Force 90HP - distance between piston and cylinder

[Frank Dettweiler]

I have to rebuild my powerhead from a Force 90HP (year 91) outboard, because the pistons are broken. A company will hone the powerhead. Therefore I need the distance between the piston and the cylinder. I ordered new pistons with oversize 0.01 inch. My dealer is unable to provide me with this information. The modell number of the outboard is H906X91C. Please advise.

 

[Spidybot]

Re: Force 90HP - distance between piston and cylinder

You really need a manual, also for rebuilding the thing after honing the cylinders.<br /><br /> http://www.iboats.com/boatproducts/mallsearch/force_manual/ <br /><br />The honing workshop should know about the new bore size, but even some of the fellow members here may have that info.

 

[wraenking]

Re: Force 90HP - distance between piston and cylinder

i just had mine rebuilt and the guy said anytime you put bigger pistons in, you MUST rejet your carbs. he said its easy, that it just screws in. but im sure you have to adjust them also. i think he took .020 off of mine and put in forged wiseco pistons, so my 98 force 120 is problably better than it was the day it came off the floor.

 

[Frank Dettweiler]

Re: Force 90HP - distance between piston and cylinder

wraenking, thankx for your info regarding rejetting of the carbs. i ordered the original oversized 0,01" pistons from mercury/force. i assume that you bought wiseco pistons with an oversize of 0.02". From reading in the forum i found out that the clearance is different from wiseco pistons to mercury/force pistons. the wiseco pistons are usally smaller and have a greater clearance. My dealer told me that the original bore is 3,375" or 3 3/8".

 

[Frank Dettweiler]

Re: Force 90HP - distance between piston and cylinder

^^

 

[Spidybot]

Re: Force 90HP - distance between piston and cylinder

As you got the new pistons from Mercury, why not ask them for the recommended bore?

 

[Frank Dettweiler]

Re: Force 90HP - distance between piston and cylinder

UU<br /><br />I will ask my dealer again, but I'm not confident if he is able to give me the right information. So I thought it's better to double check this information, because this is an important issue for boring the powerhead. <br />

 

[Spidybot]

Re: Force 90HP - distance between piston and cylinder

I understand. But nobody has better access to this info than the Merc dealers. I'd suggest you call more Merc workshops if you're not confident with the local.

 

[WhiteHorse]

Re: Force 90HP - distance between piston and cylinder

If you can return the Merc pistons, do it. Wiseco is the only way to go on rebuilt Force motors. The Quicksilver pistons have a habit of letting the rings pull right up over the pistons during break-in if you bore to Mercury spec. The wiseco pistons have the proper cylinder bore sizes right on the side of the piston box. Don't forget to change the bearings as well. The wiseco pistons come with a different wrist pin and require different bearings. <br /> I'm not sying it happens all the time, but I have rebuilt more Force powerheads than anyone I've ever met. Wiseco wins the test of time, hands down.

 

[Frank Dettweiler]

Re: Force 90HP - distance between piston and cylinder

WHM, thankx for your hints, but it's to late. I was unable to cancel the order of the mercury/quicksilver pistons. Today I got the oversized pistons 0.01". In the piston kit from Quicksilver is a boring instruction for oversized pistons. The instruction just refer to the finished hone bore size, which is 0.01" greater than the standard. So the hone bore size should be 3,385" for 0.01" oversized pistons. There is no information regarding the clearance, because the recommend finished hone bore size does take care of the necessary clearance. I agree with you that the Wiseco oversize piston is the better choice, due to good experience from other peoples. Do you have a suggestion (finished hone bore size) for me to improve my situation with the bought oversized pistons from merc ? I regret that I found to late this forum. <br /><br />---------<br />berndie61 - the force rebuild beginner