New con rods from MAP or keep the stock ones?

Searched about, couldn't find a thread that answered all my questions. Sadly, my latest aquisition- the Mongrel, as he's come to be known- seized the motor on the interstate on the way back from motorcycle club coffee. The mongrel was built from parts and never finished by a guy that died, and then my friend bought it from his estate, and then I got it, and then it sat for a while and on and on and on... so it's a february 1970 triumph t120r motor in a tr6c frame, and it's got a steve mcqueen desert sled vibe going. Nothing is original or stock or makes any sense, but the motor seems to be straight forward 1970. Slapped in a tri-spark and gave the amals a dunk in the ultrasonic and she fired right up. Timed it with a light, then checked the color on the pluggies (nice carmel color) and away we went. Ran great for about 40 miles, then the drive side piston seized. It let go and kept running on one pot, but the damage was done. Was probably quite a sight going down main street with a huge blue plume of smoke behind it for a few miles as I limped er home.

Anyhoo, started stripping it down and the motor oil looked blacker than any diesel I ever changed back in the day despite having only 40 miles on it. I guess that the modern detergent oil went to work breaking down the sludge in the probably jam packed trap and pumping it through the motor. Piston has an almost identical pattern of scuffing/scraping to the one from my Norton when the motor seized up due to oiling issues from a plugged sludge trap. Same side too. Bummer man.

Got on F/G and bought the usual stuff, pistons that were in it were already .020 over so I got .040s with rings and pins, bought new valves cause the tips were all cratered, bought new mains and a tranny rebuild kit, gaskets ect., now that I'm impoverished I figured, hey, lets get some more crap. Hence the title and the actual question- Should I get new MAP/Hepolite steel rods while I'm in there ($500! Ouch!) or just leave well enough alone and keep the stock alloy rods. Second question- do I need to rebalance the crank for the new rods? My sincerest hope is that when I mike the journals they're good with a fresh set of shells and don't need a regrind. Montana is a long way away from any reputable 1960's motorcycle crank grinders.... If I do have to send it down to mile high, I'll do rods and undersize bearings and balance anyway, but if not... Do I really need to rebalance? I couldn't find any weights for stock vs MAP rods. I'd have it miked, but this is my first triumph, so of course I need all new specialty tools to get in there. They're in the mail.

Vibration won't be too much of a concern as this bike will be doing mostly desert sled type stuff, but I would like to ocasionally ride it to work and what not, and that's about 15 miles of freeway. Ok.... FLAMEWAR BEGIN!

You will definitely have to rebalance the crank if you put steel rods in it. If you fit ‘like for like’ new Triumph rods, you won’t need to, just make sure both rods weigh the same, especially around the big end.

Do you need new rods? It really depends on the life the bike has had… your intended use… and your level of risk aversion.

Vibration is always ‘a problem’… even if rider comfort doesn’t mater, vibration is what breaks things, makes things fall off, etc…

Funnily enough, I have what you need, I was gonna build a hot rod 750 motor for my 650, NOS cases, almost as new crank, new Morgo barrels, MAP tight quench billet pistons and JS lightweight Carrillo rods! Trouble is it would cost a lot plus shipping and tariffs would be prohibitive.

Fast Eddie said:

You will definitely have to rebalance the crank if you put steel rods in it. If you fit ‘like for like’ new Triumph rods, you won’t need to, just make sure both rods weigh the same, especially around the big end.

Do you need new rods? It really depends on the life the bike has had… your intended use… and your level of risk aversion.

Vibration is always ‘a problem’… even if rider comfort doesn’t mater, vibration is what breaks things, makes things fall off, etc…

Funnily enough, I have what you need, I was gonna build a hot rod 750 motor for my 650, NOS cases, almost as new crank, new Morgo barrels, MAP tight quench billet pistons and JS lightweight Carrillo rods! Trouble is it would cost a lot plus shipping and tariffs would be prohibitive.

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The weird thing about this bike is that years after I got it I found the original motor in boxes at a friend’s place, so if I go all in it’ll be on the numbers matching motor- right now it’s hideous enough to be the perfect bitsa desert sled. I’m just trying to scab this motor back together best I can without cutting any corners, though I would like it to be quite dependable because I intend on taking it on a lot of long desolate back roads and some forest service trails. As to the motors history I’ve only got the usual automotive archeology clues to go off of- it wasn’t seized when I got it, ran super strong till it blew up and the pistons say 020 so I’m not the first. My guess is they only did the top end and never cleaned out the bottom-

Remember that it seized because something was wrong:

insufficient piston clearance in the bore,

weak mixture/fuel starvation,

ignition timing off,

fuel octane too low for the compression ratio,

making the engine pull too hard for too long in too high a gear,

what else?

You don’t want the same thing to happen again, so what will be different next time you ride it?

Running after the seizure could blacken the oil.

If new rods will make you happier, buy them. I run old rods, but I don’t advise others what to do about rods.

Triton Thrasher said:

Remember that it seized because something was wrong:

insufficient piston clearance in the bore,

weak mixture/fuel starvation,

ignition timing off,

fuel octane too low for the compression ratio,

making the engine pull too hard for too long in too high a gear,

what else?

You don’t want the same thing to happen again, so what will be different next time you ride it?

Running after the seizure could blacken the oil.

If new rods will make you happier, buy them. I run old rods, but I don’t advise others what to do about rods.

Click to expand...

Yeah it was weird- had run on flat land at about 65 mph for about 15 miles then seized outta nowhere- my bet is oil starvation on that side with a plugged up trap. Plugs got checked a few times, color looked good so not fuel, timed it twice with a light, timing good. Carbs were just ultrasonic cleaned, probably not lean. I did buy a new set of premiers to toss on when she’s back together

MAP Cycle rods and pistons come with installation instructions and rod and piston weights relative to balancing. Yes you should get the crank rebalanced for the new parts cuz the MAP Cycle steel rods are heavier, and if you go long rods, the pistons are lighter. I built a Norton750 engine with MAP Cycle long rods and pistons. Worked pretty good. However, if you want to save money use the stock parts and go +.040 pistons. If money doesn't matter then sky is the limit.

My guess is the crank might be scratched up with black oil and a seized bore limped home. If you are going out in the dirt, a lighter weight crank would be fun you might appreciate.

By the way, if you want to build a strong hot rod Triumph, you might want to talk to Lowbrow Customs. The owner knows how to build land speed record holding vintage Triumphs.

EDIT: The MAP Triumph long rods and pistons might only be for short stroke engine builds. You'll have to verify.

Test don't guess.

Do you need new rods? Measure the old ones. If the big and small ends are round, parallel, and to size, you probably don't need new ones.

Mile High doesn't dynamically balance cranks. Marine Crankshafts does though.

Schwany said:

MAP Cycle rods and pistons come with installation instructions and rod and piston weights relative to balancing. Yes you should get the crank rebalanced for the new parts cuz the MAP Cycle steel rods are heavier, and if you go long rods, the pistons are lighter. I built a Norton750 engine with MAP Cycle long rods and pistons. Worked pretty good. However, if you want to save money use the stock parts and go +.040 pistons. If money doesn't matter then sky is the limit.

My guess is the crank might be scratched up with black oil and a seized bore limped home. If you are going out in the dirt, a lighter weight crank would be fun you might appreciate.

By the way, if you want to build a strong hot rod Triumph, you might want to talk to Lowbrow Customs. The owner knows how to build land speed record holding vintage Triumphs.

EDIT: The MAP Triumph long rods and pistons might only be for short stroke engine builds. You'll have to verify.

Click to expand...

Although it seems kinda counter intuitive, actually the MAP long Triumph rods are intended for the MAP long stroke crank kits (to restore rod to stroke ration, same as JS argues) and therefore require use of their special taller cylinder barrels.

tompkins said:

Yeah it was weird- had run on flat land at about 65 mph for about 15 miles then seized outta nowhere- my bet is oil starvation on that side with a plugged up trap. Plugs got checked a few times, color looked good so not fuel, timed it twice with a light, timing good. Carbs were just ultrasonic cleaned, probably not lean. I did buy a new set of premiers to toss on when she’s back together

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You’ll see if the sludge trap is plugged up.

t ingermanson said:

Test don't guess.

Do you need new rods? Measure the old ones. If the big and small ends are round, parallel, and to size, you probably don't need new ones.

Mile High doesn't dynamically balance cranks. Marine Crankshafts does though.

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This is what I was lookin for- I’m leaning more towards keep the stock ones and blow the cash elsewhere

Schwany said:

MAP Cycle rods and pistons come with installation instructions and rod and piston weights relative to balancing. Yes you should get the crank rebalanced for the new parts cuz the MAP Cycle steel rods are heavier, and if you go long rods, the pistons are lighter. I built a Norton750 engine with MAP Cycle long rods and pistons. Worked pretty good. However, if you want to save money use the stock parts and go +.040 pistons. If money doesn't matter then sky is the limit.

My guess is the crank might be scratched up with black oil and a seized bore limped home. If you are going out in the dirt, a lighter weight crank would be fun you might appreciate.

By the way, if you want to build a strong hot rod Triumph, you might want to talk to Lowbrow Customs. The owner knows how to build land speed record holding vintage Triumphs.

EDIT: The MAP Triumph long rods and pistons might only be for short stroke engine builds. You'll have to verify.

Click to expand...

Lowbrow is where I bought the new carbies! I’m sure it won’t be the last purchase I make there. I’m hoping no scratches, but if so, it is what it is- I’m leaning toward stock rods but the lighter crank thing is intriguing- I always thought a heavy crank was better for dirt to keep inertia? I guess I never really thought about it- lighter = more revvy right?

Fast Eddie said:

Although it seems kinda counter intuitive, actually the MAP long Triumph rods are intended for the MAP long stroke crank kits (to restore rod to stroke ration, same as JS argues) and therefore require use of their special taller cylinder barrels.

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OK that's starting to get expensive.
Below is what the MAP Norton Long Rod box I have says on the outside. I have never built a Triumph Stroker. Anyway, I was just attempting to say the MAP parts can work if somebody wanted to use them.

tompkins said:

Lowbrow is where I bought the new carbies! I’m sure it won’t be the last purchase I make there. I’m hoping no scratches, but if so, it is what it is- I’m leaning toward stock rods but the lighter crank thing is intriguing- I always thought a heavy crank was better for dirt to keep inertia? I guess I never really thought about it- lighter = more revvy right?

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I have a warped sense of what works and should STFU. I like a lighter weight rotating mass. I prefer an engine that gets into the meat of the cam quicker. It would wind up a bit faster with a lighter crank, but if a very mild or stock profile cam was used the engine shouldn't fall on its face if short shifted with a bit less inertia. What might not work so great in the dirt is a lighter crank, a big cam, and running higher gearing. You might stall the engine at the worst possible time because 1st gear was too high. Ouch!! Also a lighter rotating mass, bigger cam, and higher gearing can be hard on the clutch leaving dead stops.