Aholland

Member
Jun 14, 2002
11
0
I have a 1988 RM250 which has been completely rebuilt using a wiesco piston (new crank bearings, connecting rod, piston rings etc). With the OEM factory piston I had a squish angle clearance of about 0.035". With the Weisco piston I now have a clearance of 0.017", which I belive is too little. I am going to remachine the head and need to know what the new squish clearance should be for this bike. As a side note, head is closest at the intake side of the head (0.017") but larger ar the exhaust side (0.022"). Should I worry about equalising this? Thanks Alex
 

EricGorr

Super Power AssClown
Aug 24, 2000
708
1
You need a total of .050 inch for a 250cc bike. The reason that the measurements are unequal is because the piston has a bit of freeplay on the needle bearing and at the cylinder wall. Go for .050in minimum clearance.
I think that your cylinder head was modified at some point because a stock RM has about .060 inch squish clearance.
The Wiseco pistons aren't any different than stock on the timing height (dimension from the center of the pin to the crown).
Good luck
 

Aholland

Member
Jun 14, 2002
11
0
The bike was originally modified by...TUF racing when you (Eric) were there. I believe that one of less skilled people did the original porting. When it first came back, it was totally unacceptable. Would'nt run worth a damn. I sent it back when I believe you worked on it. You cut the cylinder base down and re-ported it. Now it runs very strong in the mid and high range. Bottom end is a bit suspect. I measured the squish clearance with 4 pieces of solder (front, back and sides) at the same time to reduce any piston rock. I always record the clearances when I do a rebuild which is why I found it funny that they had closed up so much. This is the first time I have used a Weisco piston. Maybe the gaskets amount for a small amout of the clearance and new parts the rest. I'll increase the clearance to 0.050". This will probably drop the compression to 150 lbs which should still be okay.

As a second question, Do you advise advancing the timing on these bikes (altitude 4000-6000 ft). The ignition can be advanced considerably, but I read in one or Eric's technical papers that timing should not be advanced more than 0.7mm (stator rotation). This may have only applied to the 1997 bikes in the article. I can advance mine probably about 4 mm rotation. Any suggestions guys? I never ride below 4000 ft.

Thanks.
 

LJW

Member
Dec 3, 2001
77
0
Without a way to positively locate the head to the cylinder, setting squish clearance is guesswork. The installation of 2 dowels (or tube dowels around studs) will allow reassemblely with consistent concentric location. Takes the worry out of being "close".
 

Aholland

Member
Jun 14, 2002
11
0
When I measured the squish clearance, it was on a fully assembled engine. That should assure a perfect (actual) measurement, as the clearance was the actual distance between the head and the piston crown.
 

LJW

Member
Dec 3, 2001
77
0
Set the head on the cylinder and slide it from side to side. The clearance around the studs allows the squish angle to move closer to the piston dome on one side and farther away on the other. Dowels or a flat top piston are the only cure.
 

Aholland

Member
Jun 14, 2002
11
0
Thanks for all the input guys. I have remachined the head to 0.038" now must decide if I go the extra 0.012" eric suggested. Re-assemble, reachine, recheck. Better done right than whack a piston.

Alex
 
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