Honda TRX250R Forums banner

1 - 4 of 4 Posts

·
Registered
Joined
·
18 Posts
Discussion Starter #1
I have an ESR 310 prox kit on my bike. This past weekend the rod bearing let go and tore up the whole engine. I thought it went lean at first but then took it apart and saw the destruction of bearing bits come up from the bottom end and chew up everything (piston,head,cylinder). Anyway long story short I want to get a new OEM crank but there are only long rods available. On the ESR website they say that with a long rod you have to use a spacer plate, but I dont want to, I was wondering why I couldn't use the long rod crank with an 87-89 piston and no spacer? The only thing I could think of would be if the big bore piston were only available in 86 style? Also, the new bore will put me at the max of 73.5mm for the 310 kit. How bad is that? Thank!! and sorry for the long post P.S. I posted this in C-Leigh's forum but I figured more ppl may see this here.. Thanks again!!!!
 

·
Registered
Joined
·
722 Posts
you can stack a ton of base gskets under it! LOL why dont you want to use a spacer???
 

·
Registered
Joined
·
27 Posts
the long rod crank is not a longer stroke crank , the rod that gos from the crank to the piston is just longer 85 atc and 86 trx are shorter and 87-89 are longer ...... piston crown height should be the same on all 250rs you need the spacer plate because even though the stroke is not farther , the longer rod will make the piston run up and down higher in the cylinder which would A: make the piston hit your head and B: even if the engine would turn over it would throw your port timing way off , the piston would not travel far enough down the cylinder to open the ports properly..... i was thinking of doing the long rod/spacer plate to my bike because the farther away your piston is from your crank the less side load you woulkd have on your piston thus making it more durable ..... look at a chevy smallblock, 350 and 400 are same but 400 has a larger bore/stoke, but because the stroke and deck clearance they run a shorter rod which gives the piston more side load and makes them wear the cylinder like an egg scewwing the motor sooner, however i also wonder what the increase in rotating mass would do for reving, might slow it down but prolly in such a small amount you would never notice
 

·
Premium Member
Joined
·
4,273 Posts
Both the old model & the new model cylinders will have the same piston deck height, but, you got the difference in the rod lenghts from the 85/86 & then the 87/88/89.
The old model piston has 5mms of extra height from the wrist pin hole to the top because of the 5mm shorted rod. The new model piston has 5mms taken away from the wrist pin hole up to the top because of the 5mm longer rod. This is why all years of TRX250R cylinders will have the same piston deck height.
Spacer plates have to be used because with the 5mm longer rod & the 5mm taller old model piston you end up with a total of 10mms alltogether & 5 of that has got to be taken up with the plate.
A Pro-x cylinder on top of a old model short rod crank & old model piston you wouldnt need the spacer plate. A long rod crank with a new model long rod piston you wouldnt need the spacer plate either.
The new model piston on a long rod will work in the Pro-x cylinder without the spacer plate, but, you got to set it up tight piston to cylinder clearance because you dont want a loose piston flopping around in the bore for the skirt to hang up in that big intake port.
Neil
 
1 - 4 of 4 Posts
Top