Running in new Mahle piston on fresh bore ...

Hello people ...

I'm racing next weekend at Irish Valley in PA. I am also about to fit a freshly bored barrel and matched NOS piston from the machine shop and want to know if anyone has any thoughts on run-in period? How much time should I give this fresh bore and piston to run-in?

I am concerned that I am going to bolt it back together and then go straight into a race situation and over-stress the piston and bore before its had time to break in properly ... (?!?!)

I will probably have time to run it up and down in the pits but that's going to be the limit to it possibly.

Thoughts anyone?

Oh - and it's on a 1973 CR400 ... Standard steel liner. Mahle piston 1st over. 81.42mm.

im not sure if its needed..but ive always let it idle until heat is felt decently thru the barrel then cooled twice before riding after doing a fresh top end.
after that i wouldnt ring its neck until you get a half tank of gas thru it..perhaps its overly cautious but the mahle will last a long time, it deserves a good break in. a plated cylinder has a lot less break in time in my opinion.

Thanks Justintendo ... Sounds like good common sense. I'm only nervous as the last bore was done to 2 thou not 4 thou clearance and I have the NOS heat seized Mahle to prove it. Hopefully it'll be a happier result this time. Agreed on Mahle being tough as well.

If you have any chance at all...
Try to ride the bike a bit before the race...even for 15 minutes to 1/2 hour.
Do NOT ride at a steady throttle.
Try to get in 3rd gear or so...then UP and back on the throttle...up/back...up/back...
Shut off...let cool...repeat...

I live in a rural area, so I was able to do a 4 mile road loop before going to Rocket...then ran the piss out of the little 125 with a new .060 Wiseco.
No problems!

Note that I-V has a later start...get there in the morning...pits are right next to a looong straight starting area...you can ride up and back for quite a few passes...let it cool...repeat...

I have always followed a similar procedure as justintendo describes, 1st start heat into the barrel until you feel it through your glove then cool, repeat. Then ride under moderate load as to not stress the engine under heavy load... Then let cool and your ready to bang bars. I did find myself in a similar situation as you, in that I had a fresh bore and wiseco piston and only had 2 heat cycles at idle, then raced the Big Bear100 national H.S. I never had a problem with that piston bore, bike ran great.

I burnish TS-70 moly paste into the cylinder wall and the piston skirt with a soft rag. Then apply a light coat of 2t oil. Assemble it. I start it up and let it idle. Let the choke stay on so it fogs the engine with oil for a while. Then put the choke off when it almost stalls. Let it get hot, shut it off and repeat when it cools. Then test ride it. Then I rev it up and hold it to seat the rings.

In the drag race engines I set the timing, adjust the valves, adjust the carb. Then once it's up to temp it's wide open throttle for 30 seconds to seat the rings. Then I drain the start up oil, change the oil filter. Put in the racing wt oil. Then it's 1,000 miles to break it in. Change the oil and filter and she's ready to race.

you need to work as many heat cycles in as you can and you need to ride just out of the power, working gently just into the band and changing gear. just extend this process till you giving it some stick...usually a few tank fulls, your out of time and just need to do the best you can

Surprize, Bigbill, Richard, Cruisetopdown, Thank you all for your thoughts ... Seems the consensus is go easy for a while and then let the hammer down. Ill let you know how it goes. Weather forecast is rain so should be interesting!

Are you riding the I-V XC (Sat) or MX (Sun) race?

I think I'll be riding both ... Love this event. Great way to start the season.

"Great way to start the season". ... Not on a new bore it isnt....

Hmmmm, Point taken. That said though the first day of this event is a punishing (on the rider) 4 or 5 lap XC race that's a 3 mile loop through woodland. It's more a slow technical race and is pretty gentle from an rpm perspective ... Very little wide open throttle. I think that (done gently) will suffice for run-in.

yep that should work fine (till the first guy passes you and the red mist sets in.... HA Ha)

I been testing moly products since 1970. I had a buddy who worked for Dow Corning were they tested it. They put it in a over heating gear box and it ran cooler. They put a large amount of moly in a new jeeps oil. They ran it around a race track. They removed the oil & oil pan, they ran it around the race track with no oil. Put the oil and pan back on and it had no noises.

I was repairing gravely tractors at the time. They have counter balances like a Harley. I inspected the crankshaft and con rod. The oil hole was lined up. The con rod is captured between the counter balances. Figuring the oil hole is ok I bored it and assembled it with a new piston. With moly as a pre lube. I went garden tilling. By my forth garden she started acting up. When I got home I found the con rod bushing had spun. Just my luck the oil hole lined up during the rebuild. The cylinder was ok. The rings were cooked but the piston could be saved for another rebuild. Moly saved the parts. I rebuilt so many of these tractors it's just a freak thing.

In my sons Honda 125cr I burnished moly into the cylinder and piston skirt. Assembled it. Ran it all summer well hammered it all summer. In the fall we disassembled the top end to check it. It looked like the day we assembled it. No wear marks at all.