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Discussion Starter #1
Blew a piston on my 1998 summit 670, ordered new pistons, rings and gaskets. The problem is we didn't count the holes on the old gasket, so now I don't know how thick of a gasket to use,sure could use some help here. thanx. I ordered Kimpex piston has anyone had trouble with these?
 

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Find an old scrap of the base gasket and measure it (or have an autoparts measure) with a micrometer or vernier caliper. BTW you could buy 2 SPI cast pistons for less than 1 Kimpex. Find out what caused the failure befor you attempt to use the sled again.
 

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well , good advice as always from td. you could buy a .06 (6 hole) and see if the clearances are to your liking...hmmnnn..why not assemble it with no gaskets, measure the squish and go from there . i guess what i`m getting at is nows your chance to MAXIMIZE it. anyways... never thought about the assembly without the gaskets but its gotta work. right?
 

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Find an old scrap of the base gasket and measure it (or have an autoparts measure) with a micrometer or vernier caliper. BTW you could buy 2 SPI cast pistons for less than 1 Kimpex. Find out what caused the failure befor you attempt to use the sled again.[/b]
scrap base gasket long gone, I knew I should of posted about the pistons before I ordered them, I hope the kimpex will be ok. Is the 6 hole the standard gasket? I'm not looking to hop it up just stock and reliable nothing fancy
 

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i just put a .05 base gasket on my 670... it's fine. If it's an HO 670 then check into it more.

Does anyone know what the TQ specs are on the cylinder bolts? Or the head bolts too?

I noticed last night when I got home from a 40 mile ride that I blew the rear corner of the base gasket on the stator side. It's all folded out and torn on the corner where the bolt hole is. I know I didn't put it together like that.

Should I have smeared a smidge of gasket sealer on it first? Or Is 20ft/lbs not enough on the base bolts?

Thanx again guys!!! SWRules
 

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According to Rotax you should coat both side of the base gasket with Loctite 515 or 518 then torque the cylinder and head bolts to 29 N*m (21 lb*ft). Don't forget that the front cylinder bolts (M8 X 105mm) are longer than the rear (M8 X 100mm) bolts. Apply some pipe thread sealant to the two cylinder inside rear bolt threads or you will find coolant leaking into the inlet ports of the crankcase. the two cylinder head bolts over the RAVEs are shorter too.
 

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scrap base gasket long gone, I knew I should of posted about the pistons before I ordered them, I hope the kimpex will be ok. Is the 6 hole the standard gasket? I'm not looking to hop it up just stock and reliable nothing fancy[/b]
Most jobber base gaskets are .5 mm (.020") BRP part number 420 9312 33. That is right in the middle of all the available base gaskets and I would say your safe there as long as no one has monkeyed with the case height or line bored the main bearing saddles. If this is a 1998 Summit X then you need a little more measuring to do first.
 

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not to hijack the thread but im kinda new with the whole world of base gaskets but my question is which holes do you count for thickness? I've been measuring mine with a micrometer but I could've counted holes?
 

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not to hijack the thread but im kinda new with the whole world of base gaskets but my question is which holes do you count for thickness? I've been measuring mine with a micrometer but I could've counted holes?[/b]
At the front of the base gasket in between the cylinders there are rows of holes:

3 = 0.3mm
4 = 0.4mm
5 = 0.5mm
6 = 0.6mm
8 = 0.8mm

Need thicker ? now you stack them.
 

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ok one more question while ive got the 670 guru here, just rebuilt my 1996 mach1 and put the 0.4mm gasket in...too big? should I change it or just shave the head to bring the squish to .65, right now it's .85pto and .88 mag
 

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well, let's see, on that motor (1996) with twin pipes (which I'll assume you don't have) the old trick was to thicken the base gaskets to raise the transfer ports to achieve 134 degrees intake and 202 degrees exhaust then remove that amount from the head on twin piped motors. Since yours is a single pipe and you want grunt and no over rev, I'd say bring the cylinders down using a P/N 420931230 0.3mm base gasket. Install the head and find some real rosin core lead solder (not the hard alloy stuff home hardware sells) and stress to achieve 1.52mm (.060") squish at the extreme ends of the combustion chambers. You can achieve this by laying some 120 grit emery cloth on a pane of glass and work the head following a figure 8 pattern and check your progress using the heel of a Verier Caliper extended through the spark plug holes on to the same pane of glass. Go slow and get that squish spec then polish the combustion chamber and exhaust port and there will be good power and no deto. Good for 87 - 89 octane gas 91 for warm weather and lake racing (sustained WOT). How's That?.
 

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oh I have the 44mm carbs and the factory twin pipes...motor is .20 over and has about 2 hours on the recent rebuild....I also have full access to a machine shop so shaving the head will be no prob....just scratching my head with the base gasket choices....but you have been helpful...thanks!

Brian
 

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Well now that is a different animal !, that engine set up needs to spin 8200 RPM and has a thin power band. Make darn sure that the Rotary valve timing is spot on 145 degrees open and 76 degrees closing (stock 501 disk) shim the cylinders up to 0.8 mm base gasket (pair) and send head to be fly cut to achieve 1.61 (.065") squish high revs don't like peak compression ratios. Taper grind the exhaust flanges to match the exhaust port. Polish the exhaust ports. Replace your RAVE springs 420229946 and make sure the guillotines are not stiff to move (replace o-ring cushion and put Moly grease on the shafts). add .010" to the initial timing for a total of .086" BTDC. Clean out the pipes with an old speedo cable mounted in a high speed drill or remove the heat envelopes and have them "Hot Tanked" because those pipes are a little too tight stock and it doesn't take much carbon build up to make them sour. Run 91 octane all the time. Do that and replace the primary spring with a new one and go embarras some 800s on the lake.

BTW the stock tach will be generous by 200 RPM
 

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Thank you! finally the answers I was looking for...good to have people here with real world knowledge of these motors...I've done a ton of reading, asking questions and getting a ton of different answers sending me in all directions...I rebuilt the engine myself and it is a screamer, I will follow your direction and make this sled a true menace hehe now I just gotta figure out how to make it hook up with the notorious traction deficient m-10 suspension!
Thank TD I appreciate it!

Brian
 

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Discussion Starter #15
Thanx for all the help I wasn't aware of the lock tite or the pipe sealant . You guy's are first class all the way, ride safe and happy holidays.
 
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