Hoping that I don't ask a stupid question, but I hope some of you could help me.
I have nissan pistons I want to use in my engine. The pistons fit in the cilinders, that's not a problem. The issue is that the deckhight is different, only 32mm in stead op 39.6.
What is the most logical way of solving this? Bigger stroke or shortening the cilinders?
I was thinking 82mm stroke could come pretty close..
Or should I just skip the idea? Would be fun to try I think..
Glauco
Deck hight
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Re: Deck hight
Stroke it!
Shortening the cylinders opens up a huge can of worms. You will need shorter head studs. Then shorter pushrod tubes. Shorter pushrods is easy to do.
After you get the heads on, you have to pick your exhaust carefully. Can it be installed on such a narrow engine?
You'll have to trim the cylinder covers quite a bit. Then the fan shroud will be too wide. Hammer that a bit.
Hopefully you will plan to use dual carbs, because if you want to use a single carb, the manifold will be too wide.
There's more than this.....
Shortening the cylinders opens up a huge can of worms. You will need shorter head studs. Then shorter pushrod tubes. Shorter pushrods is easy to do.
After you get the heads on, you have to pick your exhaust carefully. Can it be installed on such a narrow engine?
You'll have to trim the cylinder covers quite a bit. Then the fan shroud will be too wide. Hammer that a bit.
Hopefully you will plan to use dual carbs, because if you want to use a single carb, the manifold will be too wide.
There's more than this.....
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- Joined: Mon Jun 18, 2012 11:41 am
Re: Deck hight
I was hoping for that answer. Stroking it will be then. Thank you for the feedback!
Glauco
Glauco
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Re: Deck hight
Assuming everything else (case, cylinders, rods) was stock, changing to 32mm pin-height pistons and changing to an 84mm crank, your deck height would change from 0.055" to 0.059".
An 82mm crank would only get the deck height down to 0.100", in my opinion, necessitating a longer rod (and then maybe requiring shims too) to reduce that down to a better range.
An 82mm crank would only get the deck height down to 0.100", in my opinion, necessitating a longer rod (and then maybe requiring shims too) to reduce that down to a better range.
- Marc
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Re: Deck hight
Agreed, an 84 stroke would seem almost perfect - at 82mm, you're still going to have ~.045" excess piston deck height assuming you use stock-length rods.
However, I would never use stock-length rods with 84mm stroke (the rod ratio's extremely short, raising some other serious issues). In fact, they're too short at 82mm unless you're building a thumper,say for a bus or off-roader.
5.7" rods would solve the problem with a stock stroke, but IMO that's too extreme in the other direction except for a race engine that'll spend all its time at high RPM. Plus it can clatter like a diesel at idle ...BTDT
Aftermarket rods are readily available in 5.5" and 5.6" ...I'd consider combinations using a longer rod and more moderate stroke that'll give you the right "package width" and neither an excessively low or high rod ratio.
78.4mm stroke with 5.5" rods should come out near perfect. If you can't find a 78.4, a 78 should be close (may still need to shorten the cylinders a hair, but not enough to bring on the problems Bruce mentioned above). The rod ratio would be 1.79, lower than stock but not as ugly as the 1.67 you'd have with stock-length rods on an 82 stroke or 1.63 with 84.
74mm with 5.6" also works and gives a rod ratio of 1.92, very close to stock 1.98, that would make for a happy smaller-displacement combination if you plan to build it to rev.
And there's always the option of having a custom-stroke crank built, too...I once went with 5.7" rods on a stock-based crank that was simply offset-ground to 73.025mm (the most I could get with 2" Buick journals) - that yielded a stock rod ratio, and it still made ample torque (~130 from 1879cc) at 3000 RPM while revving happily to 6500 and beyond without the need for radical heads.
Besides the higher stresses placed on the rods/wristpins and greater cylinderwall wear that results from a low rod ratio, the piston spends less time near TDC which yields less power than a long-rod combination of the same displacement. You also need a bigger intake tract to keep a short-rod motor from "running out of breath" at a comparatively low RPM.
http://victorylibrary.com/mopar/rod-tech-c.htm
However, I would never use stock-length rods with 84mm stroke (the rod ratio's extremely short, raising some other serious issues). In fact, they're too short at 82mm unless you're building a thumper,say for a bus or off-roader.
5.7" rods would solve the problem with a stock stroke, but IMO that's too extreme in the other direction except for a race engine that'll spend all its time at high RPM. Plus it can clatter like a diesel at idle ...BTDT
Aftermarket rods are readily available in 5.5" and 5.6" ...I'd consider combinations using a longer rod and more moderate stroke that'll give you the right "package width" and neither an excessively low or high rod ratio.
78.4mm stroke with 5.5" rods should come out near perfect. If you can't find a 78.4, a 78 should be close (may still need to shorten the cylinders a hair, but not enough to bring on the problems Bruce mentioned above). The rod ratio would be 1.79, lower than stock but not as ugly as the 1.67 you'd have with stock-length rods on an 82 stroke or 1.63 with 84.
74mm with 5.6" also works and gives a rod ratio of 1.92, very close to stock 1.98, that would make for a happy smaller-displacement combination if you plan to build it to rev.
And there's always the option of having a custom-stroke crank built, too...I once went with 5.7" rods on a stock-based crank that was simply offset-ground to 73.025mm (the most I could get with 2" Buick journals) - that yielded a stock rod ratio, and it still made ample torque (~130 from 1879cc) at 3000 RPM while revving happily to 6500 and beyond without the need for radical heads.
Besides the higher stresses placed on the rods/wristpins and greater cylinderwall wear that results from a low rod ratio, the piston spends less time near TDC which yields less power than a long-rod combination of the same displacement. You also need a bigger intake tract to keep a short-rod motor from "running out of breath" at a comparatively low RPM.
http://victorylibrary.com/mopar/rod-tech-c.htm
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Re: Deck hight
And if I use my 82mm crank with 5.5" rods wich I have as well, I'm coming in pretty close I guess? Or am I thinking wrong?
- Marc
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Re: Deck hight
That'd give a 1.7 rod ratio, (just acceptable IMO, but the fact you already own those parts does make it attractive) but the engine will end up wider than stock since you'll need to use spacers under the cylinders to keep the pistons from poking out...but it won't be too wide. Probably will need spacers ~.050-.060" thick like jhoefer said, most exhaust systems & throttle linkage will accommodate an extra ⅛" width with no hassle, and the head studs should be long enough too (and you can always cheat them out a couple of threads at the case if needed to get full thread engagement on the nuts). Odds are you'll need custom-length pushrods to establish ideal rockerarm geometry anyway, just allow time in your assembly schedule to obtain them when you get to that point. If stock-length ones come close enough, you might just get away with milling a bit off the rocker stands, or perhaps resorting to lashcaps. Too many variables in rockerarm geometry to predict for sure, but most likely it'll be close if the lifters are tall enough and the cam's base-circle isn't too small. At 82mm stroke, the lobes of a stock base-circle cam may have interference problems at the crank counterweights which can limit your choices there. Even if the lobes clear (.040" clearance recommended) you'll probably still need to whittle a little off the shaft and thrust flange (and possibly the case windows and barrel skirts) depending upon the rods used. That's why "dry" mockup assemblies are nearly always mandatory when you start playing around with modifications such as this. It's usually an adventure
If the piston crowns are thick enough you can also mill them down a bit to get the desired deck height instead of using spacers. Just don't be radical, you don't want the top ring to end up living too close to the combustion zone. Typically you can get away with taking off ~.100" before things start getting scary, and this build shouldn't require that much. I'm assuming it'll be conventionally-aspirated (and no nitrous), and that the crown thickness isn't going to need to be further compromised by valve-clearance notches for high lift when I say that.
If the piston crowns are thick enough you can also mill them down a bit to get the desired deck height instead of using spacers. Just don't be radical, you don't want the top ring to end up living too close to the combustion zone. Typically you can get away with taking off ~.100" before things start getting scary, and this build shouldn't require that much. I'm assuming it'll be conventionally-aspirated (and no nitrous), and that the crown thickness isn't going to need to be further compromised by valve-clearance notches for high lift when I say that.