1985 85 hp force blown piston and rebuild

[ammermanfam]

I am in process of tearing motor down to see if block is salvagable after piston grenaded.it was bottom piston and not sure why it did this. the carb didnt have a speck of dirt in the bowl. just before it blew a large plume of water was flying past my shoulder and when i looked back had a clump of tules on motor leg throwing water up. wondering if maybe it took a mouthful of water. anyways, any suggestions as to where to look on what caused it to shatter would be appreciated. also does anyone have experience only reboring and replacing bad piston and just putting new rings on other two?

 

[jason32038]

Re: 1985 85 hp force

Re: 1985 85 hp force

I rebuilt mine last summer. I had 1 cylinder overbored to .040 and the other 2 lightly honed. I didnt replace the rings. A light honing should so with the used rings. Machining was $109 including tax. (Took 4 months to get back so find someone who even wants to work on it and get it in the shop asap so you can use it this season) Made an offer on the wiseco piston for $70 on ebay and the seller accepted. I used silicone gasket maker on everything except the head gasket but i'd reccomend a gasket kit. I have an 86 85hp.

 

[jerryjerry05]

Re: 1985 85 hp force

Re: 1985 85 hp force

They don't usually just GO!!
It probably was heading south before it went bad.
There are lots of good machine shops out there.Where are you located? Profile?
I used to pay $45 a hole.That was Jet Machine Shop.Pasadena,Md.
Reasons it went bad:carbs set wrong,timing off,clogged head,clogged exhaust,no thermostat,bad thermostat,
fuel recirc system clogged????? On and on.
Bore out as little as possible..010 or .020 at the max.
Re using the rings,I won't.If a customer asks to re-use old parts like ring etc.I say NO WARRANTY with a rebuild.
A gasket kit is the only way to go.

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

those are good suspect areas. alot of them will be covered during the rebuild. i live in carson city nv so not a lot of shops in the area but sacramento is only a couple hours drive and there is a lot there. definitely will be replacing rings on other pistons as well as thermostat. cracking the crank case open today but the largest gouge i can see i can catch my fingernail in it. i think resleeving may be necessary. last night when i took mounting plate of bottom of block about a half cup thick black oily substance was in between the plate and hollow underneath #3 cylinder. i am thinking possibly sooty accumulation from bad seal w exhaust. any ways i will take a look at carbs once reassembeled and can fire up so as not to have to many areas to check if i have trouble getting it to fire up.

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

just an up date. finished cracking block open and what thought was scratches in cylinder are actually metal transfer,great news i believe. bearings and races are smooth.

 

[jerryjerry05]

Re: 1985 85 hp force

Re: 1985 85 hp force

Good,used pistons are all over the place.
No re-sleeveing on these motors.
If you have a bunch of stuff in the bottom cylinder then the recirculation system failed.
While the carbs are off:screw the air screws in and count the turns.Should be somewhere around 1 turn out.

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

block is at the shop, will be bored on monday and they will call me to let me how much over they had to go so i can order appropriate piston and then they can finish once they receive new piston. so in mean time i will be aquiring the rest of parts to finish the job. what are good sealants to use on the rebuild?? book has part numbers for different ones but i am sure somebody can tell me modern versions. thx in advance

 

[Frank Acampora]

Re: 1985 85 hp force

Re: 1985 85 hp force

Pistons are machined to have proper clearance for an exact overbore. You don't need the piston to finish the boring. Have the shop finish hone to the exact overbore of .010, .020, or .030 plus or minus a couple of ten thousands tolerance. Make your own gaskets for the reed blocks and manifolds and also the bypass port covers. Auto stores will carry the gray gasket paper 1/32 inch thick. Buy an auto thermostat gasket in the correct size for the carb mounting gaskets.

part with a couple of bucks and buy the exhaust cover gaskets (2) and block to match plate gasket.

Older engines had a space between the match plate and the exhaust. They always had accumulations of black exhaust and grease on the match plate. Your engine should have had the match plate with a land and a gasket separating the exhaust tube from the rest of the match plate. Look for a broken gasket here, allowing exhaust to enter.

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

thats good info.thank you,bound to save me a few bucks that way. the sealants i am curious about is what factory used to seal mounting plate to leg.appears original was a white sealant and there is what appears to be black sealant on plate the covers top thrust bearing. is there one that can be used for both? will good ole rtv blue be fine?

 

[jerryjerry05]

Re: 1985 85 hp force

Re: 1985 85 hp force

I use black RTV.

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

alrighty, machine shop just called and asked if i knew the piston to cylinder clearance>manual does not say anything about that so i am assuming the difference between bore size and new piston. is this correct??

 

[Frank Acampora]

Re: 1985 85 hp force

Re: 1985 85 hp force

Just tell them not to worry about clearance. Like I said above: Finish hone to the exact overbore size. The piston is machined to provide correct clearance for the exact overbore. Chrysler and Force never published piston clearance tolerances but the piston manufacturers know what to allow.

AND you can NOT go by difference between bore and piston BECAUSE: The pistons are cam ground to an oval shape with a major and minor diameter to account for uneven heating and expansion ar running temperature. They are also tapered quite a bit to account for crown expansion.

Sigh! People can never read my replies and accept them. I always need to explain further. LOL

 

[ammermanfam]

Re: 1985 85 hp force

Re: 1985 85 hp force

thx Frank for taking the time to explain further, I really do appreciate it.