TheSamba.com :: View topic - Got wrong stroker crank

Wed Apr 14, 2010 6:36 am

I'm building a 78x94 engine. With the B pistons, i have about .2"
deck. I'm in the process of trying to trade them for A pistons.

This deck is with a stock rod. A longer rod would get it closer to where it should be. I did a pretty thorough search and didn't find the info about 78mm cranks vs B pistons. Some sort of chart to get builders into the ballpark, at least, would be handy.

If you have the clearance and a decent rod ratio, i think you may be better off with the 82.

Wed Apr 14, 2010 6:42 am

this is why building with a 78 crank is such a mistake, my opinion.

Wed Apr 14, 2010 8:40 am

bartman wrote: I'm building a 78x94 engine. With the B pistons, i have about .2"
deck. I'm in the process of trying to trade them for A pistons.

This deck is with a stock rod. A longer rod would get it closer to where it should be. I did a pretty thorough search and didn't find the info about 78mm cranks vs B pistons. Some sort of chart to get builders into the ballpark, at least, would be handy.

If you have the clearance and a decent rod ratio, i think you may be better off with the 82.

If you use A pistons, you will have huge spacers and a wide engine. Why not face some material from the base of the cylinders to achieve the correct deck with the B pistons?

Wed Apr 14, 2010 8:47 am

Matthew wrote: bartman wrote: I'm building a 78x94 engine. With the B pistons, i have about .2"
deck. I'm in the process of trying to trade them for A pistons.

This deck is with a stock rod. A longer rod would get it closer to where it should be. I did a pretty thorough search and didn't find the info about 78mm cranks vs B pistons. Some sort of chart to get builders into the ballpark, at least, would be handy.

If you have the clearance and a decent rod ratio, i think you may be better off with the 82.

If you use A pistons, you will have huge spacers and a wide engine. Why not face some material from the base of the cylinders to achieve the correct deck with the B pistons?
or for the same price, go 82 stroke and have a stock witdh engine with alot less deck height and easier to assemble

Wed Apr 14, 2010 8:52 am

jamestwo wrote: No, they did you a favor. With the 78.4 you was heading for all kind of problems setting the deck.

Hp won't be less, it will just come at a lower RPM.

What kind of problems? That's what I'm working with for my 2017 build, as I was advised it was a good crank to go with due to less clearancing involved, etc.

Wed Apr 14, 2010 8:57 am

With the 78 you end up with extra deck, not really a problem just an extra step. You need to machine the bottom of the cylinders or deck the case or both if running 94's. Or if you don't have all your parts yet, get longer rods.

Wed Apr 14, 2010 8:57 am

grimace007 wrote: Matthew wrote: bartman wrote: I'm building a 78x94 engine. With the B pistons, i have about .2"
deck. ..

If you have the clearance and a decent rod ratio, i think you may be better off with the 82.

If you use A pistons, you will have huge spacers and a wide engine. Why not face some material from the base of the cylinders to achieve the correct deck with the B pistons?
or for the same price, go 82 stroke and have a stock witdh engine with alot less deck height and easier to assemble

I agree, I was just working with the 78mm crank as a given. If I had the choice I would go 82 for sure.

Wed Apr 14, 2010 9:31 am

Matthew wrote: grimace007 wrote: Matthew wrote: bartman wrote: I'm building a 78x94 engine. With the B pistons, i have about .2"
deck. ..

If you have the clearance and a decent rod ratio, i think you may be better off with the 82.

If you use A pistons, you will have huge spacers and a wide engine. Why not face some material from the base of the cylinders to achieve the correct deck with the B pistons?
or for the same price, go 82 stroke and have a stock witdh engine with alot less deck height and easier to assemble

I agree, I was just working with the 78mm crank as a given. If I had the choice I would go 82 for sure. well if it was given to you then run it by all means :)

sometimes you can get lucky in the classified and find someone wanting a shorter stroke in trade for their 82

Wed Apr 14, 2010 10:20 am

Teeroy wrote: With the 78 you end up with extra deck, not really a problem just an extra step. You need to machine the bottom of the cylinders or deck the case or both if running 94's. Or if you don't have all your parts yet, get longer rods.

No one mentioned this to me when I first started asking around for feedback on the build I've got coming together. I kept hearing that a 78.4mm build is a pretty trouble-free route to take, and a good option, but no one ever mentioned anything about deck difficulties. I'd heard about that problem with the 74 and 76mm cranks, but not 78.4 units.

I'm not terribly worried, as I know I can handle getting the deck where it needs to be, I'm just a little surprised this is the first I've heard about it.

Wed Apr 14, 2010 6:36 pm

A 78mm crank (well, my 78mm crank with my case and stock rods) with A pistons results in a deck of about -.2". This requires about half an inch of shims, total.

My combo with B pistons has about .2" of deck. This is only a problem if one lives in a boonfuck town with one machine shop that has no idea what one is talking about when one takes them a jug and asks about taking material off the bottom.

The solution, as mentioned, is to change rod length or take a trip to the big city where machine shops know what the hell they're doing.

IMO, the 82 is a better and easier build. the 78 works if one gets the rod length right.

I have stock rods and I'm considering getting a longer rod and selling my 311b's. In my case, this would probably be cheaper than machine work. I do have a guy that might trade my B pistons for a set of A's. This is the cheapest fix, requiring shimming to dial in the deck.

Wed Apr 14, 2010 7:06 pm

sweet mistake cb made,,you,ll love the torque

Thu Apr 15, 2010 8:47 am

bartman wrote: A 78mm crank (well, my 78mm crank with my case and stock rods) with A pistons results in a deck of about -.2". This requires about half an inch of shims, total.

My combo with B pistons has about .2" of deck. This is only a problem if one lives in a boonfuck town with one machine shop that has no idea what one is talking about when one takes them a jug and asks about taking material off the bottom.

The solution, as mentioned, is to change rod length or take a trip to the big city where machine shops know what the hell they're doing.

IMO, the 82 is a better and easier build. the 78 works if one gets the rod length right.

I have stock rods and I'm considering getting a longer rod and selling my 311b's. In my case, this would probably be cheaper than machine work. I do have a guy that might trade my B pistons for a set of A's. This is the cheapest fix, requiring shimming to dial in the deck.

I'd forgotten to mention earlier that I have 'B' pistons... I haven't begun to put this engine together yet, so I guess I'll see where I'm at when the time comes.