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.

 

[patprimmer]

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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[jkdmag]

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.

 

[jkdmag]

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.

 

[patprimmer]

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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[Tmoose]

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

 

[patprimmer]

I would think so, but I am not sure a full 360 degree ring around the circumference of the chamber is a good quench pad shape

Regards
Pat
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[jkdmag]

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.

 

[patprimmer]

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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[pontiacjack]

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.

 

[patprimmer]

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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[jkdmag]

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.

 

[patprimmer]

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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[BrianGar]

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.

 

[patprimmer]

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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[BrianGar]

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

Brian,

 

[jkdmag]

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.

 

[patprimmer]

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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[pontiacjack]

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?

 

[jkdmag]

Honda VFR400.