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Big Overbore Limitations
3

Big Overbore Limitations

Big Overbore Limitations

(OP)
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.

RE: Big Overbore Limitations

The problem is the head gasket not the head. Just make sure the head has no sharp edges inside the chamber. Scrape the edge and polish if necessary. Of course hole for the bore in the gasket must be big enough so it does not overhang the bore at all or it will glow red hot and cause pre-ignition..

Regards
Pat
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RE: Big Overbore Limitations

(OP)
Thanks patprimmer, head gasket was always going to a size to suit bore but was not sure that large bore/head overlap was a good idea.

RE: Big Overbore Limitations

(OP)
Another question on related subject, what is the rule for the minimum wall thickness of a cast aluminum jacket to accept a press fit steel sleeve.   It is for a normal road use water cooler motorcycle engine.

RE: Big Overbore Limitations

Despite my previous comment, often a significant air flow increase can be had by removing metal from the chamber wall where it is close to the valve seat so as to un-shroud the valve.

I am guessing, but I would think about 6mm. The restriction is often the area for head gasket seal between the bores.

I think the sleeves are probably cast iron, not steel

Regards
Pat
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RE: Big Overbore Limitations

From the description wouldn't the overlap become squish/quench area ? (if piston to head clearance is tight enough)   

RE: Big Overbore Limitations

(OP)
patprimmer is the 6mm figure related to the limit of bore to head overlap or the minimum wall thickness of jacket surrounding a pressed in sleeve.

RE: Big Overbore Limitations

I stress it is a guess, but it's related to bore thickness and gasket width between adjacent cylinders.

Some cylinder heads have a flat surface with the valves mounted in it at the deck face and the chamber is a bowel in the piston. Models that spring to mind are late 80s ish Ford 105E, V12 Jaguar, and VW Golf GTI.

Regards
Pat
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RE: Big Overbore Limitations

re: "minimum wall thickness of a cast aluminum jacket to accept a press fit steel sleeve?"
Much more information needed. As mentioned, readily-available sleeves are cast iron. Two types are generally available- wet or dry. Wet sleeves have sufficient thickness/strength to be used with zero-thickness block material surrounding them (need only to be supported at top and bottom). Dry sleeves are thinner and require considerable support by the surrounding walls of the block. In either case, the end-result walls must be "up to the task"- so more info about the "task" is required.

For whatever it's worth: my blown alky V8 (~2.23 HP/c.i.) DOES employ steel sleeves custom machined from DOM tubing (I don't recall the alloy, but strength is up in the 70Ksi range). Long-story-short: I needed more strength than cast iron could provide within my dimensional restrictions.

RE: Big Overbore Limitations

It is certainly not that uncommon on high performance modified Honda engines to bore away the aluminium bores completely and install a full cast iron wet sleeve replacement to increase bore strength.

How thick the iron must be depends on the cylinder pressures. A low compression engine can be thinner than a high compression engine. NA engine can be quite a bit thinner than a supercharged engine.

Also the top 35% of the ring travel needs to be a lot thicker than the area below the rings.  

Regards
Pat
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RE: Big Overbore Limitations

(OP)
Apologies for poorly worded questions.

Have given some thought to removing the OEM cylinder jackets and fitting wet sleeves but considered support for the sleeves in open deck block would be a problem, not to mention cost.

Was contemplating using over-sized flanged dry sleeves, flanged sleeves would offer more head gasket support at the head/bore junction.

As mentioned earlier, it is a road use motorcycle, currently 400cc, 60hp and 14,000 rpm, simply want to increase the capacity for more torque.

Can buy oversize piston kit but as the re-bore leaves maybe 2mm wall thickness in OEM sleeve thought I might be better off to replace the sleeve and get max capacity at the same time.

Was going to work backwards from cylinder jacket diameter to find largest sleeve that would be suitable while maintaining the minimum jacket wall thickness for reliability.

Hope that better explains the situation.

RE: Big Overbore Limitations

Look up Darton sleeves. At least for Honda cars which are mostly a dry sleeved aluminium open deck block, Darton make what they call a Modular Intergrated Deck Sleeve.

Basically the sleeve has a flange that locates in a machine finished surface on the inside of the water jacket wall so the sleeve contains the closed deck feature.

Regards
Pat
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RE: Big Overbore Limitations

2
Jkdmag,

Pat is correct about the dartons. I would certainly go this route rather than machining away to a slim wall to fit a dry sleeve.

Dartons are centrifugally cast ductile iron and they certainly know what they are at when it comes to making them.

They have a VERY large selection, but if you cant find one to suit, you can design your own on the site.

Be warned, if you are machining out the deck and all, get_a_good_machinist, otherwise your beautiful sleeves will be wasted.

I have no idea how may cylinders you've got, but if its just one, you could easily get a sleeve with a wide top flange and bore this flange for your coolant routes. Your original routes will have got machined away with the large rebate required for the top lip of sleeve.
Or, if you have banks, you can machine in a pocket to take a frame with coolant holes that darton also do that locates down over the top of the wet sleeves to hold them all in place. This locates both around the sleeves, and into an accurately cut pocket in your original deck.

As for dry sleeving, and the min you need for support, this is not as easy done as you think due to the water jacket sand core shifting at time of pouring.
Ive cut up a lot of engines to study casting details, and every one of them has shown some signs of core shift.
Sometimes the core will shift to your favour, ie, forward, leaving a ticker portion of bore where the most trust force support is needed during ignition. But, with other blocks, the core can shift to the back, or sides leaving you with thinner portions where you least want them.

Alloy blocks do seem to display less water jacket shift. Im guessing because of more modern foundry tech, and the alloy having less inertia than cast iron, but, it is still present and different in all blocks - even ones with the same p/ns.

Since as Pat mentioned, its at the top you need the most support boring out the existing to fit a larger dry sleeve may not seem a problem. But, by boring, and if the cores have shifted, you may run into very thin wall sections as you go down.

In cast Iron Id like to see 3mm of metal min.
In alloy, 4.5mm.

Obviously, you have no way of telling how much you have left, but chopping up a block the same as it will give you an approx Idea.

Be warned, that you may hit chaplets in the adjacent bore walls also when you go thin. These chaplets are used to stabilise the sand cores before the metal is poured. Exposing them can lead to very unsightly areas that bad things can originate from. Be it cracks, or water/coolant ingress to gasket interface.

Another time, I cut up a honda block that had been dry sleeved. The wall was very thin in one spot and there also had been some casting porosity there. The porosity would normally have been fine, and watertight, but since the tight metal had been machined away behind it, water had crept in, and entered the space between dry sleeve, and original bore. The water found its way to the sump, and the engine was binned. How it did creep between them I dont know, perhaps the sleeve moved a little with the extra thin support.
Obviously, both the casting guy, and later, the guy that bored the engine to fit the sleeves were not at fault. But, the machining guy could have been more careful, a simple check by filling the water jacket with penetrating oil or something thin would have showed up the porosity.

Im probably going a bit too far with this, but I feel its all worth a mention for the above reasons.

If I was you, Id wet sleeve it. They are not too bad money-wise either, 90euro will get you any one you want. I never saw the point in getting it all jigged up to fit a thin dry sleeve, when, with a small bit more work, you could hog it all out and fit a proper sleeve that the devil could not destroy.

See attached Image for both bore wall thickness deviation and chaplet position between bores. Granted, its cast Iron, but Ive seen the same with Al blocks too. Sometimes the only way to learn is to cut stuff up and have a proper look.

Brian,

ps, Im in no way connected to Darton, but they make amazing sleeves.




 

RE: Big Overbore Limitations

Another point when machining a cast iron dry sleeve in an aluminium block is that as the milling cutter crosses from iron to aluminium it can easily dig in or crack the iron. It is best to accurately bore the iron out completely, then take the aluminium.

Loctite make a wick in product designed to seal porous castings. It might be a good safeguard to use it at potential water leak areas, just in case.

My experience is with plastics, not metals, but in plastics, porosity only occurs in the centre of thicker sections. It is due to material not being able to flow in to make up for shrinkage due to the material solidifying, so cutting deep into thick sections is where I would most expect to find porosity.

Regards
Pat
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RE: Big Overbore Limitations

Exactly where I saw it Pat. Right where a stabilizer rib joined the bore jacket, to block exterior.

Brian,

RE: Big Overbore Limitations

(OP)
Brian & Pat, thanks for your comments.  The engine configuration is V4 with effectively 4 individual cylinders.

For many of the reasons you have listed, I have hesitated to buy the oversize piston kit.   Apparently there is a question make over the centering of the OEM sleeves when re-boring for this kit, it is quite easy to break through the sleeve wall unless one takes great care in the setup.  This suggests to me that even if done well the sleeve wall thickness remaining may be suspect for long-term reliability.  

Will investigate the wet sleeve option.

Any further thoughts on the head/bore overlap.  If I was to go wet sleeve then bore diameter could end up significantly bigger than OEM head size.   Head is 4 valve, OEM pistons domed with valve cut-outs and would be same style in big-bore version.  

RE: Big Overbore Limitations

Just un-shroud the valves to the bore line and rejoice in the extra quench area in the normal quench pad areas.

Big bore kits that leave thin walls might be good for bench racing, but even if they don't actually break, the extra cylinder wall deflection under load kills ring seal and the increased blow by quite likely kills more power than the capacity increase gives for an oil burner with less power net result.

Good thick walls with good ring seal is how you make real power.

Regards
Pat
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RE: Big Overbore Limitations

Somewhat off-topic, but curiosity is killing me- the identity of your 4-valve 400 cc V4? Sixty HP- nearly three HP/c.i. N/A?

RE: Big Overbore Limitations

(OP)
Honda VFR400.       

RE: Big Overbore Limitations

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.
 

RE: Big Overbore Limitations

(OP)
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.

RE: Big Overbore Limitations

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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RE: Big Overbore Limitations

(OP)
Hazelbrook in Blue Mountains NSW.

RE: Big Overbore Limitations

(OP)
Thanks Pat.

RE: Big Overbore Limitations

(OP)
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.

RE: Big Overbore Limitations

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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RE: Big Overbore Limitations

(OP)
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.

RE: Big Overbore Limitations

(OP)
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?

RE: Big Overbore Limitations

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,

RE: Big Overbore Limitations

(OP)
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.

 

RE: Big Overbore Limitations

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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RE: Big Overbore Limitations

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...

RE: Big Overbore Limitations

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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RE: Big Overbore Limitations

(OP)
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.

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