RM370A seizure

[micko]

Hi Guys, just completely rebuild motor on the above. Running a wossner maico piston in it and 32:1 synthetic oil in Vortex 98 fuel. Running a 310 main jet. Ran around on and off for about an hour, did a couple of plug chops to check colour of plug etc. Got up it on the straight and started to feel it lock up. Upon removal of the cylinder it nipped up on both transfer sides. Seams a heat issue. Any suggestions would be appreciated. 

[matcho mick]

if it was heat related,would it not seize fore/aft,not across the piston,(i'm guessing transferr ports are across the bore east west??),pistons usually have greater clearance along the gudgeon pin,(not familiar with wossners but),seems odd to seize there all the same?,transferr ports line up with piston?,suppose it would heat up there if nothings transferring  ,switch back to mineral 20- 1 to run it in??,good luck,
ps see guys i didn't mention the m word

[micko]

Yep, transfers are east - west. Strange there is no scoring on the exhaust port bridge.There seamed to be no shortage of oil in and around the crank area. Any way just looking for suggestions. Cheers.

[smed]

Were all the ports chamfered after a rebore? Coz they should have been ,maybe not the problem but worth checking, It's a very unusual place to nip up

[oldyzman]

Was the ring end gap checked before assembly, I think its 3 thou gap per inch of bore..
Brett

[oldfart]

Do you own a honing tool if so give it a light hone, ( use a cordless drill )  this will show up the high spots.
As per other posts the ports should have been relieve ( bevelled ) by whom ever did your bore.
Does your exhaust port have a bridge if so did you drill 2 oil holes for lubrication.

[pmc57]

A Suzuki Service Bulletin says if you use a 310 main you also need to change the Needle Jet to a Q-4 (after engine #13128). If you didn't also change the Needle Jet it may be too lean in the mid and high throttle range. Perhaps you should go back to the 340 main with Q-0 Needle Jet as specified pre the above engine # and start from scratch.

Also as Matcho Mick suggests, go back to 20:1 mineral because the standard jetting (both of above) was calculated to run with a 20:1 fuel/oil mix.

[80-85 husky]

I don't rekon oil is the issue but the jetting as disclosed. ensure the bore is round, weird to tighten across the bore rather then along it..
richen it up in the midrange and main ....try again!..if at first we don't succeed...

[Lozza]

Can you post a pic?

[Simo63]

I'm no expert on 370's but a 310 main jet sounds awfully small for a big bore ..... assuming that you have the required amount of piston/bore clearance, I would be looking at the jetting.

[micko]

Thanks for all the info everyone.
We think there wasn't enough piston to cylinder clearance and maybe a little lean. Thankfully the cylinder cleaned up with the hone and I will throw a used piston back in it and start with the 340 main jet. What piston/cylinder clearance do most of you recommend on a big bore ?
Cheers. Ps,can't work out how to post a photo 

[KTM47]

I think you are correct, not enough clearance.  The Woosner piston like Wiesco  and mahle pistons are forged pistons and need more clearance.  Also did you put spacers in the gudgeon pin holes.  Suzukis have no thrust washers on the big end bearing.  Maicos have thrust washers.  The thrust is controlled by the little end.  The width of the Suzuki conrod little end is about three mm smaller than the Maico.  When weisco sold pistons for Maico 400s and RM 370s (same piston) there were two spacer (thrust) washers included.

[pmc57]

A tip an old guy gave me a while back! 

After a piston has a pickup check the bore for any residual aluminium even after the hone and put a dab of hydrochloric acid on it. The acid reacts and dissolves any aluminium the hone didn't remove ensuring less chance of a future pickup caused by expanding micro sized bits due to the higher expansion rate of aluminium over cast iron.

I've never had the opportunity to do it but it makes sense to me!!

[RD]

Have you changed anything ie jetts or ign timing when you rebuilt the engine.
 If not you would think it is clearances or the ports not relieved.

[JohnnyO]

It's probably bored too tight. Nothing wrong with 32:1 that's more than enough oil and a 310 main is on the money.
I run 40:1 and a 300 main jet in my 370. As long as your needle setting is not too lean I'd say you have a piston to bore clearance problem.
Also the B model runs a bigger needle jet, I use a R0 with the needle in the 2nd position.