom out. I prefer a sleeve that still leaves an aluminium stump up to just below the area of the sleeve that sees cylinder pressure and major side thrust. This leaves a more stable base to help prevent sleeves sinking and water leaks into the crankcase developing.

Regards
Pat
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[jkdmag]

Overbore to improve mid-range power as a road bike and an engineering exercise.

There is an big bore piston package that makes 444cc with a re-bore but there are questions as to the reliability of boring the original liners this large. At best would leave perhaps 1.5mm of the original cast-in liner, that may not be centered on original bore or worse.

Might as well go bigger and get closer to 500cc, however wiser heads on the site have doubts that re-bore and re-sleeve or boring and plating of the original cylinder casting would be reliable with such a large bore size.

Hence option of wet sleeve conversion with closed deck for structural support.