broken skirt

[dheck88]

hi there ive been on this site a few times but just joined. i am at a loss. i have an 87 vmax that i just did a topend rebuild on which includes sending the jugs out to milenium tech tor be recoated and checked. i put two new yamaha stock pistons rings clips and wristpins in.
i took the sled out for a break in ride all day it came out to about 100 miles or so. it ran great good color on both pistons and plugs. the second day i went out i opened it up on a straight away for awhile shut it down checked the plugs all was good. im going down a winding trail coming of the throttle and the motor starts making noise knowing this isnt right i shut it down. pulled the plugs and there is aluiminuim on it so we drag it home i pull the head and the skirt on the intake side is broken off halfway up the wrist pin, what would cause this?
i forgot to mention years ago and about 2500 miles ago a reichards oil injection system was installed as well as the jugs were ported and polished to factory spec. any help would be great on why it came apart and how should i fix it, do i need to split the case or will i be fine just cleaning it from the top side thanks guys.
oh yeah does anyone know where to get comet 108 c parts now that they are out of biz

[Bob Vehring]

Well, that's certainly not a common problem, several things came to my mind.

First I would check piston to wall clearance, I would check on both sides. If you can get access to one I would check with a dial bore gauge to also check for run-out and taper.

Next I would check for how well the ports were champhered. If either the skirt on the intake side or the top of the piston on the exhaust side snagged a port, it could cock the piston, and break the skirt.

Did the exhaust side top of the piston suffer detonation? that could cause the piston to cock and jam the skirt into the wall.

When you got on it hard, did you have good syn. oil in it, or break in oil. I never get on an engine hard until its fully broke in and has the best oil in it.

[chrisrider63]

Hey Bob, the few oil companies there are make all of the brands that are out there, I was just running ESSO mix in my V-MAX because my Dad was saying its probably the same stuff that Yamaha uses or one of the good companies use but im starting to think otherwise as i was running castrol in it before and then started with the esso and thats when i started having trouble so maybe ill go get a fresh case of Yamalube..

[chrisrider63]

I was just going to message him Bob and tell him to ask one of you guys but i was thinking the same thing, something got caught or he had too much play and it rattled and snapped a skirt off.

[Bob Vehring]

From decades of racing, and of course having to fix my own mistakes, I have learned the benefits of a good synthetic oil. Back in the very early 80's I had very modified high compression Rupp 440 engines and 521 Rotax's At the time we use a pretty good shot of Nitrous on them The brand of oil I used was Golden Spectro. It made a huge difference on both the internal wear, and life span of these engines.
Today the race engine we build are small OHV engines for Karts and the 120 sled racers.
I'm no chemist, but I do see a great deal of difference in the oils, which we have many brands just made for racing.. Not all brands hold up well at the HP level we are at. These are small 4 cycle engines,which means many moving parts but we are well over the 2 hp per cu in ratio. That compares them to big block car engines making over 1000hp as far as stress levels. I have seen many oil related failures on these engines even from some of the "special Syn. Racing " oils available. On the other hand, good oils we have learned do not break down, will make your engine last and entire season with little wear.
I'm not an expert on 2 cy oils, I always used Golden Spectro but that is hard to find around here. I switched recently to Amsoil Dominator for two reasons. First I know racers who swear by it, and second, Amsoil sponsors both the National oval series and the National Snow X series. I always support people that put money back in to our sport.

[dheck88]

Thanks guys no i dont have a guage available so millenium is taking them back and are going to check the specs. i also was running yamalube with about two quarts of citgo sea and snow because i ran out of yamalube the second day. the sled had 5k on it before the rebuild do i need to do any thing with the oil pump

[Bob Vehring]

Re oil pump;
You have to be sure its is bled, and works. I'm probably the wrong person to ask. I will always admit, I have several lower performance sleds that have oil inj., and never have had a problem with any of them. When it comes to my hot rods, again with no rational reason to back this up, the thought of the little things that could go wrong scare me, so I pre mix.

[chrisrider63]

I think that all the guys on here that know way more then i do about sleds will agree when I say that you cant trail ride a racing sled. The author of this post (sorry cant recall who) said all the porting mods were done to the machine maybe from the site article index here but this required more lubrication as stated in the Yamaha document with the tunning/porting changes. "A 83 oil pump should be used to give the engine more lubrication, or a pre-mix at 50:1 should be used". I was thinking about this the last couple days, but guys dont seem to realize if you just set these AWESOME machines up stock and tweek the clutcht a litte you'll run down 700' and 800's like i seem to be able to do in trials and fields. Racing machines with porting and other adjustments need to be tunned and worked on DAILY and are made to run a couple heats on a weekend and a final at a oval or drag strip thats it. Not 3-4 hour trail rides with varying temperature and conditions. If you want a trail sled it needs to be set up stock. Correct me if im wrong but dont these machines make close to 90 HP stock??? And the stock pipes from the documents in the index show they make the most HP LOL. I dont get why people feel they gotta "improve" these things and F*$^$^&#*$^#&??!!!! them all up!!

[Vmax540]

dheck88 did milenium tech specify what rings to run with their cylinder coating ? ie. rings for Chrome bores or Sleeved ?

Chris, I think most of us (including myself) have been there done that.... Anymore a reliable sled means more to me than one that is modified. I have many times seen, well maintained and tunned stockers beat highly moddified sleds especially, when the conditions change from where they were set up to run and I know the small window along with the head aches that modified machines typically need in order to trail ride.

[dheck88]

i told mil tech that i was going to use stock yam pistons and rings and they said that would be fine.
when i said that the motor was ported i meant that it was blueprinted nothing radical the machines jugs were set to 1987 yam standards just balnced and cleaned up. and the sled ran great for ten years like that the only reason i wound up rebuilding it was the jugs were just out of spec on both sides and 1 oil line came off of the pump last year and i did not catch it when i put the carbs on. i agree with u to a point about leaving a sled stock. the manufacturer puts millions of dollars worth of research into a motor to find an even balance between performance and reliability. i dont know how it is today but back in the day u could buy two identical sleds from the same dealer and one could be a screamer and the other a dog, due to tolerences at the manufacturer needing to be re calibrated and the vmax was no different. so to be clear all that was done was blueprinting to 87 stock yamaha specs due to the sled being a dog, once this was done the sled ran like it was supposed to for a long time.

thanks for everyones help i will keep you guys posted on what i find out. hope to have it back together by the end of the month!

[Bob Vehring]

That,s a pretty relative statement. Back in the early '80's the VM had the possibility of being the fastest sled on the lake. Today it takes 250-300HP to even play. I know the guy how owns the world record on asphalt ( dragstrip) hes at 550 hp, 7.64, 178 mph
Engine building today has come along way from the '80's Todays kings are the turbo or centrifugal blown Yammi 4 strokes crank up the boost 300 hp back it down and ride the trail all day. and still be the fastest.
My kid is very well known in the 120 world, his engines are doing very well both in ovals and Snow X. Both types of racing use the same engines, but they are built differently. Oval sleds can be built for topend power and a limited power band. Snow X needs low end and a strong midrange..
Our engines can be built the same way. The VM, even stock has alot of port timing and a strong top end but it also lacks in the midrange because of it.. So far, my SRV walks away from the VM, Given a long enough lake, the VM might get it back, but I'm no longer a lake guy.
I have a friend that has built both VM and SRV engines for decades, many were for Cross Country racing. He welds up the ports, just like we do on the 4 strokes and makes them smaller, then uses 34 or 36 mm carbs. These things out run anything in the woods.
I have been racing sense the late 60's, Cars, bikes, sleds, karts. People tend to look what the fastest people are doing, drag racing or speed runs for sleds. Their geared, clutched and built just for that. Your right that wouldn't be a good trail engine. If you look in the My Project sec. I did have one of my Rotax rotary engines mocked up to go in the VM, it fit just fine, but your right, that would not have been a good trail sled so I went back to the 535. Right now, Perry is welding up the ports in a set of cyl's for my VM.
Today engine builders can build you an engine for any app you want, and it will be far superior to stock. When I put my SRV back on the trail 15 years ago, I took out all the compression, and its been trouble free for thousands of miles, starts on first pull every time

[chrisrider63]

Ya that sounds a little different sounds like the ports were just cleaned up maybe. Could we see some pics?? Sounds like a crank bearing issue to me, the thing needs to be tore down. Also with the mods you made these machines are HIGH permformance machines that need to be brought to operating temperature I warm mine up for about 3 minutes and still doesnt seem like its enough. Someone posted on here ( a think a new guy) that the YAMI oil injection overfills when it sits and; like my MAX and exciter runs real rich when you take off then lean again- and then back to normal. I noticed this so i just take it easy before I go running down 700's and 800's !!!! LOL.

[dheck88]

yes as soon as the jugs come back from millenium i will take some pics. i was going to pull the crank out any way and have it checked.

have any of you guys put a ripsaw track on this sled i need to do something. this sled feels like it wants to be a bullet just cant get it to couple up. i put one on a 700 and it worked great but im not sure how this sled would handle that heavier track or if it would even fit.

[chrisrider63]

Mine wasnt hookin up last year and sounded big and bad but the clutches were synced and set , get a marker and mark down the sheeve where the belt sits on both clutches and see if they both go to the top.

[dheck88]

well the cylinder shop called me and told me the break was not caused by a size issue. they seem to think that i just caught a bad piston from yam. do i try yam pistons again or go aftermarket?
do you guys think with a stock set up nothing radical a set of egt guages would be a good idea or is that overkill/