Fitting Imp Pistons to a 1098

Post by **wanderinstar** » Sun Jun 20, 2004 9:50 pm

I know most people will say go to a 1275,but I have a 1098 that needs reboring. my question is, whats involved.Skimming piston tops, con rods,etc. Someone must have done it in the past. Any info greatfully received. I also intend fitting a 12G295 head,1 3/4" carb onw/heated manifold.
Ian

Post by **rayofleamington** » Sun Jun 20, 2004 10:20 pm

1 3/4" carb

will probably cause a lot of trouble on sucha a small cc.
1.5" will out perform it.

Post by **wanderinstar** » Sun Jun 20, 2004 10:31 pm

I am already running 1 3/4 carb on 1098 now with no problems,but will probably run better onoverbored engine.
Ian.

Post by **Cam** » Sun Jun 20, 2004 10:32 pm

the 1 3/4 runs nice on a tuned 1380! Mind you I ran a tuned 1293 with a 1 1/2" and it was pretty good.

Post by **Scott** » Mon Jun 21, 2004 4:52 am

Ian,
I've done this mod & it's also described by Vizard in his book. It takes the engine to 1215cc.
The bores need to be offset & the block machined down to the pistons.
I'm sure 1275cc would be a better proposition, I only did it because 1275's here are pretty scarce.

Post by **Robins** » Mon Jun 21, 2004 7:53 pm

I've run a modded 998 mini on a 1 3/4 carb with no problems, but after fitting a 1 1/2 carb it went like a rocket. Saying that I know a chap that runs a 1275 moggie on a 1 1/4 carb with no problems.

I've just bought a 1098 spriget engine thats been bored out with Imp pistons in. I will be rebuilding that with the 12G295 head and 1 1/2 carb, can't wait. Although it has Imp pistons they don't look offset, how do you tell Scott?
cheers.

Post by **Scott** » Tue Jun 22, 2004 1:49 am

You probably won't be able to pick up the offset by eye.
The machine shop I used had experience with this mod & later I saw the same mod in Vizards book. I can dig up some figures for you tonight if you want.
It's even possible to fit oversize Imp pistons but the walls are pretty thin then.

Post by **wanderinstar** » Fri Jun 25, 2004 8:05 pm

Scott,
You say that you have done this mod.How long ago? If still running it have you had any problem with smoking?
The reason I ask is that I asked the bloke at Minisport who does the boring, and he said that because the Imp pistons are shorter that you eventually get a smoking engine.

Post by **Scott** » Mon Jun 28, 2004 4:23 am

Does he mean that because the pistons are shorter they'll wear quicker? Does the Imp have problems (other than being an Imp

& overheating)? I haven't heard of this problem - in Imps or Minors.

Unfortunately, premature piston/ring wear wasn't the problem with my engine. Melting #3 & #4 pistons were my biggest problem. Sorted that side out then the crankshaft decided it didn't want to be in one piece.

Post by **brewbags** » Sat Apr 16, 2005 5:45 am

Hi; I ran a 1098 bored to 1230 using Imp pistons some years ago. Great engine, plenty of torque and really noticeable increase in power. Problem encountered was lots of blowby, presumably the walls of the bore flexing thru lack of sufficient metal after the big overbore. This meant pressurised crankcase, and some degree of oil blowing out the breather - not excessive but blowing oil nevertheless. Engine was used for 3 years as daily transport.
Sadly, the engine came to grief on a trip from Melbourne to Brisbane (Australia) & limped all the way home. Problem was a circlip jumping out of a piston, badly scoring the bore. Engine was a U/S.
I would do the mod again, but use teflon buttons instead of circlips. Even Vizard advises that the inertia created with such a long stroke causes the circlips to pop out.

Post by **bmcecosse** » Sat Apr 16, 2005 4:07 pm

'Inertia' with a long stroke ?? I would think the biggest problem would be getting hold of Imp pistons these days !!!

Post by **wanderinstar** » Sat Apr 16, 2005 6:40 pm

I could lay my hands on some s/hand pistons. Question would be how worn are they. When a rebore is nessesary its mainly because bore wears, right?, so does that mean pistons would be OK to put in an original size bore?.

Post by **les** » Sat Apr 16, 2005 9:59 pm

Don't forget that pistons can also wear, the rings loose their snug fit in the grooves.

Post by **brewbags** » Sun Apr 17, 2005 12:55 am

les wrote:Don't forget that pistons can also wear, the rings loose their snug fit in the grooves.

Don't even think about using second hand pistons; you are asking for trouble. JD Pisitons here in Adelaide, South Australia still make Hillman imp pistons.
You might want to consider pistons from a Ford Fiesta, Ford Escort 1989, 1119cc engine. The bore size of 68.715 will give you 1215cc. The compression height of the piston is 29.4mm - much more like what it needs to be (30.3mm is 1098 compression height). The imp piston is so short on compression height you need to have the block machined approx 5mm from memory - scary stuff. I haven't tried the Ford pistons as yet, but the spec's all look good. If you go with the Ford pistons, you would need to do away with the bushes in the little end of the con rod, and have the little end accurately machined so the gudgeon pin is a .003" interference press fit in the con rod (ie just like 1275 pistons are).
I have a set of new Ford pistons ready for a future project.

Post by **bmcecosse** » Sun Apr 17, 2005 7:42 pm

Sounds a bit scarey with the gudgeon pin arrangements - but otherwise it could be ok. But is it all worth it ? Why not just re-bore to the next standard size and use the correct pistons ?

Morris Minor Owners Club

Fitting Imp Pistons to a 1098

Fitting Imp Pistons to a 1098

Imp Pistons to 1098